The Move Reference

Welcome to Move, a next generation language for secure asset programming. Its primary use case is in blockchain environments, where Move programs are used to construct state changes. Move allows developers to write programs that flexibly manage and transfer assets, while providing the security and protections against attacks on those assets. However, Move has been developed with use cases in mind outside a blockchain context as well.

Move takes its cue from Rust by using resource types with move (hence the name) semantics as an explicit representation of digital assets, such as currency.

Modules

Modules are the core program unit that define types along with functions that operate on these types. Struct types define the schema of Move's storage, and module functions define the rules interacting with values of those types. While modules themselves are also stored in storage, they are not accessible from within a Move program. In a blockchain environment, the modules are stored on chain in a process typically referred to as "publishing". After being published, entry and public functions can be invoked according to the rules of that particular Move instance.

Syntax

A module has the following syntax:

module <address>::<identifier> {
    (<use> | <type> | <function> | <constant>)*
}

where <address> is a valid address specifying the module's package.

For example:

module 0x42::test {
    public struct Example has copy, drop { i: u64 }

    use std::debug;

    const ONE: u64 = 1;

    public fun print(x: u64) {
        let sum = x + ONE;
        let example = Example { i: sum };
        debug::print(&sum)
    }
}

Names

The module test_addr::test part specifies that the module test will be published under the numerical address value assigned for the name test_addr in the package settings.

Modules should normally be declared using named addresses (as opposed to using the numerical value directly). For example:

module test_addr::test {
    public struct Example has copy, drop { a: address }

    friend test_addr::another_test;

    public fun print() {
        let example = Example { a: @test_addr };
        debug::print(&example)
    }
}

These named addresses commonly match the name of the package.

Because named addresses only exist at the source language level and during compilation, named addresses will be fully substituted for their value at the bytecode level. For example if we had the following code:

fun example() {
    my_addr::m::foo(@my_addr);
}

and we compiled it with my_addr set to 0xC0FFEE, then it would be operationally equivalent to the following:

fun example() {
    0xC0FFEE::m::foo(@0xC0FFEE);
}

While at the source level these two different accesses are equivalent, it is a best practice to always use the named address and not the numerical value assigned to that address.

Module names can start with a lowercase letter from a to z or an uppercase letter from A to Z. After the first character, module names can contain underscores _, letters a to z, letters A to Z, or digits 0 to 9.

module a::my_module {}
module a::foo_bar_42 {}

Typically, module names start with a lowercase letter. A module named my_module should be stored in a source file named my_module.move.

Members

All members inside a module block can appear in any order. Fundamentally, a module is a collection of types and functions. The use keyword refers to members from other modules. The const keyword defines constants that can be used in the functions of a module.

The friend syntax is a deprecated concept for specifying a list of trusted modules. The concept has been superseded by public(package)

Primitive Types

The primitive types are the basic building blocks of the language.

These primitive types can be used on their own or can be used be used to build more complex, user-defined types, e.g. in a struct.

• Integers
• Bool
• Address
• Vector

These primitive types are used in conjunction with other types

• References
• Tuples and Unit

Integers

Move supports six unsigned integer types: u8, u16, u32, u64, u128, and u256. Values of these types range from 0 to a maximum that depends on the size of the type.

Literals

Literal values for these types are specified either as a sequence of digits (e.g.,112) or as hex literals, e.g., 0xFF. The type of the literal can optionally be added as a suffix, e.g., 112u8. If the type is not specified, the compiler will try to infer the type from the context where the literal is used. If the type cannot be inferred, it is assumed to be u64.

Number literals can be separated by underscores for grouping and readability. (e.g.,1_234_5678, 1_000u128, 0xAB_CD_12_35).

If a literal is too large for its specified (or inferred) size range, an error is reported.

Examples

// literals with explicit annotations;
let explicit_u8 = 1u8;
let explicit_u16 = 1u16;
let explicit_u32 = 1u32;
let explicit_u64 = 2u64;
let explicit_u128 = 3u128;
let explicit_u256 = 1u256;
let explicit_u64_underscored = 154_322_973u64;

// literals with simple inference
let simple_u8: u8 = 1;
let simple_u16: u16 = 1;
let simple_u32: u32 = 1;
let simple_u64: u64 = 2;
let simple_u128: u128 = 3;
let simple_u256: u256 = 1;

// literals with more complex inference
let complex_u8 = 1; // inferred: u8
// right hand argument to shift must be u8
let _unused = 10 << complex_u8;

let x: u8 = 38;
let complex_u8 = 2; // inferred: u8
// arguments to `+` must have the same type
let _unused = x + complex_u8;

let complex_u128 = 133_876; // inferred: u128
// inferred from function argument type
function_that_takes_u128(complex_u128);

// literals can be written in hex
let hex_u8: u8 = 0x1;
let hex_u16: u16 = 0x1BAE;
let hex_u32: u32 = 0xDEAD80;
let hex_u64: u64 = 0xCAFE;
let hex_u128: u128 = 0xDEADBEEF;
let hex_u256: u256 = 0x1123_456A_BCDE_F;

Operations

Arithmetic

Each of these types supports the same set of checked arithmetic operations. For all of these operations, both arguments (the left and right side operands) must be of the same type. If you need to operate over values of different types, you will need to first perform a cast. Similarly, if you expect the result of the operation to be too large for the integer type, perform a cast to a larger size before performing the operation.

All arithmetic operations abort instead of behaving in a way that mathematical integers would not (e.g., overflow, underflow, divide-by-zero).

Bitwise

The integer types support the following bitwise operations that treat each number as a series of individual bits, either 0 or 1, instead of as numerical integer values.

Bitwise operations do not abort.

Bit Shifts

Similar to the bitwise operations, each integer type supports bit shifts. But unlike the other operations, the righthand side operand (how many bits to shift by) must always be a u8 and need not match the left side operand (the number you are shifting).

Bit shifts can abort if the number of bits to shift by is greater than or equal to 8, 16, 32, 64, 128 or 256 for u8, u16, u32, u64, u128 and u256 respectively.

Comparisons

Integer types are the only types in Move that can use the comparison operators. Both arguments need to be of the same type. If you need to compare integers of different types, you must cast one of them first.

Comparison operations do not abort.

Equality

Like all types with drop, all integer types support the "equal" and "not equal" operations. Both arguments need to be of the same type. If you need to compare integers of different types, you must cast one of them first.

Equality operations do not abort.

For more details see the section on equality

Casting

Integer types of one size can be cast to integer types of another size. Integers are the only types in Move that support casting.

Casts do not truncate. Casting aborts if the result is too large for the specified type.

Here, the type of e must be 8, 16, 32, 64, 128 or 256 and T must be u8, u16, u32, u64, u128, or u256.

For example:

• (x as u8)
• (y as u16)
• (873u16 as u32)
• (2u8 as u64)
• (1 + 3 as u128)
• (4/2 + 12345 as u256)

Ownership

As with the other scalar values built-in to the language, integer values are implicitly copyable, meaning they can be copied without an explicit instruction such as copy.

Bool

bool is Move's primitive type for boolean true and false values.

Literals

Literals for bool are either true or false.

Operations

Logical

bool supports three logical operations:

Control Flow

bool values are used in several of Move's control-flow constructs:

• if (bool) { ... }
• while (bool) { .. }
• assert!(bool, u64)

Ownership

As with the other scalar values built-in to the language, boolean values are implicitly copyable, meaning they can be copied without an explicit instruction such as copy.

Address

address is a built-in type in Move that is used to represent locations (sometimes called accounts) in storage. An address value is a 256-bit (32 byte) identifier. Move uses addresses to differentiate packages of modules, where each package has its own address and modules. Specific deployments of Move might also use the address value for storage operations.

For Sui, address is used to represent "accounts", and also objects via strong type wrappers (with sui::object::UID and sui::object::ID).

Although an address is a 256 bit integer under the hood, Move addresses are intentionally opaque---they cannot be created from integers, they do not support arithmetic operations, and they cannot be modified. Specific deployments of Move might have native functions to enable some of these operations (e.g., creating an address from bytes vector<u8>), but these are not part of the Move language itself.

While there are runtime address values (values of type address), they cannot be used to access modules at runtime.

Addresses and Their Syntax

Addresses come in two flavors, named or numerical. The syntax for a named address follows the same rules for any named identifier in Move. The syntax of a numerical address is not restricted to hex-encoded values, and any valid u256 numerical value can be used as an address value, e.g., 42, 0xCAFE, and 10_000 are all valid numerical address literals.

To distinguish when an address is being used in an expression context or not, the syntax when using an address differs depending on the context where it's used:

• When an address is used as an expression, the address must be prefixed by the @ character, i.e., @<numerical_value> or @<named_address_identifier>.
• Outside of expression contexts, the address may be written without the leading @ character, i.e., <numerical_value> or <named_address_identifier>.

In general, you can think of @ as an operator that takes an address from being a namespace item to being an expression item.

Named Addresses

Named addresses are a feature that allow identifiers to be used in place of numerical values in any spot where addresses are used, and not just at the value level. Named addresses are declared and bound as top level elements (outside of modules and scripts) in Move packages, or passed as arguments to the Move compiler.

Named addresses only exist at the source language level and will be fully substituted for their value at the bytecode level. Because of this, modules and module members should be accessed through the module's named address and not through the numerical value assigned to the named address during compilation. So while use my_addr::foo is equivalent to use 0x2::foo (if my_addr is assigned 0x2), it is a best practice to always use the my_addr name.

Examples

// shorthand for
// 0x0000000000000000000000000000000000000000000000000000000000000001
let a1: address = @0x1;
// shorthand for
// 0x0000000000000000000000000000000000000000000000000000000000000042
let a2: address = @0x42;
// shorthand for
// 0x00000000000000000000000000000000000000000000000000000000DEADBEEF
let a3: address = @0xDEADBEEF;
// shorthand for
// 0x000000000000000000000000000000000000000000000000000000000000000A
let a4: address = @0x0000000000000000000000000000000A;
// Assigns `a5` the value of the named address `std`
let a5: address = @std;
// Any valid numerical value can be used as an address
let a6: address = @66;
let a7: address = @42_000;

module 66::some_module {   // Not in expression context, so no @ needed
    use 0x1::other_module; // Not in expression context so no @ needed
    use std::vector;       // Can use a named address as a namespace item
    ...
}

module std::other_module {  // Can use a named address when declaring a module
    ...
}

Vector

vector<T> is the only primitive collection type provided by Move. A vector<T> is a homogeneous collection of T's that can grow or shrink by pushing/popping values off the "end".

A vector<T> can be instantiated with any type T. For example, vector<u64>, vector<address>, vector<0x42::my_module::MyData>, and vector<vector<u8>> are all valid vector types.

Literals

General vector Literals

Vectors of any type can be created with vector literals.

In these cases, the type of the vector is inferred, either from the element type or from the vector's usage. If the type cannot be inferred, or simply for added clarity, the type can be specified explicitly:

vector<T>[]: vector<T>
vector<T>[e1, ..., en]: vector<T>

Example Vector Literals

(vector[]: vector<bool>);
(vector[0u8, 1u8, 2u8]: vector<u8>);
(vector<u128>[]: vector<u128>);
(vector<address>[@0x42, @0x100]: vector<address>);

vector<u8> literals

A common use-case for vectors in Move is to represent "byte arrays", which are represented with vector<u8>. These values are often used for cryptographic purposes, such as a public key or a hash result. These values are so common that specific syntax is provided to make the values more readable, as opposed to having to use vector[] where each individual u8 value is specified in numeric form.

There are currently two supported types of vector<u8> literals, byte strings and hex strings.

Byte Strings

Byte strings are quoted string literals prefixed by a b, e.g. b"Hello!\n".

These are ASCII encoded strings that allow for escape sequences. Currently, the supported escape sequences are:

Hex Strings

Hex strings are quoted string literals prefixed by a x, e.g. x"48656C6C6F210A".

Each byte pair, ranging from 00 to FF, is interpreted as hex encoded u8 value. So each byte pair corresponds to a single entry in the resulting vector<u8>.

Example String Literals

fun byte_and_hex_strings() {
    assert!(b"" == x"", 0);
    assert!(b"Hello!\n" == x"48656C6C6F210A", 1);
    assert!(b"\x48\x65\x6C\x6C\x6F\x21\x0A" == x"48656C6C6F210A", 2);
    assert!(
        b"\"Hello\tworld!\"\n \r \\Null=\0" ==
            x"2248656C6C6F09776F726C6421220A200D205C4E756C6C3D00",
        3
    );
}

Operations

vector supports the following operations via the std::vector module in the Move standard library:

More operations may be added over time.

Example

use std::vector;

let mut v = vector::empty<u64>();
vector::push_back(&mut v, 5);
vector::push_back(&mut v, 6);

assert!(*vector::borrow(&v, 0) == 5, 42);
assert!(*vector::borrow(&v, 1) == 6, 42);
assert!(vector::pop_back(&mut v) == 6, 42);
assert!(vector::pop_back(&mut v) == 5, 42);

Destroying and copying vectors

Some behaviors of vector<T> depend on the abilities of the element type, T. For example, vectors containing elements that do not have drop cannot be implicitly discarded like v in the example above--they must be explicitly destroyed with vector::destroy_empty.

Note that vector::destroy_empty will abort at runtime unless vec contains zero elements:

fun destroy_any_vector<T>(vec: vector<T>) {
    vector::destroy_empty(vec) // deleting this line will cause a compiler error
}

But no error would occur for dropping a vector that contains elements with drop:

fun destroy_droppable_vector<T: drop>(vec: vector<T>) {
    // valid!
    // nothing needs to be done explicitly to destroy the vector
}

Similarly, vectors cannot be copied unless the element type has copy. In other words, a vector<T> has copy if and only if T has copy. Note that it will be implicitly copied if needed:

let x = vector[10];
let y = x; // implicit copy
let z = x;
(y, z)

Keep in mind, copies of large vectors can be expensive. If this is a concern, annotating the intended usage can prevent accidental copies. For example,

let x = vector[10];
let y = move x;
let z = x; // ERROR! x has been moved
(y, z)

For more details see the sections on type abilities and generics.

Ownership

As mentioned above, vector values can be copied only if the elements can be copied. In that case, the copy can be done via a copy or a dereference *.

References

Move has two types of references: immutable & and mutable &mut. Immutable references are read only, and cannot modify the underlying value (or any of its fields). Mutable references allow for modifications via a write through that reference. Move's type system enforces an ownership discipline that prevents reference errors.

Reference Operators

Move provides operators for creating and extending references as well as converting a mutable reference to an immutable one. Here and elsewhere, we use the notation e: T for "expression e has type T".

The &e.f and &mut e.f operators can be used both to create a new reference into a struct or to extend an existing reference:

let s = S { f: 10 };
let f_ref1: &u64 = &s.f; // works
let s_ref: &S = &s;
let f_ref2: &u64 = &s_ref.f // also works

A reference expression with multiple fields works as long as both structs are in the same module:

public struct A { b: B }
public struct B { c : u64 }
fun f(a: &A): &u64 {
    &a.b.c
}

Finally, note that references to references are not allowed:

let x = 7;
let y: &u64 = &x;
let z: &&u64 = &y; // ERROR! will not compile

Reading and Writing Through References

Both mutable and immutable references can be read to produce a copy of the referenced value.

Only mutable references can be written. A write *x = v discards the value previously stored in x and updates it with v.

Both operations use the C-like * syntax. However, note that a read is an expression, whereas a write is a mutation that must occur on the left hand side of an equals.

In order for a reference to be read, the underlying type must have the copy ability as reading the reference creates a new copy of the value. This rule prevents the copying of assets:

fun copy_coin_via_ref_bad(c: Coin) {
    let c_ref = &c;
    let counterfeit: Coin = *c_ref; // not allowed!
    pay(c);
    pay(counterfeit);
}

Dually: in order for a reference to be written to, the underlying type must have the drop ability as writing to the reference will discard (or "drop") the old value. This rule prevents the destruction of resource values:

fun destroy_coin_via_ref_bad(mut ten_coins: Coin, c: Coin) {
    let ref = &mut ten_coins;
    *ref = c; // ERROR! not allowed--would destroy 10 coins!
}

freeze inference

A mutable reference can be used in a context where an immutable reference is expected:

let mut x = 7;
let y: &u64 = &mut x;

This works because the under the hood, the compiler inserts freeze instructions where they are needed. Here are a few more examples of freeze inference in action:

fun takes_immut_returns_immut(x: &u64): &u64 { x }

// freeze inference on return value
fun takes_mut_returns_immut(x: &mut u64): &u64 { x }

fun expression_examples() {
    let mut x = 0;
    let mut y = 0;
    takes_immut_returns_immut(&x); // no inference
    takes_immut_returns_immut(&mut x); // inferred freeze(&mut x)
    takes_mut_returns_immut(&mut x); // no inference

    assert!(&x == &mut y, 42); // inferred freeze(&mut y)
}

fun assignment_examples() {
    let x = 0;
    let y = 0;
    let imm_ref: &u64 = &x;

    imm_ref = &x; // no inference
    imm_ref = &mut y; // inferred freeze(&mut y)
}

Subtyping

With this freeze inference, the Move type checker can view &mut T as a subtype of &T. As shown above, this means that anywhere for any expression where a &T value is used, a &mut T value can also be used. This terminology is used in error messages to concisely indicate that a &mut T was needed where a &T was supplied. For example

module a::example {
    fun read_and_assign(store: &mut u64, new_value: &u64) {
        *store = *new_value
    }

    fun subtype_examples() {
        let mut x: &u64 = &0;
        let mut y: &mut u64 = &mut 1;

        x = &mut 1; // valid
        y = &2; // ERROR! invalid!

        read_and_assign(y, x); // valid
        read_and_assign(x, y); // ERROR! invalid!
    }
}

will yield the following error messages

error:

    ┌── example.move:11:9 ───
    │
 12 │         y = &2; // invalid!
    │         ^ Invalid assignment to local 'y'
    ·
 12 │         y = &2; // invalid!
    │             -- The type: '&{integer}'
    ·
  9 │         let mut y: &mut u64 = &mut 1;
    │                    -------- Is not a subtype of: '&mut u64'
    │

error:

    ┌── example.move:14:9 ───
    │
 15 │         read_and_assign(x, y); // invalid!
    │         ^^^^^^^^^^^^^^^^^^^^^ Invalid call of 'a::example::read_and_assign'. Invalid argument for parameter 'store'
    ·
  8 │         let mut x: &u64 = &0;
    │                    ---- The type: '&u64'
    ·
  3 │     fun read_and_assign(store: &mut u64, new_value: &u64) {
    │                                -------- Is not a subtype of: '&mut u64'
    │

The only other types that currently have subtyping are tuples

Ownership

Both mutable and immutable references can always be copied and extended even if there are existing copies or extensions of the same reference:

fun reference_copies(s: &mut S) {
  let s_copy1 = s; // ok
  let s_extension = &mut s.f; // also ok
  let s_copy2 = s; // still ok
  ...
}

This might be surprising for programmers familiar with Rust's ownership system, which would reject the code above. Move's type system is more permissive in its treatment of copies, but equally strict in ensuring unique ownership of mutable references before writes.

References Cannot Be Stored

References and tuples are the only types that cannot be stored as a field value of structs, which also means that they cannot exist in storage or objects. All references created during program execution will be destroyed when a Move program terminates; they are entirely ephemeral. This also applies to all types without the store ability: any value of a non-store type must be destroyed before the program terminates. ability, but note that references and tuples go a step further by never being allowed in structs in the first place.

This is another difference between Move and Rust, which allows references to be stored inside of structs.

One could imagine a fancier, more expressive, type system that would allow references to be stored in structs. We could allow references inside of structs that do not have the store ability, but the core difficulty is that Move has a fairly complex system for tracking static reference safety. This aspect of the type system would also have to be extended to support storing references inside of structs. In short, Move's reference safety system would have to expand to support stored references, and it is something we are keeping an eye on as the language evolves.

Tuples and Unit

Move does not fully support tuples as one might expect coming from another language with them as a first-class value. However, in order to support multiple return values, Move has tuple-like expressions. These expressions do not result in a concrete value at runtime (there are no tuples in the bytecode), and as a result they are very limited:

• They can only appear in expressions (usually in the return position for a function).
• They cannot be bound to local variables.
• They cannot be bound to local variables.
• They cannot be stored in structs.
• Tuple types cannot be used to instantiate generics.

Similarly, unit () is a type created by the Move source language in order to be expression based. The unit value () does not result in any runtime value. We can consider unit() to be an empty tuple, and any restrictions that apply to tuples also apply to unit.

It might feel weird to have tuples in the language at all given these restrictions. But one of the most common use cases for tuples in other languages is for functions to allow functions to return multiple values. Some languages work around this by forcing the users to write structs that contain the multiple return values. However in Move, you cannot put references inside of structs. This required Move to support multiple return values. These multiple return values are all pushed on the stack at the bytecode level. At the source level, these multiple return values are represented using tuples.

Literals

Tuples are created by a comma separated list of expressions inside of parentheses.

Note that (e) does not have type (e): (t), in other words there is no tuple with one element. If there is only a single element inside of the parentheses, the parentheses are only used for disambiguation and do not carry any other special meaning.

Sometimes, tuples with two elements are called "pairs" and tuples with three elements are called "triples."

Examples

module 0x42::example {
    // all 3 of these functions are equivalent

    // when no return type is provided, it is assumed to be `()`
    fun returns_unit_1() { }

    // there is an implicit () value in empty expression blocks
    fun returns_unit_2(): () { }

    // explicit version of `returns_unit_1` and `returns_unit_2`
    fun returns_unit_3(): () { () }


    fun returns_3_values(): (u64, bool, address) {
        (0, false, @0x42)
    }
    fun returns_4_values(x: &u64): (&u64, u8, u128, vector<u8>) {
        (x, 0, 1, b"foobar")
    }
}

Operations

The only operation that can be done on tuples currently is destructuring.

Destructuring

For tuples of any size, they can be destructured in either a let binding or in an assignment.

For example:

module 0x42::example {
    // all 3 of these functions are equivalent
    fun returns_unit() {}
    fun returns_2_values(): (bool, bool) { (true, false) }
    fun returns_4_values(x: &u64): (&u64, u8, u128, vector<u8>) { (x, 0, 1, b"foobar") }

    fun examples(cond: bool) {
        let () = ();
        let (mut x, mut y): (u8, u64) = (0, 1);
        let (mut a, mut b, mut c, mut d) = (@0x0, 0, false, b"");

        () = ();
        (x, y) = if (cond) (1, 2) else (3, 4);
        (a, b, c, d) = (@0x1, 1, true, b"1");
    }

    fun examples_with_function_calls() {
        let () = returns_unit();
        let (mut x, mut y): (bool, bool) = returns_2_values();
        let (mut a, mut b, mut c, mut d) = returns_4_values(&0);

        () = returns_unit();
        (x, y) = returns_2_values();
        (a, b, c, d) = returns_4_values(&1);
    }
}

For more details, see Move Variables.

Subtyping

Along with references, tuples are the only types that have subtyping in Move. Tuples have subtyping only in the sense that subtype with references (in a covariant way).

For example:

let x: &u64 = &0;
let y: &mut u64 = &mut 1;

// (&u64, &mut u64) is a subtype of (&u64, &u64)
// since &mut u64 is a subtype of &u64
let (a, b): (&u64, &u64) = (x, y);

// (&mut u64, &mut u64) is a subtype of (&u64, &u64)
// since &mut u64 is a subtype of &u64
let (c, d): (&u64, &u64) = (y, y);

// ERROR! (&u64, &mut u64) is NOT a subtype of (&mut u64, &mut u64)
// since &u64 is NOT a subtype of &mut u64
let (e, f): (&mut u64, &mut u64) = (x, y);

Ownership

As mentioned above, tuple values don't really exist at runtime. And currently they cannot be stored into local variables because of this (but it is likely that this feature will come at some point in the future). As such, tuples can only be moved currently, as copying them would require putting them into a local variable first.

Local Variables and Scope

Local variables in Move are lexically (statically) scoped. New variables are introduced with the keyword let, which will shadow any previous local with the same name. Locals marked as mut are mutable and can be updated both directly and via a mutable reference.

Declaring Local Variables

let bindings

Move programs use let to bind variable names to values:

let x = 1;
let y = x + x:

let can also be used without binding a value to the local.

let x;

The local can then be assigned a value later.

let x;
if (cond) {
  x = 1
} else {
  x = 0
}

This can be very helpful when trying to extract a value from a loop when a default value cannot be provided.

let x;
let cond = true;
let i = 0;
loop {
    let (res, cond) = foo(i);
    if (!cond) {
        x = res;
        break;
    };
    i = i + 1;
}

To modify a local variable after it is assigned, or to borrow it mutably &mut, it must be declared as mut.

let mut x = 0;
if (cond) x = x + 1;
foo(&mut x);

For more details see the section on assignments below.

Variables must be assigned before use

Move's type system prevents a local variable from being used before it has been assigned.

let x;
x + x // ERROR! x is used before being assigned

let x;
if (cond) x = 0;
x + x // ERROR! x does not have a value in all cases

let x;
while (cond) x = 0;
x + x // ERROR! x does not have a value in all cases

Valid variable names

Variable names can contain underscores _, letters a to z, letters A to Z, and digits 0 to 9. Variable names must start with either an underscore _ or a letter a through z. They cannot start with uppercase letters.

// all valid
let x = e;
let _x = e;
let _A = e;
let x0 = e;
let xA = e;
let foobar_123 = e;

// all invalid
let X = e; // ERROR!
let Foo = e; // ERROR!

Type annotations

The type of a local variable can almost always be inferred by Move's type system. However, Move allows explicit type annotations that can be useful for readability, clarity, or debuggability. The syntax for adding a type annotation is:

let x: T = e; // "Variable x of type T is initialized to expression e"

Some examples of explicit type annotations:

module 0x42::example {

    public struct S { f: u64, g: u64 }

    fun annotated() {
        let u: u8 = 0;
        let b: vector<u8> = b"hello";
        let a: address = @0x0;
        let (x, y): (&u64, &mut u64) = (&0, &mut 1);
        let S { f, g: f2 }: S = S { f: 0, g: 1 };
    }
}

Note that the type annotations must always be to the right of the pattern:

// ERROR! should be let (x, y): (&u64, &mut u64) = ...
let (x: &u64, y: &mut u64) = (&0, &mut 1);

When annotations are necessary

In some cases, a local type annotation is required if the type system cannot infer the type. This commonly occurs when the type argument for a generic type cannot be inferred. For example:

let _v1 = vector[]; // ERROR!
//        ^^^^^^^^ Could not infer this type. Try adding an annotation
let v2: vector<u64> = vector[]; // no error

In a rarer case, the type system might not be able to infer a type for divergent code (where all the following code is unreachable). Both return and abort are expressions and can have any type. A loop has type () if it has a break (or T if has a break e where e: T), but if there is no break out of the loop, it could have any type. If these types cannot be inferred, a type annotation is required. For example, this code:

let a: u8 = return ();
let b: bool = abort 0;
let c: signer = loop ();

let x = return (); // ERROR!
//  ^ Could not infer this type. Try adding an annotation
let y = abort 0; // ERROR!
//  ^ Could not infer this type. Try adding an annotation
let z = loop (); // ERROR!
//  ^ Could not infer this type. Try adding an annotation

Adding type annotations to this code will expose other errors about dead code or unused local variables, but the example is still helpful for understanding this problem.

Multiple declarations with tuples

let can introduce more than one local at a time using tuples. The locals declared inside the parenthesis are initialized to the corresponding values from the tuple.

let () = ();
let (x0, x1) = (0, 1);
let (y0, y1, y2) = (0, 1, 2);
let (z0, z1, z2, z3) = (0, 1, 2, 3);

The type of the expression must match the arity of the tuple pattern exactly.

let (x, y) = (0, 1, 2); // ERROR!
let (x, y, z, q) = (0, 1, 2); // ERROR!

You cannot declare more than one local with the same name in a single let.

let (x, x) = 0; // ERROR!

The mutability of the local variables declared can be mixed.

let (mut x, y) = (0, 1);
x = 1;

Multiple declarations with structs

let can also introduce more than one local at a time when destructuring (or matching against) a struct. In this form, the let creates a set of local variables that are initialized to the values of the fields from a struct. The syntax looks like this:

public struct T { f1: u64, f2: u64 }

let T { f1: local1, f2: local2 } = T { f1: 1, f2: 2 };
// local1: u64
// local2: u64

Similarly for positional structs

public struct P(u64, u64)

and

let P(local1, local2) = T { f1: 1, f2: 2 };
// local1: u64
// local2: u64

Here is a more complicated example:

module 0x42::example {
    public struct X(u64)
    public struct Y { x1: X, x2: X }

    fun new_x(): X {
        X(1)
    }

    fun example() {
        let Y { x1: X(f), x2 } = Y { x1: new_x(), x2: new_x() };
        assert!(f + x2.f == 2, 42);

        let Y { x1: X(f1), x2: X(f2) } = Y { x1: new_x(), x2: new_x() };
        assert!(f1 + f2 == 2, 42);
    }
}

Fields of structs can serve double duty, identifying the field to bind and the name of the variable. This is sometimes referred to as punning.

let Y { x1, x2 } = e;

is equivalent to:

let Y { x1: x1, x2: x2 } = e;

As shown with tuples, you cannot declare more than one local with the same name in a single let.

let Y { x1: x, x2: x } = e; // ERROR!

And as with tuples, the mutability of the local variables declared can be mixed.

let Y { x1: mut x1, x2 } = e;

Furthermore, the mutability of annotation can be applied to the punned fields. Giving the equivalent example

let Y { mut x1, x2 } = e;

Destructuring against references

In the examples above for structs, the bound value in the let was moved, destroying the struct value and binding its fields.

public struct T { f1: u64, f2: u64 }

let T { f1: local1, f2: local2 } = T { f1: 1, f2: 2 };
// local1: u64
// local2: u64

In this scenario the struct value T { f1: 1, f2: 2 } no longer exists after the let.

If you wish instead to not move and destroy the struct value, you can borrow each of its fields. For example:

let t = T { f1: 1, f2: 2 };
let T { f1: local1, f2: local2 } = &t;
// local1: &u64
// local2: &u64

And similarly with mutable references:

let mut t = T { f1: 1, f2: 2 };
let T { f1: local1, f2: local2 } = &mut t;
// local1: &mut u64
// local2: &mut u64

This behavior can also work with nested structs.

module 0x42::example {
    public struct X(u64)
    public struct Y { x1: X, x2: X }

    fun new_x(): X {
        X(1)
    }

    fun example() {
        let mut y = Y { x1: new_x(), x2: new_x() };

        let Y { x1: X(f), x2 } = &y;
        assert!(*f + x2.f == 2, 42);

        let Y { x1: X(f1), x2: X(f2) } = &mut y;
        *f1 = *f1 + 1;
        *f2 = *f2 + 1;
        assert!(*f1 + *f2 == 4, 42);
    }
}

Ignoring Values

In let bindings, it is often helpful to ignore some values. Local variables that start with _ will be ignored and not introduce a new variable

fun three(): (u64, u64, u64) {
    (0, 1, 2)
}

let (x1, _, z1) = three();
let (x2, _y, z2) = three();
assert!(x1 + z1 == x2 + z2, 42);

This can be necessary at times as the compiler will warn on unused local variables

let (x1, y, z1) = three(); // WARNING!
//       ^ unused local 'y'

General let grammar

All of the different structures in let can be combined! With that we arrive at this general grammar for let statements:

let-binding → let pattern-or-list type-annotation_opt > initializer_opt > pattern-or-list → pattern | ( pattern-list ) > pattern-list → pattern ,_opt | pattern , pattern-list > type-annotation → : type initializer → = expression

The general term for the item that introduces the bindings is a pattern. The pattern serves to both destructure data (possibly recursively) and introduce the bindings. The pattern grammar is as follows:

pattern → local-variable | struct-type { field-binding-list } > field-binding-list → field-binding ,_opt | field-binding , > field-binding-list > field-binding → field | field : pattern

A few concrete examples with this grammar applied:

    let (x, y): (u64, u64) = (0, 1);
//       ^                           local-variable
//       ^                           pattern
//          ^                        local-variable
//          ^                        pattern
//          ^                        pattern-list
//       ^^^^                        pattern-list
//      ^^^^^^                       pattern-or-list
//            ^^^^^^^^^^^^           type-annotation
//                         ^^^^^^^^  initializer
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ let-binding

    let Foo { f, g: x } = Foo { f: 0, g: 1 };
//      ^^^                                    struct-type
//            ^                                field
//            ^                                field-binding
//               ^                             field
//                  ^                          local-variable
//                  ^                          pattern
//               ^^^^                          field-binding
//            ^^^^^^^                          field-binding-list
//      ^^^^^^^^^^^^^^^                        pattern
//      ^^^^^^^^^^^^^^^                        pattern-or-list
//                      ^^^^^^^^^^^^^^^^^^^^   initializer
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ let-binding

Mutations

Assignments

After the local is introduced (either by let or as a function parameter), a mut local can be modified via an assignment:

x = e

Unlike let bindings, assignments are expressions. In some languages, assignments return the value that was assigned, but in Move, the type of any assignment is always ().

(x = e: ())

Practically, assignments being expressions means that they can be used without adding a new expression block with braces ({...}).

let x;
if (cond) x = 1 else x = 2;

The assignment uses the similar pattern syntax scheme as let bindings, but with absence of mut:

module 0x42::example {
    public struct X { f: u64 }

    fun new_x(): X {
        X { f: 1 }
    }

    // Note: this example will complain about unused variables and assignments.
    fun example() {
       let (mut x, mut y, mut f, mut g) = (0, 0, 0, 0);

       (X { f }, X { f: x }) = (new_x(), new_x());
       assert!(f + x == 2, 42);

       (x, y, f, _, g) = (0, 0, 0, 0, 0);
    }
}

Note that a local variable can only have one type, so the type of the local cannot change between assignments.

let mut x;
x = 0;
x = false; // ERROR!

Mutating through a reference

In addition to directly modifying a local with assignment, a mut local can be modified via a mutable reference &mut.

let mut x = 0;
let r = &mut x;
*r = 1;
assert!(x == 1, 42);

This is particularly useful if either:

(1) You want to modify different variables depending on some condition.

let mut x = 0;
let mut y = 1;
let r = if (cond) &mut x else &mut y;
*r = *r + 1;

(2) You want another function to modify your local value.

let mut x = 0;
modify_ref(&mut x);

This sort of modification is how you modify structs and vectors!

let mut v = vector[];
vector::push_back(&mut v, 100);
assert!(*vector::borrow(&v, 0) == 100, 42);

For more details, see Move references.

Scopes

Any local declared with let is available for any subsequent expression, within that scope. Scopes are declared with expression blocks, {...}.

Locals cannot be used outside of the declared scope.

let x = 0;
{
    let y = 1;
};
x + y // ERROR!
//  ^ unbound local 'y'

But, locals from an outer scope can be used in a nested scope.

{
    let x = 0;
    {
        let y = x + 1; // valid
    }
}

Locals can be mutated in any scope where they are accessible. That mutation survives with the local, regardless of the scope that performed the mutation.

let mut x = 0;
x = x + 1;
assert!(x == 1, 42);
{
    x = x + 1;
    assert!(x == 2, 42);
};
assert!(x == 2, 42);

Expression Blocks

An expression block is a series of statements separated by semicolons (;). The resulting value of an expression block is the value of the last expression in the block.

{ let x = 1; let y = 1; x + y }

In this example, the result of the block is x + y.

A statement can be either a let declaration or an expression. Remember that assignments (x = e) are expressions of type ().

{ let x; let y = 1; x = 1; x + y }

Function calls are another common expression of type (). Function calls that modify data are commonly used as statements.

{ let v = vector[]; vector::push_back(&mut v, 1); v }

This is not just limited to () types---any expression can be used as a statement in a sequence!

{
    let x = 0;
    x + 1; // value is discarded
    x + 2; // value is discarded
    b"hello"; // value is discarded
}

But! If the expression contains a resource (a value without the drop ability), you will get an error. This is because Move's type system guarantees that any value that is dropped has the drop ability. (Ownership must be transferred or the value must be explicitly destroyed within its declaring module.)

{
    let x = 0;
    Coin { value: x }; // ERROR!
//  ^^^^^^^^^^^^^^^^^ unused value without the `drop` ability
    x
}

If a final expression is not present in a block---that is, if there is a trailing semicolon ;, there is an implicit unit () value. Similarly, if the expression block is empty, there is an implicit unit () value.

Both are equivalent

{ x = x + 1; 1 / x; }

{ x = x + 1; 1 / x; () }

Similarly both are equivalent

{ }

{ () }

An expression block is itself an expression and can be used anyplace an expression is used. (Note: The body of a function is also an expression block, but the function body cannot be replaced by another expression.)

let my_vector: vector<vector<u8>> = {
    let mut v = vector[];
    vector::push_back(&mut v, b"hello");
    vector::push_back(&mut v, b"goodbye");
    v
};

(The type annotation is not needed in this example and only added for clarity.)

Shadowing

If a let introduces a local variable with a name already in scope, that previous variable can no longer be accessed for the rest of this scope. This is called shadowing.

let x = 0;
assert!(x == 0, 42);

let x = 1; // x is shadowed
assert!(x == 1, 42);

When a local is shadowed, it does not need to retain the same type as before.

let x = 0;
assert!(x == 0, 42);

let x = b"hello"; // x is shadowed
assert!(x == b"hello", 42);

After a local is shadowed, the value stored in the local still exists, but will no longer be accessible. This is important to keep in mind with values of types without the drop ability, as ownership of the value must be transferred by the end of the function.

module 0x42::example {
    public struct Coin has store { value: u64 }

    fun unused_coin(): Coin {
        let x = Coin { value: 0 }; // ERROR!
//          ^ This local still contains a value without the `drop` ability
        x.value = 1;
        let x = Coin { value: 10 };
        x
//      ^ Invalid return
    }
}

When a local is shadowed inside a scope, the shadowing only remains for that scope. The shadowing is gone once that scope ends.

let x = 0;
{
    let x = 1;
    assert!(x == 1, 42);
};
assert!(x == 0, 42);

Remember, locals can change type when they are shadowed.

let x = 0;
{
    let x = b"hello";
    assert!(x = b"hello", 42);
};
assert!(x == 0, 42);

Move and Copy

All local variables in Move can be used in two ways, either by move or copy. If one or the other is not specified, the Move compiler is able to infer whether a copy or a move should be used. This means that in all of the examples above, a move or a copy would be inserted by the compiler. A local variable cannot be used without the use of move or copy.

copy will likely feel the most familiar coming from other programming languages, as it creates a new copy of the value inside of the variable to use in that expression. With copy, the local variable can be used more than once.

let x = 0;
let y = copy x + 1;
let z = copy x + 2;

Any value with the copy ability can be copied in this way, and will be copied implicitly unless a move is specified.

move takes the value out of the local variable without copying the data. After a move occurs, the local variable is unavailable, even if the value's type has the copy ability.

let x = 1;
let y = move x + 1;
//      ------ Local was moved here
let z = move x + 2; // Error!
//      ^^^^^^ Invalid usage of local 'x'
y + z

Safety

Move's type system will prevent a value from being used after it is moved. This is the same safety check described in let declaration that prevents local variables from being used before it is assigned a value.

Inference

As mentioned above, the Move compiler will infer a copy or move if one is not indicated. The algorithm for doing so is quite simple:

• Any value with the copy ability is given a copy.
• Any reference (both mutable &mut and immutable &) is given a copy.
  • Except under special circumstances where it is made a move for predictable borrow checker errors. This will happen once the reference is no longer used.
• Any other value is given a move.

Given the structs

public struct Foo has copy, drop, store { f: u64 }
public struct Coin has store { value: u64 }

we have the following example

let s = b"hello";
let foo = Foo { f: 0 };
let coin = Coin { value: 0 };
let coins = vector[Coin { value: 0 }, Coin { value: 0 }];

let s2 = s; // copy
let foo2 = foo; // copy
let coin2 = coin; // move
let coins2 = coin; // move

let x = 0;
let b = false;
let addr = @0x42;
let x_ref = &x;
let coin_ref = &mut coin2;

let x2 = x; // copy
let b2 = b; // copy
let addr2 = @0x42; // copy
let x_ref2 = x_ref; // copy
let coin_ref2 = coin_ref; // copy

Equality

Move supports two equality operations == and !=

Operations

Typing

Both the equal (==) and not-equal (!=) operations only work if both operands are the same type

0 == 0; // `true`
1u128 == 2u128; // `false`
b"hello" != x"00"; // `true`

Equality and non-equality also work over all user defined types!

module 0x42::example {
    public struct S has copy, drop { f: u64, s: vector<u8> }

    fun always_true(): bool {
        let s = S { f: 0, s: b"" };
        s == s
    }

    fun always_false(): bool {
        let s = S { f: 0, s: b"" };
        s != s
    }
}

If the operands have different types, there is a type checking error

1u8 == 1u128; // ERROR!
//     ^^^^^ expected an argument of type 'u8'
b"" != 0; // ERROR!
//     ^ expected an argument of type 'vector<u8>'

Typing with references

When comparing references, the type of the reference (immutable or mutable) does not matter. This means that you can compare an immutable & reference with a mutable one &mut of the same underlying type.

let i = &0;
let m = &mut 1;

i == m; // `false`
m == i; // `false`
m == m; // `true`
i == i; // `true`

The above is equivalent to applying an explicit freeze to each mutable reference where needed

let i = &0;
let m = &mut 1;

i == freeze(m); // `false`
freeze(m) == i; // `false`
m == m; // `true`
i == i; // `true`

But again, the underlying type must be the same type

let i = &0;
let s = &b"";

i == s; // ERROR!
//   ^ expected an argument of type '&u64'

Automatic Borrowing

Starting in Move 2024 edition, the == and != operators automatically borrow their operands if one of the operands is a reference and the other is not. This means that the following code works without any errors:

let r = &0;

// In all cases, `0` is automatically borrowed as `&0`
r == 0; // `true`
0 == r; // `true`
r != 0; // `false`
0 != r; // `false`

This automatic borrow is always an immutable borrow.

Restrictions

Both == and != consume the value when comparing them. As a result, the type system enforces that the type must have drop. Recall that without the drop ability, ownership must be transferred by the end of the function, and such values can only be explicitly destroyed within their declaring module. If these were used directly with either equality == or non-equality !=, the value would be destroyed which would break drop ability safety guarantees!

module 0x42::example {
    public struct Coin has store { value: u64 }
    fun invalid(c1: Coin, c2: Coin) {
        c1 == c2 // ERROR!
//      ^^    ^^ These assets would be destroyed!
    }
}

But, a programmer can always borrow the value first instead of directly comparing the value, and reference types have the drop ability. For example

module 0x42::example {
    public struct Coin as store { value: u64 }
    fun swap_if_equal(c1: Coin, c2: Coin): (Coin, Coin) {
        let are_equal = &c1 == c2; // valid, note `c2` is automatically borrowed
        if (are_equal) (c2, c1) else (c1, c2)
    }
}

Avoid Extra Copies

While a programmer can compare any value whose type has drop, a programmer should often compare by reference to avoid expensive copies.

let v1: vector<u8> = function_that_returns_vector();
let v2: vector<u8> = function_that_returns_vector();
assert!(copy v1 == copy v2, 42);
//      ^^^^       ^^^^
use_two_vectors(v1, v2);

let s1: Foo = function_that_returns_large_struct();
let s2: Foo = function_that_returns_large_struct();
assert!(copy s1 == copy s2, 42);
//      ^^^^       ^^^^
use_two_foos(s1, s2);

This code is perfectly acceptable (assuming Foo has drop), just not efficient. The highlighted copies can be removed and replaced with borrows

let v1: vector<u8> = function_that_returns_vector();
let v2: vector<u8> = function_that_returns_vector();
assert!(&v1 == &v2, 42);
//      ^      ^
use_two_vectors(v1, v2);

let s1: Foo = function_that_returns_large_struct();
let s2: Foo = function_that_returns_large_struct();
assert!(&s1 == &s2, 42);
//      ^      ^
use_two_foos(s1, s2);

The efficiency of the == itself remains the same, but the copys are removed and thus the program is more efficient.

Abort and Assert

return and abort are two control flow constructs that end execution, one for the current function and one for the entire transaction.

More information on return can be found in the linked section

abort

abort is an expression that takes one argument: an abort code of type u64. For example:

abort 42

The abort expression halts execution the current function and reverts all changes made to state by the current transaction (note though that this guarantee must be upheld by the adapter of the specific deployment of Move). There is no mechanism for "catching" or otherwise handling an abort.

Luckily, in Move transactions are all or nothing, meaning any changes to storage are made all at once only if the transaction succeeds. For Sui, this means no objects are modified.

Because of this transactional commitment of changes, after an abort there is no need to worry about backing out changes. While this approach is lacking in flexibility, it is incredibly simple and predictable.

Similar to return, abort is useful for exiting control flow when some condition cannot be met.

In this example, the function will pop two items off of the vector, but will abort early if the vector does not have two items

use std::vector;

fun pop_twice<T>(v: &mut vector<T>): (T, T) {
    if (vector::length(v) < 2) abort 42;
    (vector::pop_back(v), vector::pop_back(v))
}

This is even more useful deep inside a control-flow construct. For example, this function checks that all numbers in the vector are less than the specified bound. And aborts otherwise

use std::vector;
fun check_vec(v: &vector<u64>, bound: u64) {
    let i = 0;
    let n = vector::length(v);
    while (i < n) {
        let cur = *vector::borrow(v, i);
        if (cur > bound) abort 42;
        i = i + 1;
    }
}

assert

assert is a builtin, macro operation provided by the Move compiler. It takes two arguments, a condition of type bool and a code of type u64

assert!(condition: bool, code: u64)

Since the operation is a macro, it must be invoked with the !. This is to convey that the arguments to assert are call-by-expression. In other words, assert is not a normal function and does not exist at the bytecode level. It is replaced inside the compiler with

if (condition) () else abort code

assert is more commonly used than just abort by itself. The abort examples above can be rewritten using assert

use std::vector;
fun pop_twice<T>(v: &mut vector<T>): (T, T) {
    assert!(vector::length(v) >= 2, 42); // Now uses 'assert'
    (vector::pop_back(v), vector::pop_back(v))
}

and

use std::vector;
fun check_vec(v: &vector<u64>, bound: u64) {
    let i = 0;
    let n = vector::length(v);
    while (i < n) {
        let cur = *vector::borrow(v, i);
        assert!(cur <= bound, 42); // Now uses 'assert'
        i = i + 1;
    }
}

Note that because the operation is replaced with this if-else, the argument for the code is not always evaluated. For example:

assert!(true, 1 / 0)

Will not result in an arithmetic error, it is equivalent to

if (true) () else (1 / 0)

So the arithmetic expression is never evaluated!

Abort codes in the Move VM

When using abort, it is important to understand how the u64 code will be used by the VM.

Normally, after successful execution, the Move VM, and the adapter for the specific deployment, determine the changes made to storage.

If an abort is reached, the VM will instead indicate an error. Included in that error will be two pieces of information:

• The module that produced the abort (package/address value and module name)
• The abort code.

For example

module 0x2::example {
    public fun aborts() {
        abort 42
    }
}

module 0x3::invoker {
    public fun always_aborts() {
        0x2::example::aborts()
    }
}

If a transaction, such as the function always_aborts above, calls 0x2::example::aborts, the VM would produce an error that indicated the module 0x2::example and the code 42.

This can be useful for having multiple aborts being grouped together inside a module.

In this example, the module has two separate error codes used in multiple functions

module 0x42::example {

    use std::vector;

    const EEmptyVector: u64 = 0;
    const EIndexOutOfBounds: u64 = 1;

    // move i to j, move j to k, move k to i
    public fun rotate_three<T>(v: &mut vector<T>, i: u64, j: u64, k: u64) {
        let n = vector::length(v);
        assert!(n > 0, EEmptyVector);
        assert!(i < n, EIndexOutOfBounds);
        assert!(j < n, EIndexOutOfBounds);
        assert!(k < n, EIndexOutOfBounds);

        vector::swap(v, i, k);
        vector::swap(v, j, k);
    }

    public fun remove_twice<T>(v: &mut vector<T>, i: u64, j: u64): (T, T) {
        let n = vector::length(v);
        assert!(n > 0, EEmptyVector);
        assert!(i < n, EIndexOutOfBounds);
        assert!(j < n, EIndexOutOfBounds);
        assert!(i > j, EIndexOutOfBounds);

        (vector::remove<T>(v, i), vector::remove<T>(v, j))
    }
}

The type of abort

The abort i expression can have any type! This is because both constructs break from the normal control flow, so they never need to evaluate to the value of that type.

The following are not useful, but they will type check

let y: address = abort 0;

This behavior can be helpful in situations where you have a branching instruction that produces a value on some branches, but not all. For example:

let b =
    if (x == 0) false
    else if (x == 1) true
    else abort 42;
//       ^^^^^^^^ `abort 42` has type `bool`

Control Flow

Move offers multiple constructs for control flow based on boolean expressions, including common programming constructs such as if expressions and while and for loops, along with advanced control flow structures including labels for loops and escapable named blocks.

• Conditional Expressions
• Loops
• Labeled Control FLow

Conditional if Expressions

An if expression specifies that some code should only be evaluated if a certain condition is true. For example:

if (x > 5) x = x - 5

The condition must be an expression of type bool.

An if expression can optionally include an else clause to specify another expression to evaluate when the condition is false.

if (y <= 10) y = y + 1 else y = 10

Either the "true" branch or the "false" branch will be evaluated, but not both. Either branch can be a single expression or an expression block.

The conditional expressions may produce values so that the if expression has a result.

let z = if (x < 100) x else 100;

The expressions in the true and false branches must have compatible types. For example:

// x and y must be u64 integers
let maximum: u64 = if (x > y) x else y;

// ERROR! branches different types
let z = if (maximum < 10) 10u8 else 100u64;

// ERROR! branches different types, as default false-branch is () not u64
if (maximum >= 10) maximum;

If the else clause is not specified, the false branch defaults to the unit value. The following are equivalent:

if (condition) true_branch // implied default: else ()
if (condition) true_branch else ()

Commonly, if expressions are used in conjunction with expression blocks.

let maximum = if (x > y) x else y;
if (maximum < 10) {
    x = x + 10;
    y = y + 10;
} else if (x >= 10 && y >= 10) {
    x = x - 10;
    y = y - 10;
}

Grammar for Conditionals

if-expression → if ( expression ) expression else-clause_opt > else-clause → else expression

Loop Constructs in Move

Many programs require iteration over values, and Move provides while and loop forms to allow you to write code in these situations. In addition, you can also modify control flow of these loops during execution by using break (to exit the loop) and continue (to skip the remainder of this iteration and return to the top of the control flow structure).

while Loops

The while construct repeats the body (an expression of type unit) until the condition (an expression of type bool) evaluates to false.

Here is an example of simple while loop that computes the sum of the numbers from 1 to n:

fun sum(n: u64): u64 {
    let mut sum = 0;
    let mut i = 1;
    while (i <= n) {
        sum = sum + i;
        i = i + 1
    };

    sum
}

Infinite while loops are also allowed:

fun foo() {
    while (true) { }
}

Using break Inside of while Loops

In Move, while loops can use break to exit early. For example, suppose we were looking for the position of a value in a vector, and would like to break if we find it:

fun find_position(values: &vector<u64>, target_value: u64): Option<u64> {
    let size = vector::length(values);
    let mut i = 0;
    let mut found = false;

    while (i < size) {
        if (vector::borrow(values, i) == &target_value) {
            found = true;
            break
        };
        i = i + 1
    };

    if (found) {
        Option::Some(i)
    } else {
        Option::None
    }
}

Here, if the borrowed vector value is equal to our target value, we set the found flag to true and then call break, which will cause the program to exit the loop.

Finally, note that break for while loops cannot take a value: while loops always return the unit type () and thus break does, too.

Using continue Inside of while Loops

Similar to break, Move's while loops can invoke continue to skip over part of the loop body. This allows us to skip part of a computation if a condition is not met, such as in the following example:

fun sum_even(values: &vector<u64>): u64 {
    let size = vector::length(values);
    let mut i = 0;
    let mut even_sum = 0;

    while (i < size) {
        let number = *vector::borrow(values, i);
        i = i + 1;
        if (number % 2 == 1) continue;
        even_sum = even_sum + number;
    };
    even_sum
}

This code will iterate over the provided vector. For each entry, if that entry is an even number, it will add it to the even_sum. If it is not, however, it will call continue, skipping the sum operation and returning to the while loop conditional check.

loop Expressions

The loop expression repeats the loop body (an expression with type ()) until it hits a break:

fun sum(n: u64): u64 {
    let mut sum = 0;
    let mut i = 1;

    loop {
       i = i + 1;
       if (i >= n) break;
       sum = sum + i;
    };

    sum
}

Without a break, the loop will continue forever. In the example below, the program will run forever because the loop does not have a break:

fun foo() {
    let mut i = 0;
    loop { i = i + 1 }
}

Here is an example that uses loop to write the sum function:

fun sum(n: u64): u64 {
    let sum = 0;
    let i = 0;
    loop {
        i = i + 1;
        if (i > n) break;
        sum = sum + i
    };

    sum
}

Using break with Values in loop

Unlike while loops, which always return (), a loop may return a value using break. In doing so, the overall loop expression evaluates to a value of that type. For example, we can rewrite find_position from above using loop and break, immediately returning the index if we find it:

fun find_position(values: &vector<u64>, target_value: u64): Option<u64> {
    let size = vector::length(values);
    let mut i = 0;

    loop {
        if (vector::borrow(values, i) == &target_value) {
            break Option::Some(i)
        } else if (i >= size) {
            break Option::None
        };
        i = i + 1;
    }
}

This loop will break with an option result, and, as the last expression in the function body, will produce that value as the final function result.

Using continue Inside of loop Expressions

As you might expect, continue can also be used inside a loop. Here is the previous sum_even function rewritten using loop with break and continue instead of while.

fun sum_even(values: &vector<u64>): u64 {
    let size = vector::length(values);
    let mut i = 0;
    let mut even_sum = 0;

    loop {
        if (i >= size) break;
        let number = *vector::borrow(values, i);
        i = i + 1;
        if (number % 2 == 1) continue;
        even_sum = even_sum + number;
    };
    even_sum
}

The Type of while and loop

In Move, loops are typed expressions. A while expression always has type ().

let () = while (i < 10) { i = i + 1 };

If a loop contains a break, the expression has the type of the break. A break with no value has the unit type ().

(loop { if (i < 10) i = i + 1 else break }: ());
let () = loop { if (i < 10) i = i + 1 else break };

let x: u64 = loop { if (i < 10) i = i + 1 else break 5 };
let x: u64 = loop { if (i < 10) { i = i + 1; continue} else break 5 };

In addition, if a loop contains multiple breaks, they must all return the same type:

// invalid -- first break returns (), second returns 5
let x: u64 = loop { if (i < 10) break else break 5 };

If loop does not have a break, loop can have any type much like return, abort, break, and continue.

(loop (): u64);
(loop (): address);
(loop (): &vector<vector<u8>>);

If you need even more-precise control flow, such as breaking out of nested loops, the next chapter presents the use of labeled control flow in Move.

Labeled Control Flow

Move supports labeled control flow, allowing you to define and transfer control to specific labels in a function. For example, we can nest two loops and use break and continue with those labels to precisely specify control flow. You can prefix any loop or while form with a 'label: form to allow breaking or continuing directly there.

To demonstrate this behavior, consider a function that takes nested vectors of numbers (i.e., vector<vector<u64>>) to sum against some threshold, which behaves as follows:

• If the sum of all the numbers are under the threshold, return that sum.
• If adding a number to the current sum would surpass the threshold, return the current sum.

We can write this by iterating over the vector of vectors as nested loops and labelling the outer one. If any addition in the inner loop would push us over the threshold, we can use break with the outer label to escape both loops at once:

fun sum_until_threshold(input: &vector<vector<u64>>, threshold: u64): u64 {
    let mut sum = 0;
    let mut i = 0;
    let input_size = vector::length(vec);

    'outer: loop {
        // breaks to outer since it is the closest enclosing loop
        if (i >= input_size) break sum;

        let vec = vector::borrow(input, i);
        let size = vector::length(vec);
        let mut j = 0;

        while (j < size) {
            let v_entry = *vector::borrow(vec, j);
            if (sum + v_entry < threshold) {
                sum = sum + v_entry;
            } else {
                // the next element we saw would break the threshold,
                // so we return the current sum
                break 'outer sum
            };
            j = j + 1;
        };
        i = i + 1;
    }
}

These sorts of labels can also be used with a nested loop form, providing precise control in larger bodies of code. For example, if we were processing a large table where each entry required iteration that might see us continuing the inner or outer loop, we could express that code using labels:

'outer: loop {
    ...
    'inner: while (cond) {
        ...
        if (cond0) { break 'outer value };
        ...
        if (cond1) { continue 'inner }
        else if (cond2) { continue 'outer };
        ...
    }
    ...
}

Functions

Functions are declared inside of modules and define the logic and behavior of the module. Functions can be reused, either being called from other functions or as entry points for execution.

Declaration

Functions are declared with the fun keyword followed by the function name, type parameters, parameters, a return type, and finally the function body.

<visibility>? <entry>? fun <identifier><[type_parameters: constraint],*>([identifier: type],*): <return_type> <function_body>

For example

fun foo<T1, T2>(x: u64, y: T1, z: T2): (T2, T1, u64) { (z, y, x) }

Visibility

Module functions, by default, can only be called within the same module. These internal (sometimes called private) functions cannot be called from other modules or as entry points.

module a::m {
    fun foo(): u64 { 0 }
    fun calls_foo(): u64 { foo() } // valid
}

module b::other {
    fun calls_m_foo(): u64 {
        a::m::foo() // ERROR!
//      ^^^^^^^^^^^ 'foo' is internal to 'a::m'
    }
}

To allow access from other modules, the function must be declared public or public(package). Tangential to visibility, an entry function can be called as an entry point for execution.

public visibility

A public function can be called by any function defined in any module. As shown in the following example, a public function can be called by:

• other functions defined in the same module,
• functions defined in another module, or
• as an entry point for execution.

module a::m {
    public fun foo(): u64 { 0 }
    fun calls_foo(): u64 { foo() } // valid
}

module b::other {
    fun calls_m_foo(): u64 {
        a::m::foo() // valid
    }
}

Fore more details on the entry point to execution see the section below.

public(package) visibility

The public(package) visibility modifier is a more restricted form of the public modifier to give more control about where a function can be used. A public(package) function can be called by:

• other functions defined in the same module, or
• other functions defined in the same package (the same address)

module a::m {
    public(package) fun foo(): u64 { 0 }
    fun calls_foo(): u64 { foo() } // valid
}

module a::n {
    fun calls_m_foo(): u64 {
        a::m::foo() // valid, also in `a`
    }
}

module b::other {
    fun calls_m_foo(): u64 {
        b::m::foo() // ERROR!
//      ^^^^^^^^^^^ 'foo' can only be called from a module in `a`
    }
}

DEPRECATED public(friend) visibility

Before the addition of public(package), public(friend) was used to allow limited public access to functions in the same package, but where the list of allowed modules had to be explicitly enumerated by the callee's module. see Friends for more details.

entry modifier

In addition to public functions, you might have some functions in your modules that you want to use as the entry point to execution. The entry modifier is designed to allow module functions to initiate execution, without having to expose the functionality to other modules.

Essentially, the combination of pbulic and entry functions define the "main" functions of a module, and they specify where Move programs can start executing.

Keep in mind though, an entry function can still be called by other Move functions. So while they can serve as the start of a Move program, they aren't restricted to that case.

For example:

module a::m {
    entry fun foo(): u64 { 0 }
    fun calls_foo(): u64 { foo() } // valid!
}

module a::n {
    fun calls_m_foo(): u64 {
        a::m::foo() // ERROR!
//      ^^^^^^^^^^^ 'foo' is internal to 'a::m'
    }
}

entry functions may have restrictions on their parameters and return types. Although, these restrictions are specific to each individual deployment of Move.

The documentation for entry functions on Sui can be found here..

To enable easier testing, entry functions can be called from #[test] and #[test_only] contexts.

module a::m {
    entry fun foo(): u64 { 0 }
}
module a::m_test {
    #[test]
    fun my_test(): u64 { a::m::foo() } // valid!
    #[test_only]
    fun my_test_helper(): u64 { a::m::foo() } // valid!
}

Name

Function names can start with letters a to z. After the first character, function names can contain underscores _, letters a to z, letters A to Z, or digits 0 to 9.

fun fOO() {}
fun bar_42() {}
fun bAZ_19() {}

Type Parameters

After the name, functions can have type parameters

fun id<T>(x: T): T { x }
fun example<T1: copy, T2>(x: T1, y: T2): (T1, T1, T2) { (copy x, x, y) }

For more details, see Move generics.

Parameters

Functions parameters are declared with a local variable name followed by a type annotation

fun add(x: u64, y: u64): u64 { x + y }

We read this as x has type u64

A function does not have to have any parameters at all.

fun useless() { }

This is very common for functions that create new or empty data structures

module a::example {
  public struct Counter { count: u64 }

  fun new_counter(): Counter {
      Counter { count: 0 }
  }
}

Return type

After the parameters, a function specifies its return type.

fun zero(): u64 { 0 }

Here : u64 indicates that the function's return type is u64.

Using tuples, a function can return multiple values:

fun one_two_three(): (u64, u64, u64) { (0, 1, 2) }

If no return type is specified, the function has an implicit return type of unit (). These functions are equivalent:

fun just_unit(): () { () }
fun just_unit() { () }
fun just_unit() { }

As mentioned in the tuples section, these tuple "values" do not exist as runtime values. This means that a function that returns unit () does not return any value during execution.

Function body

A function's body is an expression block. The return value of the function is the last value in the sequence

fun example(): u64 {
    let x = 0;
    x = x + 1;
    x // returns 'x'
}

See the section below for more information on returns

For more information on expression blocks, see Move variables.

Native Functions

Some functions do not have a body specified, and instead have the body provided by the VM. These functions are marked native.

Without modifying the VM source code, a programmer cannot add new native functions. Furthermore, it is the intent that native functions are used for either standard library code or for functionality needed for the given Move environment.

Most native functions you will likely see are in standard library code, such as vector

module std::vector {
    native public fun length<Element>(v: &vector<Element>): u64;
    ...
}

Calling

When calling a function, the name can be specified either through an alias or fully qualified

module a::example {
    public fun zero(): u64 { 0 }
}

module b::other {
    use a::example::{Self, zero};
    fun call_zero() {
        // With the `use` above all of these calls are equivalent
        a::example::zero();
        example::zero();
        zero();
    }
}

When calling a function, an argument must be given for every parameter.

module a::example {
    public fun takes_none(): u64 { 0 }
    public fun takes_one(x: u64): u64 { x }
    public fun takes_two(x: u64, y: u64): u64 { x + y }
    public fun takes_three(x: u64, y: u64, z: u64): u64 { x + y + z }
}

module b::other {
    fun call_all() {
        a::example::takes_none();
        a::example::takes_one(0);
        a::example::takes_two(0, 1);
        a::example::takes_three(0, 1, 2);
    }
}

Type arguments can be either specified or inferred. Both calls are equivalent.

module aexample {
    public fun id<T>(x: T): T { x }
}

module b::other {
    fun call_all() {
        a::example::id(0);
        a::example::id<u64>(0);
    }
}

For more details, see Move generics.

Returning values

The result of a function, its "return value", is the final value of its function body. For example

fun add(x: u64, y: u64): u64 {
    x + y
}

The return value here is the result of x + y.

As mentioned above, the function's body is an expression block. The expression block can sequence various statements, and the final expression in the block will be be the value of that block

fun double_and_add(x: u64, y: u64): u64 {
    let double_x = x * 2;
    let double_y = y * 2;
    double_x + double_y
}

The return value here is the result of double_x + double_y

return expression

A function implicitly returns the value that its body evaluates to. However, functions can also use the explicit return expression:

fun f1(): u64 { return 0 }
fun f2(): u64 { 0 }

These two functions are equivalent. In this slightly more involved example, the function subtracts two u64 values, but returns early with 0 if the second value is too large:

fun safe_sub(x: u64, y: u64): u64 {
    if (y > x) return 0;
    x - y
}

Note that the body of this function could also have been written as if (y > x) 0 else x - y.

However return really shines is in exiting deep within other control flow constructs. In this example, the function iterates through a vector to find the index of a given value:

use std::vector;
use std::option::{Self, Option};
fun index_of<T>(v: &vector<T>, target: &T): Option<u64> {
    let i = 0;
    let n = vector::length(v);
    while (i < n) {
        if (vector::borrow(v, i) == target) return option::some(i);
        i = i + 1
    };

    option::none()
}

Using return without an argument is shorthand for return (). That is, the following two functions are equivalent:

fun foo() { return }
fun foo() { return () }

Structs and Resources

A struct is a user-defined data structure containing typed fields. Structs can store any non-reference, non-tuple type, including other structs.

Structs can be used to define all "asset" values or unrestricted values, where the operations performed on those values can be controlled by the struct's abilities. By default, structs are linear and ephemeral. By this we mean that they: cannot be copied, cannot be dropped, and cannot be stored in storage. This means that all values have to have ownership transferred (linear) and the values must be dealt with by the end of the program's execution (ephemeral). We can relax this behavior by giving the struct abilities which allow values to be copied or dropped and also to be stored in storage or to define storage schemas.

Defining Structs

Structs must be defined inside a module, and the struct's fields can either be named or positional:

module a::m {
    public struct Foo { x: u64, y: bool }
    public struct Bar {}
    public struct Baz { foo: Foo, }
    //                          ^ note: it is fine to have a trailing comma

    public struct PosFoo(u64, bool)
    public struct PosBar()
    public struct PosBaz(Foo)
}

Structs cannot be recursive, so the following definitions are invalid:

public struct Foo { x: Foo }
//                     ^ ERROR! recursive definition

public struct A { b: B }
public struct B { a: A }
//                   ^ ERROR! recursive definition

public struct D(D)
//              ^ ERROR! recursive definition

Visibility

As you may have noticed, all structs are declared as public. This means that the type of the struct can be referred to from any other module. However, the fields of the struct, and the ability to create or destroy the struct, are still internal to the module that defines the struct.

In the future, we plan on adding to declare structs as public(package) or as internal, much like functions.

Abilities

As mentioned above: by default, a struct declaration is linear and ephemeral. So to allow the value to be used in these ways (e.g., copied, dropped, stored in an object, or used to define a storable object), structs can be granted abilities by annotating them with has <ability>:

module a::m {
    public struct Foo has copy, drop { x: u64, y: bool }
}

The ability declaration can occur either before or after the struct's fields. However, only one or the other can be used, and not both. If declared after the struct's fields, the ability declaration must be terminated with a semicolon:

module a::m {
    public PreNamedAbilities has copy, drop { x: u64, y: bool }
    public struct PostNamedAbilities { x: u64, y: bool } has copy, drop;
    public struct PostNamedAbilitiesInvalid { x: u64, y: bool } has copy, drop
    //                                                                        ^ ERROR! missing semicolon

    public struct NamedInvalidAbilities has copy { x: u64, y: bool } has drop;
    //                                                               ^ ERROR! duplicate ability declaration

    public PrePositionalAbilities has copy, drop (u64, bool)
    public struct PostPositionalAbilities (u64, bool) has copy, drop;
    public struct PostPositionalAbilitiesInvalid (u64, bool) has copy, drop
    //                                                                     ^ ERROR! missing semicolon
    public struct InvalidAbilities has copy (u64, bool) has drop;
    //                                                  ^ ERROR! duplicate ability declaration
}

For more details, see the section on annotating a struct's abilities.

Naming

Structs must start with a capital letter A to Z. After the first letter, struct names can contain underscores _, letters a to z, letters A to Z, or digits 0 to 9.

public struct Foo {}
public struct BAR {}
public struct B_a_z_4_2 {}
public struct P_o_s_Foo()

This naming restriction of starting with A to Z is in place to give room for future language features. It may or may not be removed later.

Using Structs

Creating Structs

Values of a struct type can be created (or "packed") by indicating the struct name, followed by value for each field.

For a struct with named fields, the order of the fields does not matter, but the field name needs to be provided. For a struct with positional fields, the order of the fields must match the order of the fields in the struct definition, and it must be created using () instead of {} to enclose the parameters.

module a::m {
    public struct Foo has drop { x: u64, y: bool }
    public struct Baz has drop { foo: Foo }
    public struct Positional(u64, bool) has drop;

    fun example() {
        let foo = Foo { x: 0, y: false };
        let baz = Baz { foo: foo };
        // Note: positional struct values are created using parentheses and
        // based on position instead of name.
        let pos = Positional(0, false);
        let pos_invalid = Positional(false, 0);
        //                           ^ ERROR! Fields are out of order and the types don't match.
    }
}

For structs with named fields, you can use the following shorthand if you have a local variable with the same name as the field:

let baz = Baz { foo: foo };
// is equivalent to
let baz = Baz { foo };

This is sometimes called "field name punning".

Destroying Structs via Pattern Matching

Struct values can be destroyed by binding or assigning them in patterns using similar syntax to constructing them.

module a::m {
    public struct Foo { x: u64, y: bool }
    public struct Bar(Foo)
    public struct Baz {}
    public struct Qux()

    fun example_destroy_foo() {
        let foo = Foo { x: 3, y: false };
        let Foo { x, y: foo_y } = foo;
        //        ^ shorthand for `x: x`

        // two new bindings
        //   x: u64 = 3
        //   foo_y: bool = false
    }

    fun example_destroy_foo_wildcard() {
        let foo = Foo { x: 3, y: false };
        let Foo { x, y: _ } = foo;

        // only one new binding since y was bound to a wildcard
        //   x: u64 = 3
    }

    fun example_destroy_foo_assignment() {
        let x: u64;
        let y: bool;
        Foo { x, y } = Foo { x: 3, y: false };

        // mutating existing variables x and y
        //   x = 3, y = false
    }

    fun example_foo_ref() {
        let foo = Foo { x: 3, y: false };
        let Foo { x, y } = &foo;

        // two new bindings
        //   x: &u64
        //   y: &bool
    }

    fun example_foo_ref_mut() {
        let foo = Foo { x: 3, y: false };
        let Foo { x, y } = &mut foo;

        // two new bindings
        //   x: &mut u64
        //   y: &mut bool
    }

    fun example_destroy_bar() {
        let bar = Bar(Foo { x: 3, y: false });
        let Bar(Foo { x, y }) = bar;
        //            ^ nested pattern

        // two new bindings
        //   x: u64 = 3
        //   y: bool = false
    }

    fun example_destroy_baz() {
        let baz = Baz {};
        let Baz {} = baz;
    }

    fun example_destroy_qux() {
        let qux = Qux();
        let Qux() = qux;
    }
}

Accessing Struct Fields

Fields of a struct can be accessed using the dot operator ..

For structs with named fields, the fields can be accessed by their name:

public struct Foo { x: u64, y: bool }
let foo = Foo { x: 3, y: true };
let x = foo.x;  // x == 3
let y = foo.y;  // y == true

For positional structs, fields can be accessed by their position in the struct definition:

public struct PosFoo(u64, bool)
let pos_foo = PosFoo(3, true);
let x = pos_foo.0;  // x == 3
let y = pos_foo.1;  // y == true

Accessing struct fields without borrowing or copying them is subject to the field's ability constraints. For more details see the sections on borrowing structs and fields and reading and writing fields for more information.

Borrowing Structs and Fields

The & and &mut operator can be used to create references to structs or fields. These examples include some optional type annotations (e.g., : &Foo) to demonstrate the type of operations.

let foo = Foo { x: 3, y: true };
let foo_ref: &Foo = &foo;
let y: bool = foo_ref.y;         // reading a field via a reference to the struct
let x_ref: &u64 = &foo.x;        // borrowing a field by extending a reference to the struct

let x_ref_mut: &mut u64 = &mut foo.x;
*x_ref_mut = 42;            // modifying a field via a mutable reference

It is possible to borrow inner fields of nested structs:

let foo = Foo { x: 3, y: true };
let bar = Bar(foo);

let x_ref = &bar.0.x;

You can also borrow a field via a reference to a struct:

let foo = Foo { x: 3, y: true };
let foo_ref = &foo;
let x_ref = &foo_ref.x;
// this has the same effect as let x_ref = &foo.x

Reading and Writing Fields

If you need to read and copy a field's value, you can then dereference the borrowed field:

let foo = Foo { x: 3, y: true };
let bar = Bar(copy foo);
let x: u64 = *&foo.x;
let y: bool = *&foo.y;
let foo2: Foo = *&bar.0;

More canonically, the dot operator can be used to read fields of a struct without any borrowing. As is true with dereferencing, the field type must have the copy ability.

let foo = Foo { x: 3, y: true };
let x = foo.x;  // x == 3
let y = foo.y;  // y == true

Dot operators can be chained to access nested fields:

let bar = Bar(Foo { x: 3, y: true });
let x = baz.0.x; // x = 3;

However, this is not permitted for fields that contain non-primitive types, such a vector or another struct:

let foo = Foo { x: 3, y: true };
let bar = Bar(foo);
let foo2: Foo = *&bar.0;
let foo3: Foo = bar.0; // error! must add an explicit copy with *&

We can mutably borrow a field to a struct to assign it a new value:

let mut foo = Foo { x: 3, y: true };
*&mut foo.x = 42;     // foo = Foo { x: 42, y: true }
*&mut foo.y = !foo.y; // foo = Foo { x: 42, y: false }
let mut bar = Bar(foo);               // bar = Bar(Foo { x: 42, y: false })
*&mut bar.0.x = 52;                   // bar = Bar(Foo { x: 52, y: false })
*&mut bar.0 = Foo { x: 62, y: true }; // bar = Bar(Foo { x: 62, y: true })

Similar to dereferencing, we can instead directly use the dot operator to modify a field. And in both cases, the field type must have the drop ability.

let mut foo = Foo { x: 3, y: true };
foo.x = 42;     // foo = Foo { x: 42, y: true }
foo.y = !foo.y; // foo = Foo { x: 42, y: false }
let mut bar = Bar(foo);         // bar = Bar(Foo { x: 42, y: false })
bar.0.x = 52;                   // bar = Bar(Foo { x: 52, y: false })
bar.0 = Foo { x: 62, y: true }; // bar = Bar(Foo { x: 62, y: true })

The dot syntax for assignment also works via a reference to a struct:

let foo = Foo { x: 3, y: true };
let foo_ref = &mut foo;
foo_ref.x = foo_ref.x + 1;

Privileged Struct Operations

Most struct operations on a struct type T can only be performed inside the module that declares T:

• Struct types can only be created ("packed"), destroyed ("unpacked") inside the module that defines the struct.
• The fields of a struct are only accessible inside the module that defines the struct.

Following these rules, if you want to modify your struct outside the module, you will need to provide public APIs for them. The end of the chapter contains some examples of this.

However as stated in the visibility section above, struct types are always visible to another module

module a::m {
    public struct Foo has drop { x: u64 }

    public fun new_foo(): Foo {
        Foo { x: 42 }
    }
}

module a::n {
    use a::m::Foo;

    public struct Wrapper has drop {
        foo: Foo
        //   ^ valid the type is public

    }

    fun f1(foo: Foo) {
        let x = foo.x;
        //      ^ ERROR! cannot access fields of `Foo` outside of `a::m`
    }

    fun f2() {
        let foo_wrapper = Wrapper { foo: m::new_foo() };
        //                               ^ valid the function is public
    }
}

Ownership

As mentioned above in Defining Structs, structs are by default linear and ephemeral. This means they cannot be copied or dropped. This property can be very useful when modeling real world assets like money, as you do not want money to be duplicated or get lost in circulation.

module a::m {
    public struct Foo { x: u64 }

    public fun copying() {
        let foo = Foo { x: 100 };
        let foo_copy = copy foo; // ERROR! 'copy'-ing requires the 'copy' ability
        let foo_ref = &foo;
        let another_copy = *foo_ref // ERROR! dereference requires the 'copy' ability
    }

    public fun destroying_1() {
        let foo = Foo { x: 100 };

        // error! when the function returns, foo still contains a value.
        // This destruction requires the 'drop' ability
    }

    public fun destroying_2(f: &mut Foo) {
        *f = Foo { x: 100 } // error!
                            // destroying the old value via a write requires the 'drop' ability
    }
}

To fix the example fun destroying_1, you would need to manually "unpack" the value:

module a::m {
    public struct Foo { x: u64 }

    public fun destroying_1_fixed() {
        let foo = Foo { x: 100 };
        let Foo { x: _ } = foo;
    }
}

Recall that you are only able to deconstruct a struct within the module in which it is defined. This can be leveraged to enforce certain invariants in a system, for example, conservation of money.

If on the other hand, your struct does not represent something valuable, you can add the abilities copy and drop to get a struct value that might feel more familiar from other programming languages:

module a::m {
    public struct Foo has copy, drop { x: u64 }

    public fun run() {
        let foo = Foo { x: 100 };
        let foo_copy = foo;
        //             ^ this code copies foo,
        //             whereas `let x = move foo` would move foo

        let x = foo.x;            // x = 100
        let x_copy = foo_copy.x;  // x = 100

        // both foo and foo_copy are implicitly discarded when the function returns
    }
}

Storage

Structs can be used to define storage schemas, but the details are different per deployment of Move. See the documentation for the key ability and Sui objects for more details.

Constants

Constants are a way of giving a name to shared, static values inside of a module.

The constant's value must be known at compilation. The constant's value is stored in the compiled module. And each time the constant is used, a new copy of that value is made.

Declaration

Constant declarations begin with the const keyword, followed by a name, a type, and a value.

const <name>: <type> = <expression>;

For example

module a::example {
    const MY_ADDRESS: address = @a;

    public fun permissioned(addr: address) {
        assert!(addr == MY_ADDRESS, 0);
    }
}

Naming

Constants must start with a capital letter A to Z. After the first letter, constant names can contain underscores _, letters a to z, letters A to Z, or digits 0 to 9.

const FLAG: bool = false;
const EMyErrorCode: u64 = 0;
const ADDRESS_42: address = @0x42;

Even though you can use letters a to z in a constant. The general style guidelines are to use just uppercase letters A to Z, with underscores _ between each word. For error codes, we use E as a prefix and then upper camel case (also known as Pascal case) for the rest of the name, as seen in EMyErrorCode.

The current naming restriction of starting with A to Z is in place to give room for future language features.

Visibility

public or public(package) constants are not currently supported. const values can be used only in the declaring module. However, as a convenience, they can be used across modules in unit tests attributes.

Valid Expressions

Currently, constants are limited to the primitive types bool, u8, u16, u32, u64, u128, u256, address, and vector<T>, where T is the valid type for a constant.

Values

Commonly, consts are assigned a simple value, or literal, of their type. For example

const MY_BOOL: bool = false;
const MY_ADDRESS: address = @0x70DD;
const BYTES: vector<u8> = b"hello world";
const HEX_BYTES: vector<u8> = x"DEADBEEF";

Complex Expressions

In addition to literals, constants can include more complex expressions, as long as the compiler is able to reduce the expression to a value at compile time.

Currently, equality operations, all boolean operations, all bitwise operations, and all arithmetic operations can be used.

const RULE: bool = true && false;
const CAP: u64 = 10 * 100 + 1;
const SHIFTY: u8 = {
    (1 << 1) * (1 << 2) * (1 << 3) * (1 << 4)
};
const HALF_MAX: u128 = 340282366920938463463374607431768211455 / 2;
const REM: u256 =
    57896044618658097711785492504343953926634992332820282019728792003956564819968 % 654321;
const EQUAL: bool = 1 == 1;

If the operation would result in a runtime exception, the compiler will give an error that it is unable to generate the constant's value

const DIV_BY_ZERO: u64 = 1 / 0; // ERROR!
const SHIFT_BY_A_LOT: u64 = 1 << 100; // ERROR!
const NEGATIVE_U64: u64 = 0 - 1; // ERROR!

Additionally, constants can refer to other constants within the same module.

const BASE: u8 = 4;
const SQUARE: u8 = BASE * BASE;

Note though, that any cycle in the constant definitions results in an error.

const A: u16 = B + 1;
const B: u16 = A + 1; // ERROR!

Generics

Generics can be used to define functions and structs over different input data types. This language feature is sometimes referred to as parametric polymorphism. In Move, we will often use the term generics interchangeably with type parameters and type arguments.

Generics are commonly used in library code, such as in vector, to declare code that works over any possible type (that satisfies the specified constraints). This sort of parameterization allows you to reuse the same implementation across multiple types and situations.

Declaring Type Parameters

Both functions and structs can take a list of type parameters in their signatures, enclosed by a pair of angle brackets <...>.

Generic Functions

Type parameters for functions are placed after the function name and before the (value) parameter list. The following code defines a generic identity function that takes a value of any type and returns that value unchanged.

fun id<T>(x: T): T {
    // this type annotation is unnecessary but valid
    (x: T)
}

Once defined, the type parameter T can be used in parameter types, return types, and inside the function body.

Generic Structs

Type parameters for structs are placed after the struct name, and can be used to name the types of the fields.

public struct Foo<T> has copy, drop { x: T }

public struct Bar<T1, T2> has copy, drop {
    x: T1,
    y: vector<T2>,
}

Note that type parameters do not have to be used

Type Arguments

Calling Generic Functions

When calling a generic function, one can specify the type arguments for the function's type parameters in a list enclosed by a pair of angle brackets.

fun foo() {
    let x = id<bool>(true);
}

If you do not specify the type arguments, Move's type inference will supply them for you.

Using Generic Structs

Similarly, one can attach a list of type arguments for the struct's type parameters when constructing or destructing values of generic types.

fun foo() {
    // type arguments on construction
    let foo = Foo<bool> { x: true };
    let bar = Bar<u64, u8> { x: 0, y: vector<u8>[] };

    // type arguments on destruction
    let Foo<bool> { x } = foo;
    let Bar<u64, u8> { x, y } = bar;
}

In any case if you do not specify the type arguments, Move's type inference will supply them for you.

Type Argument Mismatch

If you specify the type arguments and they conflict with the actual values supplied, an error will be given:

fun foo() {
    let x = id<u64>(true); // ERROR! true is not a u64
}

and similarly:

fun foo() {
    let foo = Foo<bool> { x: 0 }; // ERROR! 0 is not a bool
    let Foo<address> { x } = foo; // ERROR! bool is incompatible with address
}

Type Inference

In most cases, the Move compiler will be able to infer the type arguments so you don't have to write them down explicitly. Here's what the examples above would look like if we omit the type arguments:

fun foo() {
    let x = id(true);
    //        ^ <bool> is inferred

    let foo = Foo { x: true };
    //           ^ <bool> is inferred

    let Foo { x } = foo;
    //     ^ <bool> is inferred
}

Note: when the compiler is unable to infer the types, you'll need annotate them manually. A common scenario is to call a function with type parameters appearing only at return positions.

module a::m {

    fun foo() {
        let v = vector[]; // ERROR!
        //            ^ The compiler cannot figure out the element type, since it is never used

        let v = vector<u64>[];
        //            ^~~~~ Must annotate manually in this case.
    }
}

Note that these cases are a bit contrived since the vector[] is never used, ad as such, Move's type inference cannot infer the type.

However, the compiler will be able to infer the type if that value is used later in that function:

module a::m {
    fun foo() {
        let v = vector[];
        //            ^ <u64> is inferred
        vector::push_back(&mut v, 42);
        //               ^ <u64> is inferred
    }
}

Integers

In Move, the integer types u8, u16, u32, u64, u128, and u256 are all distinct types. However, each one of these types can be created with the same numerical value syntax. In other words, if a type suffix is not provided, the compiler will infer the integer type based on the usage of the value.

let x8: u8 = 0;
let x16: u16 = 0;
let x32: u32 = 0;
let x64: u64 = 0;
let x128: u128 = 0;
let x256: u256 = 0;

If the value is not used in a context that requires a specific integer type, u64 is taken as a default.

let x = 0;
//      ^ u64 is used by default

If the value however is too large for the inferred type, an error will be given

let i: u8 = 256; // ERROR!
//          ^^^ too large for u8
let x = 340282366920938463463374607431768211454;
//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ too large for u64

In cases where the number is too large, you might need to annotate it explicitly

let x = 340282366920938463463374607431768211454u128;
//                                             ^^^^ valid!

Unused Type Parameters

For a struct definition, an unused type parameter is one that does not appear in any field defined in the struct, but is checked statically at compile time. Move allows unused type parameters so the following struct definition is valid:

public struct Foo<T> {
    foo: u64
}

This can be convenient when modeling certain concepts. Here is an example:

module a::m {
    // Currency Specifiers
    public struct A {}
    public struct B {}

    // A generic coin type that can be instantiated using a currency
    // specifier type.
    //   e.g. Coin<A>, Coin<B> etc.
    public struct Coin<Currency> has store {
        value: u64
    }

    // Write code generically about all currencies
    public fun mint_generic<Currency>(value: u64): Coin<Currency> {
        Coin { value }
    }

    // Write code concretely about one currency
    public fun mint_a(value: u64): Coin<A> {
        mint_generic(value)
    }
    public fun mint_b(value: u64): Coin<B> {
        mint_generic(value)
    }
}

In this example, Coin<Currency> is generic on the Currency type parameter, which specifies the currency of the coin and allows code to be written either generically on any currency or concretely on a specific currency. This generality applies even when the Currency type parameter does not appear in any of the fields defined in Coin.

Phantom Type Parameters

In the example above, although struct Coin asks for the store ability, neither Coin<A> nor Coin<B> will have the store ability. This is because of the rules for Conditional Abilities and Generic Types and the fact that A and B don't have the store ability, despite the fact that they are not even used in the body of struct Coin. This might cause some unpleasant consequences. For example, we are unable to put Coin<A> into a wallet in storage.

One possible solution would be to add spurious ability annotations to A and B (i.e., public struct Currency1 has store {}). But, this might lead to bugs or security vulnerabilities because it weakens the types with unnecessary ability declarations. For example, we would never expect a value in the storage to have a field in type A, but this would be possible with the spurious store ability. Moreover, the spurious annotations would be infectious, requiring many functions generic on the unused type parameter to also include the necessary constraints.

Phantom type parameters solve this problem. Unused type parameters can be marked as phantom type parameters, which do not participate in the ability derivation for structs. In this way, arguments to phantom type parameters are not considered when deriving the abilities for generic types, thus avoiding the need for spurious ability annotations. For this relaxed rule to be sound, Move's type system guarantees that a parameter declared as phantom is either not used at all in the struct definition, or it is only used as an argument to type parameters also declared as phantom.

Declaration

In a struct definition a type parameter can be declared as phantom by adding the phantom keyword before its declaration.

public struct Coin<phantom Currency> has store {
    value: u64
}

If a type parameter is declared as phantom we say it is a phantom type parameter. When defining a struct, Move's type checker ensures that every phantom type parameter is either not used inside the struct definition or it is only used as an argument to a phantom type parameter.

public struct S1<phantom T1, T2> { f: u64 }
//               ^^^^^^^ valid, T1 does not appear inside the struct definition

public struct S2<phantom T1, T2> { f: S1<T1, T2> }
//               ^^^^^^^ valid, T1 appears in phantom position

The following code shows examples of violations of the rule:

public struct S1<phantom T> { f: T }
//               ^^^^^^^ ERROR!  ^ Not a phantom position

public struct S2<T> { f: T }
public struct S3<phantom T> { f: S2<T> }
//               ^^^^^^^ ERROR!     ^ Not a phantom position

More formally, if a type is used as an argument to a phantom type parameter we say the type appears in phantom position. With this definition in place, the rule for the correct use of phantom parameters can be specified as follows: A phantom type parameter can only appear in phantom position.

Note that specifying phantom is not required, but the compiler will warn if a type parameter could be phantom but was not marked as such.

Instantiation

When instantiating a struct, the arguments to phantom parameters are excluded when deriving the struct abilities. For example, consider the following code:

public struct S<T1, phantom T2> has copy { f: T1 }
public struct NoCopy {}
public struct HasCopy has copy {}

Consider now the type S<HasCopy, NoCopy>. Since S is defined with copy and all non-phantom arguments have copy then S<HasCopy, NoCopy> also has copy.

Phantom Type Parameters with Ability Constraints

Ability constraints and phantom type parameters are orthogonal features in the sense that phantom parameters can be declared with ability constraints.

public struct S<phantom T: copy> {}

When instantiating a phantom type parameter with an ability constraint, the type argument has to satisfy that constraint, even though the parameter is phantom. The usual restrictions apply and T can only be instantiated with arguments having copy.

Constraints

In the examples above, we have demonstrated how one can use type parameters to define "unknown" types that can be plugged in by callers at a later time. This however means the type system has little information about the type and has to perform checks in a very conservative way. In some sense, the type system must assume the worst case scenario for an unconstrained generic--a type with no abilities.

Constraints offer a way to specify what properties these unknown types have so the type system can allow operations that would otherwise be unsafe.

Declaring Constraints

Constraints can be imposed on type parameters using the following syntax.

// T is the name of the type parameter
T: <ability> (+ <ability>)*

The <ability> can be any of the four abilities, and a type parameter can be constrained with multiple abilities at once. So all of the following would be valid type parameter declarations:

T: copy
T: copy + drop
T: copy + drop + store + key

Verifying Constraints

Constraints are checked at instantiation sites

public struct Foo<T: copy> { x: T }

public struct Bar { x: Foo<u8> }
//                         ^^ valid, u8 has `copy`

public struct Baz<T> { x: Foo<T> }
//                            ^ ERROR! T does not have 'copy'

And similarly for functions

fun unsafe_consume<T>(x: T) {
    // ERROR! x does not have 'drop'
}

fun consume<T: drop>(x: T) {
    // valid, x will be dropped automatically
}

public struct NoAbilities {}

fun foo() {
    let r = NoAbilities {};
    consume<NoAbilities>(NoAbilities);
    //      ^^^^^^^^^^^ ERROR! NoAbilities does not have 'drop'
}

And some similar examples, but with copy

fun unsafe_double<T>(x: T) {
    (copy x, x)
    // ERROR! T does not have 'copy'
}

fun double<T: copy>(x: T) {
    (copy x, x) // valid, T has 'copy'
}

public struct NoAbilities {}

fun foo(): (NoAbilities, NoAbilities) {
    let r = NoAbilities {};
    double<NoAbilities>(r)
    //     ^ ERROR! NoAbilities does not have 'copy'
}

For more information, see the abilities section on conditional abilities and generic types.

Limitations on Recursions

Recursive Structs

Generic structs can not contain fields of the same type, either directly or indirectly, even with different type arguments. All of the following struct definitions are invalid:

public struct Foo<T> {
    x: Foo<u64> // ERROR! 'Foo' containing 'Foo'
}

public struct Bar<T> {
    x: Bar<T> // ERROR! 'Bar' containing 'Bar'
}

// ERROR! 'A' and 'B' forming a cycle, which is not allowed either.
public struct A<T> {
    x: B<T, u64>
}

public struct B<T1, T2> {
    x: A<T1>
    y: A<T2>
}

Advanced Topic: Type-level Recursions

Move allows generic functions to be called recursively. However, when used in combination with generic structs, this could create an infinite number of types in certain cases, and allowing this means adding unnecessary complexity to the compiler, vm and other language components. Therefore, such recursions are forbidden.

This restriction might be relaxed in the future, but for now, the following examples should give you an idea of what is allowed and what is not.

module a::m {
    public struct A<T> {}

    // Finitely many types -- allowed.
    // foo<T> -> foo<T> -> foo<T> -> ... is valid
    fun foo<T>() {
        foo<T>();
    }

    // Finitely many types -- allowed.
    // foo<T> -> foo<A<u64>> -> foo<A<u64>> -> ... is valid
    fun foo<T>() {
        foo<A<u64>>();
    }
}

Not allowed:

module a::m {
    public struct A<T> {}

    // Infinitely many types -- NOT allowed.
    // error!
    // foo<T> -> foo<A<T>> -> foo<A<A<T>>> -> ...
    fun foo<T>() {
        foo<Foo<T>>();
    }
}

And similarly, not allowed:

module a::n {
    public struct A<T> {}

    // Infinitely many types -- NOT allowed.
    // error!
    // foo<T1, T2> -> bar<T2, T1> -> foo<T2, A<T1>>
    //   -> bar<A<T1>, T2> -> foo<A<T1>, A<T2>>
    //   -> bar<A<T2>, A<T1>> -> foo<A<T2>, A<A<T1>>>
    //   -> ...
    fun foo<T1, T2>() {
        bar<T2, T1>();
    }

    fun bar<T1, T2> {
        foo<T1, A<T2>>();
    }
}

Note, the check for type level recursions is based on a conservative analysis on the call sites and does NOT take control flow or runtime values into account.

module a::m {
    public struct A<T> {}

    // Infinitely many types -- NOT allowed.
    // error!
    fun foo<T>(n: u64) {
        if (n > 0) foo<A<T>>(n - 1);
    }
}

The function in the example above will technically terminate for any given input and therefore only creating finitely many types, but it is still considered invalid by Move's type system.

Abilities

Abilities are a typing feature in Move that control what actions are permissible for values of a given type. This system grants fine grained control over the "linear" typing behavior of values, as well as if and how values are used in storage (as defined by the specific deployment of Move, e.g. the notion of storage for the blockchain). This is implemented by gating access to certain bytecode instructions so that for a value to be used with the bytecode instruction, it must have the ability required (if one is required at all—not every instruction is gated by an ability).

For Sui, key is used to signify an object. Objects are the basic unit of storage where each object has a unique, 32-byte ID. store is then used to both indicate what data can be stored inside of an object, and is also used to indicate what types can be transferred outside of their defining module.

The Four Abilities

The four abilities are:

• copy
  • Allows values of types with this ability to be copied.
• drop
  • Allows values of types with this ability to be popped/dropped.
• store
  • Allows values of types with this ability to exist inside a value in storage.
  • For Sui, store controls what data can be stored inside of an object. store also controls what types can be transferred outside of their defining module.
• key
  • Allows the type to serve as a "key" for storage. Ostensibly this means the value can be a top-level value in storage; in other words, it does not need to be contained in another value to be in storage.
  • For Sui, key is used to signify an object.

copy

The copy ability allows values of types with that ability to be copied. It gates the ability to copy values out of local variables with the copy operator and to copy values via references with dereference *e.

If a value has copy, all values contained inside of that value have copy.

drop

The drop ability allows values of types with that ability to be dropped. By dropped, we mean that value is not transferred and is effectively destroyed as the Move program executes. As such, this ability gates the ability to ignore values in a multitude of locations, including:

• not using the value in a local variable or parameter
• not using the value in a sequence via ;
• overwriting values in variables in assignments
• overwriting values via references when writing *e1 = e2.

If a value has drop, all values contained inside of that value have drop.

store

The store ability allows values of types with this ability to exist inside of a value in storage, but not necessarily as a top-level value in storage. This is the only ability that does not directly gate an operation. Instead it gates the existence in storage when used in tandem with key.

If a value has store, all values contained inside of that value have store.

For Sui, store serves double duty. It controls what values can appear inside of an object, and what objects can be transferred outside of their defining module.

key

The key ability allows the type to serve as a key for storage operations as defined by the deployment of Move. While it is specific per Move instance, it serves to gates all storage operations, so in order for a type to be used with storage primitives, the type must have the key ability.

If a value has key, all values contained inside of that value have store. This is the only ability with this sort of asymmetry.

For Sui, key is used to signify an object.

Builtin Types

All primitive, builtin types have copy, drop, and store.

• bool, u8, u16, u32, u64, u128, u256, and address all have copy, drop, and store.
• vector<T> may have copy, drop, and store depending on the abilities of T.
  • See Conditional Abilities and Generic Types for more details.
• Immutable references & and mutable references &mut both have copy and drop.
  • This refers to copying and dropping the reference itself, not what they refer to.
  • References cannot appear in global storage, hence they do not have store.

Note that none of the primitive types have key, meaning none of them can be used directly with storage operations.

Annotating Structs

To declare that a struct has an ability, it is declared with has <ability> after the struct name and either before or after the fields. For example:

public struct Ignorable has drop { f: u64 }
public struct Pair has copy, drop, store { x: u64, y: u64 }
public struct MyVec(vector<u64>) has copy, drop, store;

In this case: Ignorable has the drop ability. Pair and MyVec both have copy, drop, and store.

All of these abilities have strong guarantees over these gated operations. The operation can be performed on the value only if it has that ability; even if the value is deeply nested inside of some other collection!

As such: when declaring a struct’s abilities, certain requirements are placed on the fields. All fields must satisfy these constraints. These rules are necessary so that structs satisfy the reachability rules for the abilities given above. If a struct is declared with the ability...

• copy, all fields must have copy.
• drop, all fields must have drop.
• store, all fields must have store.
• key, all fields must have store.
  • key is the only ability currently that doesn’t require itself.

For example:

// A struct without any abilities
public struct NoAbilities {}

public struct WantsCopy has copy {
    f: NoAbilities, // ERROR 'NoAbilities' does not have 'copy'
}

and similarly:

// A struct without any abilities
public struct NoAbilities {}

public struct MyData has key {
    f: NoAbilities, // Error 'NoAbilities' does not have 'store'
}

Conditional Abilities and Generic Types

When abilities are annotated on a generic type, not all instances of that type are guaranteed to have that ability. Consider this struct declaration:

public struct Cup<T> has copy, drop, store, key { item: T }

It might be very helpful if Cup could hold any type, regardless of its abilities. The type system can see the type parameter, so it should be able to remove abilities from Cup if it sees a type parameter that would violate the guarantees for that ability.

This behavior might sound a bit confusing at first, but it might be more understandable if we think about collection types. We could consider the builtin type vector to have the following type declaration:

vector<T> has copy, drop, store;

We want vectors to work with any type. We don't want separate vector types for different abilities. So what are the rules we would want? Precisely the same that we would want with the field rules above. So, it would be safe to copy a vector value only if the inner elements can be copied. It would be safe to ignore a vector value only if the inner elements can be ignored/dropped. And, it would be safe to put a vector in storage only if the inner elements can be in storage.

To have this extra expressiveness, a type might not have all the abilities it was declared with depending on the instantiation of that type; instead, the abilities a type will have depends on both its declaration and its type arguments. For any type, type parameters are pessimistically assumed to be used inside of the struct, so the abilities are only granted if the type parameters meet the requirements described above for fields. Taking Cup from above as an example:

• Cup has the ability copy only if T has copy.
• It has drop only if T has drop.
• It has store only if T has store.
• It has key only if T has store.

Here are examples for this conditional system for each ability:

Example: conditional copy

public struct NoAbilities {}
public struct S has copy, drop { f: bool }
public struct Cup<T> has copy, drop, store { item: T }

fun example(c_x: Cup<u64>, c_s: Cup<S>) {
    // Valid, 'Cup<u64>' has 'copy' because 'u64' has 'copy'
    let c_x2 = copy c_x;
    // Valid, 'Cup<S>' has 'copy' because 'S' has 'copy'
    let c_s2 = copy c_s;
}

fun invalid(c_account: Cup<signer>, c_n: Cup<NoAbilities>) {
    // Invalid, 'Cup<signer>' does not have 'copy'.
    // Even though 'Cup' was declared with copy, the instance does not have 'copy'
    // because 'signer' does not have 'copy'
    let c_account2 = copy c_account;
    // Invalid, 'Cup<NoAbilities>' does not have 'copy'
    // because 'NoAbilities' does not have 'copy'
    let c_n2 = copy c_n;
}

Example: conditional drop

public struct NoAbilities {}
public struct S has copy, drop { f: bool }
public struct Cup<T> has copy, drop, store { item: T }

fun unused() {
    Cup<bool> { item: true }; // Valid, 'Cup<bool>' has 'drop'
    Cup<S> { item: S { f: false }}; // Valid, 'Cup<S>' has 'drop'
}

fun left_in_local(c_account: Cup<signer>): u64 {
    let c_b = Cup<bool> { item: true };
    let c_s = Cup<S> { item: S { f: false }};
    // Valid return: 'c_account', 'c_b', and 'c_s' have values
    // but 'Cup<signer>', 'Cup<bool>', and 'Cup<S>' have 'drop'
    0
}

fun invalid_unused() {
    // Invalid, Cannot ignore 'Cup<NoAbilities>' because it does not have 'drop'.
    // Even though 'Cup' was declared with 'drop', the instance does not have 'drop'
    // because 'NoAbilities' does not have 'drop'
    Cup<NoAbilities> { item: NoAbilities {} };
}

fun invalid_left_in_local(): u64 {
    let n = Cup<NoAbilities> { item: NoAbilities {} };
    // Invalid return: 'c_n' has a value
    // and 'Cup<NoAbilities>' does not have 'drop'
    0
}

Example: conditional store

public struct Cup<T> has copy, drop, store { item: T }

// 'MyInnerData is declared with 'store' so all fields need 'store'
struct MyInnerData has store {
    yes: Cup<u64>, // Valid, 'Cup<u64>' has 'store'
    // no: Cup<signer>, Invalid, 'Cup<signer>' does not have 'store'
}

// 'MyData' is declared with 'key' so all fields need 'store'
struct MyData has key {
    yes: Cup<u64>, // Valid, 'Cup<u64>' has 'store'
    inner: Cup<MyInnerData>, // Valid, 'Cup<MyInnerData>' has 'store'
    // no: Cup<signer>, Invalid, 'Cup<signer>' does not have 'store'
}

Example: conditional key

public struct NoAbilities {}
public struct MyData<T> has key { f: T }

fun valid(addr: address) acquires MyData {
    // Valid, 'MyData<u64>' has 'key'
    transfer(addr, MyData<u64> { f: 0 });
}

fun invalid(addr: address) {
   // Invalid, 'MyData<NoAbilities>' does not have 'key'
   transfer(addr, MyData<NoAbilities> { f: NoAbilities {} })
   // Invalid, 'MyData<NoAbilities>' does not have 'key'
   borrow<NoAbilities>(addr);
   // Invalid, 'MyData<NoAbilities>' does not have 'key'
   borrow_mut<NoAbilities>(addr);
}

// Mock storage operation
native public fun transfer<T: key>(addr: address, value: T);

Uses and Aliases

The use syntax can be used to create aliases to members in other modules. use can be used to create aliases that last either for the entire module, or for a given expression block scope.

Syntax

There are several different syntax cases for use. Starting with the most simple, we have the following for creating aliases to other modules

use <address>::<module name>;
use <address>::<module name> as <module alias name>;

For example

use std::vector;
use std::option as o;

use std::vector; introduces an alias vector for std::vector. This means that anywhere you would want to use the module name std::vector (assuming this use is in scope), you could use vector instead. use std::vector; is equivalent to use std::vector as vector;

Similarly use std::option as o; would let you use o instead of std::option

use std::vector;
use std::option as o;

fun new_vec(): vector<o::Option<u8>> {
    let mut v = vector[];
    vector::push_back(&mut v, o::some(0));
    vector::push_back(&mut v, o::none());
    v
}

If you want to import a specific module member (such as a function or struct). You can use the following syntax.

use <address>::<module name>::<module member>;
use <address>::<module name>::<module member> as <member alias>;

For example

use std::vector::push_back;
use std::option::some as s;

This would let you use the function std::vector::push_back without full qualification. Similarly for std::option::some with s. Instead you could use push_back and s respectively. Again, use std::vector::push_back; is equivalent to use std::vector::push_back as push_back;

use std::vector::push_back;
use std::option::some as s;

fun new_vec(): vector<std::option::Option<u8>> {
    let mut v = vector[];
    vector::push_back(&mut v, s(0));
    vector::push_back(&mut v, std::option::none());
    v
}

Multiple Aliases

If you want to add aliases for multiple module members at once, you can do so with the following syntax

use <address>::<module name>::{<module member>, <module member> as <member alias> ... };

For example

use std::vector::push_back;
use std::option::{some as s, none as n};

fun new_vec(): vector<std::option::Option<u8>> {
    let mut v = vector[];
    push_back(&mut v, s(0));
    push_back(&mut v, n());
    v
}

Self aliases

If you need to add an alias to the Module itself in addition to module members, you can do that in a single use using Self. Self is a member of sorts that refers to the module.

use std::option::{Self, some, none};

For clarity, all of the following are equivalent:

use std::option;
use std::option as option;
use std::option::Self;
use std::option::Self as option;
use std::option::{Self};
use std::option::{Self as option};

Multiple Aliases for the Same Definition

If needed, you can have as many aliases for any item as you like

use std::vector::push_back;
use std::option::{Option, some, none};

fun new_vec(): vector<Option<u8>> {
    let mut v = vector[];
    push_back(&mut v, some(0));
    push_back(&mut v, none());
    v
}

Nested imports

In Move, you can also import multiple names with the same use declaration. This brings all provided names into scope:

use std::{
    vector::{Self as vec, push_back},
    string::{String, Self as str}
};

fun example(s: &mut String) {
    let mut v = vec::empty();
    push_back(&mut v, 0);
    push_back(&mut v, 10);
    str::append_utf8(s, v);
}

Inside a module

Inside of a module all use declarations are usable regardless of the order of declaration.

module a::example {
    use std::vector;

    fun new_vec(): vector<Option<u8>> {
        let mut v = vector[];
        vector::push_back(&mut v, 0);
        vector::push_back(&mut v, 10);
        v
    }

    use std::option::{Option, some, none};
}

The aliases declared by use in the module usable within that module.

Additionally, the aliases introduced cannot conflict with other module members. See Uniqueness for more details

Inside an expression

You can add use declarations to the beginning of any expression block

module a::example {
    fun new_vec(): vector<Option<u8>> {
        use std::vector::push_back;
        use std::option::{Option, some, none};

        let mut v = vector[];
        push_back(&mut v, some(0));
        push_back(&mut v, none());
        v
    }
}

As with let, the aliases introduced by use in an expression block are removed at the end of that block.

module a::example {
    fun new_vec(): vector<Option<u8>> {
        let result = {
            use std::vector::push_back;
            use std::option::{Option, some, none};

            let mut v = vector[];
            push_back(&mut v, some(0));
            push_back(&mut v, none());
            v
        };
        result
    }
}

Attempting to use the alias after the block ends will result in an error

    fun new_vec(): vector<Option<u8>> {
        let mut result = {
            use std::vector::push_back;
            use std::option::{Option, some, none};

            let mut v = vector[];
            push_back(&mut v, some(0));
            v
        };
        push_back(&mut result, std::option::none());
        // ^^^^^^ ERROR! unbound function 'push_back'
        result
    }

Any use must be the first item in the block. If the use comes after any expression or let, it will result in a parsing error

{
    let mut v = vector[];
    use std::vector; // ERROR!
}

This allows you to shorten your import blocks in many situations. Note that these imports, as the previous ones, are all subject to the naming and uniqueness rules described in the following sections.

Naming rules

Aliases must follow the same rules as other module members. This means that aliases to structs (and constants) must start with A to Z

module a::data {
    public struct S {}
    const FLAG: bool = false;
    public fun foo() {}
}
module a::example {
    use a::data::{
        S as s, // ERROR!
        FLAG as fLAG, // ERROR!
        foo as FOO,  // valid
        foo as bar, // valid
    };
}

Uniqueness

Inside a given scope, all aliases introduced by use declarations must be unique.

For a module, this means aliases introduced by use cannot overlap

module a::example {
    use std::option::{none as foo, some as foo}; // ERROR!
    //                                     ^^^ duplicate 'foo'

    use std::option::none as bar;

    use std::option::some as bar; // ERROR!
    //                       ^^^ duplicate 'bar'

}

And, they cannot overlap with any of the module's other members

module a::data {
    public struct S {}
}
module example {
    use a::data::S;

    public struct S { value: u64 } // ERROR!
    //            ^ conflicts with alias 'S' above
}
}

Inside of an expression block, they cannot overlap with each other, but they can shadow other aliases or names from an outer scope

Shadowing

use aliases inside of an expression block can shadow names (module members or aliases) from the outer scope. As with shadowing of locals, the shadowing ends at the end of the expression block;

module a::example {

    public struct WrappedVector { vec: vector<u64> }

    public fun empty(): WrappedVector {
        WrappedVector { vec: std::vector::empty() }
    }

    public fun push_back(v: &mut WrappedVector, value: u64) {
        std::vector::push_back(&mut v.vec, value);
    }

    fun example1(): WrappedVector {
        use std::vector::push_back;
        // 'push_back' now refers to std::vector::push_back
        let mut vec = vector[];
        push_back(&mut vec, 0);
        push_back(&mut vec, 1);
        push_back(&mut vec, 10);
        WrappedVector { vec }
    }

    fun example2(): WrappedVector {
        let vec = {
            use std::vector::push_back;
            // 'push_back' now refers to std::vector::push_back

            let mut v = vector[];
            push_back(&mut v, 0);
            push_back(&mut v, 1);
            v
        };
        // 'push_back' now refers to Self::push_back
        let mut res = WrappedVector { vec };
        push_back(&mut res, 10);
        res
    }
}

Unused Use or Alias

An unused use will result in a warning

module a::example {
    use std::option::{some, none}; // Warning!
    //                      ^^^^ unused alias 'none'

    public fun example(): std::option::Option<u8> {
        some(0)
    }
}

Methods

As a syntactic convenience, some functions in Move can be called as "methods" on a value. This is done by using the . operator to call the function, where the value on the left-hand side of the . is the first argument to the function (sometimes called the receiver). The type of that value statically determines which function is called. This is an important difference from some other languages, where this syntax might indicate a dynamic call, where the function to be called is determined at runtime. In Move, all function calls are statically determined.

In short, this syntax exists to make it easier to call functions without having to create an alias with use, and without having to explicitly borrow the first argument to the function. Additionally, this can make code more readable, as it reduces the amount of boilerplate needed to call a function and makes it easier to chain function calls.

Syntax

The syntax for calling a method is as follows:

<expression> . <identifier> <[type_arguments],*> ( <arguments> )

For example

coin.value();
*nums.borrow_mut(i) = 5;

Method Resolution

When a method is called, the compiler will statically determine which function is called based on the type of the receiver (the argument on the left-hand side of the .). The compiler maintains a mapping from type and method name to the module and function name that should be called. This mapping is created form the use fun aliases that are currently in scope, and from the appropriate functions in the receiver type's defining module. In all cases, the receiver type is the first argument to the function, whether by-value or by-reference.

In this section, when we say a method "resolves" to a function, we mean that the compiler will statically replace the method with a normal function call. For example if we have x.foo(e) with foo resolving to a::m::foo, the compiler will replace x.foo(e) with a::m::foo(x, e), potentially automatically borrowing x.

Functions in the Defining Module

In a type’s defining module, the compiler will automatically create a method alias for any function declaration for its types when the type is the first argument in the function. For example,

module a::m {
    public struct X() has copy, drop, store;
    public fun foo(x: &X) { ... }
    public fun bar(flag: bool, x: &X) { ... }
}

The function foo can be called as a method on a value of type X. However, not the first argument (and one is not created for bool since bool is not defined in that module). For example,

fun example(x: a::m::X) {
    x.foo(); // valid
    // x.bar(true); ERROR!
}

use fun Aliases

Like a traditional use, a use fun statement creates an alias local to its current scope. This could be for the current module or the current expression block. However, the alias is associated to a type.

The syntax for a use fun statement is as follows:

use fun <function> as <type>.<method alias>;

This creates an alias for the <function>, which the <type> can receive as <method alias>.

For example

module a::cup {
    public struct Cup<T>(T) has copy, drop, store;

    public fun cup_borrow<T>(c: &Cup<T>): &T {
        &c.0
    }

    public fun cup_value<T>(c: Cup<T>): T {
        let Cup(t) = c;
        t
    }

    public fun cup_swap<T: drop>(c: &mut Cup<T>, t: T) {
        c.0 = t;
    }
}

We can now create use fun aliases to these functions

module b::example {
    use fun a::cup::cup_borrow as Cup.borrow;
    use fun a::cup::cup_value as Cup.value;
    use fun a::cup::cup_swap as Cup.set;

    fun example(c: &mut Cup<u64>) {
        let _ = c.borrow(); // resolves to a::cup::cup_borrow
        let v = c.value(); // resolves to a::cup::cup_value
        c.set(v * 2); // resolves to a::cup::cup_swap
    }
}

Note that the <function> in the use fun does not have to be a fully resolved path, and an alias can be used instead, so the declarations in the above example could equivalently be written as

    use a::cup::{Self, cup_swap};

    use fun cup::cup_borrow as Cup.borrow;
    use fun cup::cup_value as Cup.value;
    use fun cup_swap as Cup.set;

While these examples are cute for renaming the functions in the current module, the feature is perhaps more useful for declaring methods on types from other modules. For example, if we wanted to add a new utility to Cup, we could do so with a use fun alias and still use method syntax

module b::example {

    fun double(c: &Cup<u64>): Cup<u64> {
        let v = c.value();
        Cup::new(v * 2)
    }

}

Normally, we would be stuck having to call it as double(&c) because b::example did not define Cup, but instead we can use a use fun alias

    fun double_double(c: Cup<u64>): (Cup<u64>, Cup<u64>) {
        use fun b::example::double as Cup.dub;
        (c.dub(), c.dub()) // resolves to b::example::double in both calls
    }

While use fun can be made in any scope, the target <function> of the use fun must have a first argument that is the same as the <type>.

public struct X() has copy, drop, store;

fun new(): X { X() }
fun flag(flag: bool): u8 { if (flag) 1 else 0 }

use fun new as X.new; // ERROR!
use fun flag as X.flag; // ERROR!
// Neither `new` nor `flag` has first argument of type `X`

But any first argument of the <type> can be used, including references and mutable references

public struct X() has copy, drop, store;

public fun by_val(_: X) {}
public fun by_ref(_: &X) {}
public fun by_mut(_: &mut X) {}

// All 3 valid, in any scope
use fun by_val as X.v;
use fun by_ref as X.r;
use fun by_mut as X.m;

Note for generics, the methods are associated for all instances of the generic type. You cannot overload the method to resolve to different functions depending on the instantiation.

public struct Cup<T>(T) has copy, drop, store;

public fun value<T: copy>(c: &Cup<T>): T {
    c.0
}

use fun value as Cup<bool>.flag; // ERROR!
use fun value as Cup<u64>.num; // ERROR!
// In both cases, `use fun` aliases cannot be generic, they must work for all instances of the type

public use fun Aliases

Unlike a traditional use, the use fun statement can be made public, which allows it to be used outside of its declared scope. A use fun can be made public if it is declared in the module that defines the receivers type, much like the method aliases that are automatically created for functions in the defining module. Or conversely, one can think that an implicit public use fun is created automatically for every function in the defining module that has a first argument of the receiver type (if it is defined in that module). Both of these views are equivalent.

module a::cup {
    public struct Cup<T>(T) has copy, drop, store;

    public use fun cup_borrow as Cup.borrow;
    public fun cup_borrow<T>(c: &Cup<T>): &T {
        &c.0
    }
}

In this example, a public method alias is created for a::cup::Cup.borrow and a::cup::Cup.cup_borrow. Both resolve to a::cup::cup_borrow. And both are "public" in the sense that they can be used outside of a::cup, without an additional use or use fun.

module b::example {

    fun example<T: drop>(c: a::cup::Cup<u64>) {
        c.borrow(); // resolves to a::cup::cup_borrow
        c.cup_borrow(); // resolves to a::cup::cup_borrow
    }
}

The public use fun declarations thus serve as a way of renaming a function if you want to give it a cleaner name for use with method syntax. This is especially helpful if you have a module with multiple types, and similarly named functions for each type.

module a::shapes {

    public struct Rectangle { base: u64, height: u64 }
    public struct Box { base: u64, height: u64, depth: u64 }

    // Rectangle and Box can have methods with the same name

    public use fun rectangle_base as Rectangle.base;
    public fun rectangle_base(rectangle: &Rectangle): u64 {
        rectangle.base
    }

    public use fun box_base as Box.base;
    public fun box_base(box: &Box): u64 {
        box.base
    }

}

Another use for public use fun is adding methods to types from other modules. This can be helpful in conjunction with functions spread out across a single package.

module a::cup {
    public struct Cup<T>(T) has copy, drop, store;

    public fun new<T>(t: T): Cup<T> { Cup(t) }
    public fun borrow<T>(c: &Cup<T>): &T {
        &c.0
    }
    // `public use fun` to a function defined in another module
    public use fun a::utils::split as Cup.split;
}

module a::utils {
    use a::m::{Self, Cup};

    public fun split<u64>(c: Cup<u64>): (Cup<u64>, Cup<u64>) {
        let Cup(t) = c;
        let half = t / 2;
        let rem = if (t > 0) t - half else 0;
        (cup::new(half), cup::new(rem))
    }

}

And note that this public use fun does not create a circular dependency, as the use fun is not present after the module is compiled--all methods are resolved statically.

Interactions with use Aliases

A small detail to note is that method aliases respect normal use aliases.

module a::cup {
    public struct Cup<T>(T) has copy, drop, store;

    public fun cup_borrow<T>(c: &Cup<T>): &T {
        &c.0
    }
}

module b::other {
    use a::cup::{Cup, cup_borrow as borrow};

    fun example(c: &Cup<u64>) {
        c.borrow(); // resolves to a::cup::cup_borrow
    }
}

A helpful way to think about this is that use creates an implicit use fun alias for the function whenever it can. In this case the use a::cup::cup_borrow as borrow creates an implicit use fun a::cup::cup_borrow as Cup.borrow because it would be a valid use fun alias. Both views are equivalent. This line of reasoning can inform how specific methods will resolve with shadowing. See the cases in Scoping for more details.

Scoping

If not public, a use fun alias is local to its scope, much like a normal use. For example

module a::m {
    public struct X() has copy, drop, store;
    public fun foo(_: &X) {}
    public fun bar(_: &X) {}
}

module b::other {
    use a::m::X;

    use fun a::m::foo as X.f;

    fun example(x: &X) {
        x.f(); // resolves to a::m::foo
        {
            use a::m::bar as f;
            x.f(); // resolves to a::m::bar
        };
        x.f(); // still resolves to a::m::foo
        {
            use fun a::m::bar as X.f;
            x.f(); // resolves to a::m::bar
        }
    }

Automatic Borrowing

When resolving a method, the compiler will automatically borrow the receiver if the function expects a reference. For example

module a::m {
    public struct X() has copy, drop;
    public fun by_val(_: X) {}
    public fun by_ref(_: &X) {}
    public fun by_mut(_: &mut X) {}

    fun example(mut x: X) {
        x.by_ref(); // resolves to a::m::by_ref(&x)
        x.by_mut(); // resolves to a::m::by_mut(&mut x)
    }
}

In these examples, x was automatically borrowed to &x and &mut x respectively. This will also work through field access

module a::m {
    public struct X() has copy, drop;
    public fun by_val(_: X) {}
    public fun by_ref(_: &X) {}
    public fun by_mut(_: &mut X) {}

    public struct Y has drop { x: X }

    fun example(mut y: Y) {
        y.x.by_ref(); // resolves to a::m::by_ref(&y.x)
        y.x.by_mut(); // resolves to a::m::by_mut(&mut y.x)
    }
}

Note that in both examples, the local variable had to be labeled as mut to allow for the &mut borrow. Without this, there would be an error saying that x (or y in the second example) is not mutable.

Keep in mind that without a reference, normal rules for variable and field access come into play. Meaning a value might be moved or copied if it is not borrowed.

module a::m {
    public struct X() has copy, drop;
    public fun by_val(_: X) {}
    public fun by_ref(_: &X) {}
    public fun by_mut(_: &mut X) {}

    public struct Y has drop { x: X }
    public fun drop_y(y: Y) { y }

    fun example(y: Y) {
        y.x.by_val(); // copies `y.x` since `by_val` is by-value and `X` has `copy`
        y.drop_y(); // moves `y` since `drop_y` is by-value and `Y` does _not_ have `copy`
    }
}

Chaining

Method calls can be chained, because any expression can be the receiver of the method.

module a::shapes {
    public struct Point has copy, drop, store { x: u64, y: u64 }
    public struct Line has copy, drop, store { start: Point, end: Point }

    public fun x(p: &Point): u64 { p.x }
    public fun y(p: &Point): u64 { p.y }

    public fun start(l: &Line): &Point { &l.start }
    public fun end(l: &Line): &Point { &l.end }

}

module b::example {
    use a::shapes::Line;

    public fun x_values(l: Line): (u64, u64) {
        (l.start().x(), l.end().x())
    }

}

In this example for l.start().x(), the compiler first resolves l.start() to a::shapes::start(&l). Then .x() is resolved to a::shapes::x(a::shapes::start(&l)). Similarly for l.end().x(). Keep in mind, this feature is not "special"--the left-hand side of the . can be any expression, and the compiler will resolve the method call as normal. We simply draw attention to this sort of "chaining" because it is a common practice to increase readability.

Index Syntax

Move provides syntax attributes to allow you to define operations that look and feel like native Move code, lowering these operations into your user-provided definitions.

Our first syntax method, index, allows you to define a group of operations that can be used as custom index accessors for your datatypes, such as accessing a matrix element as m[i,j], by annotating functions that should be used for these index operations. Moreover, these definitions are bespoke per-type and available implicitly for any programmer using your type.

Overview and Summary

To start, consider a Matrix type that uses a vector of vectors to represent its values. You can write a small library using index syntax annotations on the borrow and borrow_mut functions as follows:

module matrix {

    public struct Matrix<T> { v: vector<vector<T>> }

    #[syntax(index)]
    public fun borrow<T>(s: &Matrix<T>, i: u64, j: u64): &T {
        vector::borrow(vector::borrow(&s.v, i), j)
    }

    #[syntax(index)]
    public fun borrow_mut<T>(s: &mut Matrix<T>, i: u64, j: u64): &mut T {
        vector::borrow_mut(vector::borrow_mut(&mut s.v, i), j)
    }

    public fun make_matrix<T>(v: vector<vector<T>>):  Matrix<T> {
        Matrix { v }
    }

}

Now anyone using this Matrix type has access to index syntax for it:

let mut m = matrix::make_matrix(vector[
    vector[1, 0, 0],
    vector[0, 1, 0],
    vector[0, 0, 1],
]);

let mut i = 0;
while (i < 3) {
    let mut j = 0;
    while (j < 3) {
        if (i == j) {
            assert!(m[i, j] == 1, 1);
        } else {
            assert!(m[i, j] == 0, 0);
        };
        *(&mut m[i,j]) = 2;
        j = j + 1;
    };
    i = i + 1;
}

Usage

As the example indicates, if you define a datatype and an associated index syntax method, anyone can invoke that method by writing index syntax on a value of that type:

let mat = matrix::make_matrix(...);
let m_0_0 = mat[0, 0];

During compilation, the compiler translates these into the appropriate function invocations based on the position and mutable usage of the expression:

let mut mat = matrix::make_matrix(...);

let m_0_0 = mat[0, 0];
// translates to `copy matrix::borrow(&mat, 0, 0)`

let m_0_0 = &mat[0, 0];
// translates to `matrix::borrow(&mat, 0, 0)`

let m_0_0 = &mut mat[0, 0];
// translates to `matrix::borrow_mut(&mut mat, 0, 0)`
``

You can also intermix index expressions with field accesses:

```move
public struct V { v: vector<u64> }

public struct Vs { vs: vector<V> }

fun borrow_first(input: &Vs): &u64 {
    &input.vs[0].v[0]
    // translates to `vector::borrow(&vector::borrow(&input.vs, 0).v, 0)`
}

Index Functions Take Flexible Arguments

Note that, aside from the definition and type limitations described in the rest of this chapter, Move places no restrictions on the values your index syntax method takes as parameters. This allows you to implement intricate programmatic behavior when defining index syntax, such as a data structure that takes a default value if the index is out of bounds:

#[syntax(index)]
public fun borrow_or_set<Key: copy, Value: drop>(
    input: &mut MTable<Key, Value>,
    key: Key,
    default: Value
): &mut Value {
    if (contains(input, key)) {
        borrow(input, key)
    } else {
        insert(input, key, default);
        borrow(input, key)
    }
}

Now, when you index into MTable, you must also provide a default value:

let string_key: String = ...;
let mut table: MTable<String, u64> = m_table::make_table();
let entry: &mut u64 = &mut table[string_key, 0];

This sort of extensible power allows you to write precise index interfaces for your types, concretely enforcing bespoke behavior.

Defining Index Syntax Functions

This powerful syntax form allows all of your user-defined datatypes to behave in this way, assuming your definitions adhere to the following rules:

1. The #[syntax(index)] attribute is added to the designated functions defined in the same module as the subject type.
2. The designated functions have public visibility.
3. The functions take a reference type as its subject type (its first argument) and returns a matching references type (mut if the subject was mut).
4. Each type has only a single mutable and single immutable definition.
5. Immutable and mutable versions have type agreement:
   • The subject types match, differing only in mutability.
   • The return types match the mutability of their subject types.
   • Type parameters, if present, have identical constraints between both versions.
   • All parameters beyond the subject type are identical.

The following content and additional examples describe these rules in greater detail.

Declaration

To declare an index syntax method, add the #[syntax(index)] attribute above the relevant function definition in the same module as the subject type's definition. This signals to the compiler that the function is an index accessor for the specified type.

Immutable Accessor

The immutable index syntax method is defined for read-only access. It takes an immutable reference of the subject type and returns an immutable reference to the element type. The borrow function defined in std::vector is an example of this:

#[syntax(index)]
public native fun borrow<Element>(v: &vector<Element>, i: u64): &Element;

Mutable Accessor

The mutable index syntax method is the dual of the immutable one, allowing for both read and write operations. It takes a mutable reference of the subject type and returns a mutable reference to the element type. The borrow_mut function defined in std::vector is an example of this:

#[syntax(index)]
public native fun borrow_mut<Element>(v: &mut vector<Element>, i: u64): &mut Element;

Visibility

To ensure that indexing functions are available anywhere the type is used, all index syntax methods must have public visibility. This ensures ergonomic usage of indexing across modules and packages in Move.

No Duplicates

In addition to the above requirements, we restrict each subject base type to defining a single index syntax method for immutable references and a single index syntax method for mutable references. For example, you cannot define a specialized version for a polymorphic type:

#[syntax(index)]
public fun borrow_matrix_u64(s: &Matrix<u64>, i: u64, j: u64): &u64 { ... }

#[syntax(index)]
public fun borrow_matrix<T>(s: &Matrix<T>, i: u64, j: u64): &T { ... }
    // ERROR! Matrix already has a definition
    // for its immutable index syntax method

This ensures that you can always tell which method is being invoked, without the need to inspect type instantiation.

Type Constraints

By default, an index syntax method has the following type constraints:

Its subject type (first argument) must be a reference to a single type defined in the same module as the marked function. This means that you cannot define index syntax methods for tuples, type parameters, or values:

#[syntax(index)]
public fun borrow_fst(x: &(u64, u64), ...): &u64 { ... }
    // ERROR because the subject type is a tuple

#[syntax(index)]
public fun borrow_tyarg<T>(x: &T, ...): &T { ... }
    // ERROR because the subject type is a type parameter

#[syntax(index)]
public fun borrow_value(x: Matrix<u64>, ...): &u64 { ... }
    // ERROR because x is not a reference

The subject type must match mutability with the return type. This restriction allows you to clarify the expected behavior when borrowing an indexed expression as &vec[i] versus &mut vec[i]. The Move compiler uses the mutability marker to determine which borrow form to call to produce a reference of the appropriate mutability. As a result, we disallow index syntax methods whose subject and return mutability differ:

#[syntax(index)]
public fun borrow_imm(x: &mut Matrix<u64>, ...): &u64 { ... }
    // ERROR! incompatible mutability
    // expected a mutable reference '&mut' return type

Type Compatibility

When defining an immutable and mutable index syntax method pair, they are subject to a number of compatibility constraints:

1. They must take the same number of type parameters, those type parameters must have the same constraints.
2. Type parameters must be used the same by position, not name.
3. Their subject types must match exactly except for the mutability.
4. Their return types must match exactly except for the mutability.
5. All other parameter types must match exactly.

These constraints are to ensure that index syntax behaves identically regardless of being in a mutable or immutable position.

To illustrate some of these errors, recall the previous Matrix definition:

#[syntax(index)]
public fun borrow<T>(s: &Matrix<T>, i: u64, j: u64): &T {
    vector::borrow(vector::borrow(&s.v, i), j)
}

All of the following are type-incompatible definitions of the mutable version:

#[syntax(index)]
public fun borrow_mut<T: drop>(s: &mut Matrix<T>, i: u64, j: u64): &mut T { ... }
    // ERROR! `T` has `drop` here, but no in the immutable version

#[syntax(index)]
public fun borrow_mut(s: &mut Matrix<u64>, i: u64, j: u64): &mut u64 { ... }
    // ERROR! This takes a different number of type parameters

#[syntax(index)]
public fun borrow_mut<T, U>(s: &mut Matrix<U>, i: u64, j: u64): &mut U { ... }
    // ERROR! This takes a different number of type parameters

#[syntax(index)]
public fun borrow_mut<U>(s: &mut Matrix<U>, i_j: (u64, u64)): &mut U { ... }
    // ERROR! This takes a different number of arguments

#[syntax(index)]
public fun borrow_mut<U>(s: &mut Matrix<U>, i: u64, j: u32): &mut U { ... }
    // ERROR! `j` is a different type

Again, the goal here is to make the usage across the immutable and mutable versions consistent. This allows index syntax methods to work without changing out the behavior or constraints based on mutable versus immutable usage, ultimately ensuring a consistent interface to program against.

Packages

Packages allow Move programmers to more easily re-use code and share it across projects. The Move package system allows programmers to easily:

• Define a package containing Move code;
• Parameterize a package by named addresses;
• Import and use packages in other Move code and instantiate named addresses;
• Build packages and generate associated compilation artifacts from packages; and
• Work with a common interface around compiled Move artifacts.

Package Layout and Manifest Syntax

A Move package source directory contains a Move.toml package manifest file, a generated Move.lock file, and a set of subdirectories:

a_move_package
├── Move.toml      (required)
├── Move.lock      (generated)
├── sources        (required)
├── doc_templates  (optional)
├── examples       (optional, test & dev mode)
└── tests          (optional, test mode)

The directories and files labeled "required" must be present for a directory to be considered a Move package and built. Optional directories may be present, and if so, they will be included in the compilation process depending on the mode used to build the package. For instance, when built in "dev" or "test" modes, the tests and examples directories will also be included.

Going through each of these in turn:

1. The Move.toml file is the package manifest and is required for a directory to be considered a Move package. This file contains metadata about the package, such as name, dependencies, and so on.
2. The Move.lock file is generated by the Move CLI and contains the fixed build versions of the package and its dependencies. It is used to ensure consistent versions are used across different builds and that changes in dependencies are apparent as a change in this file.
3. The sources directory is required and contains the Move modules that make up the package. Modules in this directory will always be included in the compilation process.
4. The doc_templates directory can contain documentation templates that will be used when generating documentation for the package.
5. The examples directory can hold additional code to be used only for development and/or tutorials, this will not be included when compiled outside of test or dev modes.
6. The tests directory can contain Move modules that are only included when compiled in test mode or when Move unit tests are run.

Move.toml

The Move package manifest is defined within the Move.toml file and has the following syntax. Optional fields are marked with *, + denotes one or more elements:

[package]
name = <string>
edition* = <string>      # e.g., "2024.alpha" to use the Move 2024 edition,
                         # currently in alpha. Will default to the latest stable edition if not specified.
license* = <string>              # e.g., "MIT", "GPL", "Apache 2.0"
authors* = [<string>,+]  # e.g., ["Joe Smith (joesmith@noemail.com)", "John Snow (johnsnow@noemail.com)"]

# Additional fields may be added to this section by external tools. E.g., on Sui the following sections are added:
published-at* = "<hex-address>" # The address that the package is published at. Should be set after the first publication.

[dependencies] # (Optional section) Paths to dependencies
# One or more lines declaring dependencies in the following format

# ##### Local Dependencies #####
# For local dependencies use `local = path`. Path is relative to the package root
# Local = { local = "../path/to" }
# To resolve a version conflict and force a specific version for dependency
# override you can use `override = true`
# Override = { local = "../conflicting/version", override = true }
# To instantiate address values in a dependency, use `addr_subst`
<string> = {
    local = <string>,
    override* = <bool>,
    addr_subst* = { (<string> = (<string> | "<hex_address>"))+ }
}

# ##### Git Dependencies #####
# For remote import, use the `{ git = "...", subdir = "...", rev = "..." }`.
# Revision must be supplied, it can be a branch, a tag, or a commit hash.
# If no `subdir` is specified, the root of the repository is used.
# MyRemotePackage = { git = "https://some.remote/host.git", subdir = "remote/path", rev = "main" }
<string> = {
    git = <URL ending in .git>,
    subdir=<path to dir containing Move.toml inside git repo>,
    rev=<git commit hash>,
    override* = <bool>,
    addr_subst* = { (<string> = (<string> | "<hex_address>"))+ }
}

[addresses]  # (Optional section) Declares named addresses in this package
# One or more lines declaring named addresses in the following format
# Addresses that match the name of the package must be set to `"0x0"` or they will be unable to be published.
<addr_name> = "_" | "<hex_address>" # e.g., std = "_" or my_addr = "0xC0FFEECAFE"

# Named addresses will be accessible in Move as `@name`. They're also exported:
# for example, `std = "0x1"` is exported by the Standard Library.
# alice = "0xA11CE"

[dev-dependencies] # (Optional section) Same as [dependencies] section, but only included in "dev" and "test" modes
# The dev-dependencies section allows overriding dependencies for `--test` and
# `--dev` modes. You can e.g., introduce test-only dependencies here.
# Local = { local = "../path/to/dev-build" }
<string> = {
    local = <string>,
    override* = <bool>,
    addr_subst* = { (<string> = (<string> | "<hex_address>"))+ }
}
<string> = {
    git = <URL ending in .git>,
    subdir=<path to dir containing Move.toml inside git repo>,
    rev=<git commit hash>,
    override* = <bool>,
    addr_subst* = { (<string> = (<string> | "<hex_address>"))+ }
}

[dev-addresses] # (Optional section) Same as [addresses] section, but only included in "dev" and "test" modes
# The dev-addresses section allows overwriting named addresses for the `--test`
# and `--dev` modes.
<addr_name> = "<hex_address>" # e.g., alice = "0xB0B"

An example of a minimal package manifest:

[package]
name = "AName"

An example of a more standard package manifest that also includes the Move standard library and instantiates the named address std from the LocalDep package with the address value 0x1:

[package]
name = "AName"
license = "Apache 2.0"

[addresses]
address_to_be_filled_in = "_"
specified_address = "0xB0B"

[dependencies]
# Local dependency
LocalDep = { local = "projects/move-awesomeness", addr_subst = { "std" = "0x1" } }
# Git dependency
MoveStdlib = { git = "https://github.com/MystenLabs/sui.git", subdir = "crates/sui-framework/packages/move-stdlib", rev = "framework/mainnet" }

[dev-addresses] # For use when developing this module
address_to_be_filled_in = "0x101010101"

Most of the sections in the package manifest are self explanatory, but named addresses can be a bit difficult to understand so we examine them in more detail in Named Addresses During Compilation.

Named Addresses During Compilation

Recall that Move has named addresses and that named addresses cannot be declared in Move. Instead they are declared at the package level: in the manifest file (Move.toml) for a Move package you declare named addresses in the package, instantiate other named addresses, and rename named addresses from other packages within the Move package system.

Let's go through each of these actions, and how they are performed in the package's manifest one-by-one:

Declaring Named Addresses

Let's say we have a Move module in example_pkg/sources/A.move as follows:

module named_addr::a {
    public fun x(): address { @named_addr }
}

We could in example_pkg/Move.toml declare the named address named_addr in two different ways. The first:

[package]
name = "example_pkg"
...
[addresses]
named_addr = "_"

Declares named_addr as a named address in the package example_pkg and that this address can be any valid address value. In particular, an importing package can pick the value of the named address named_addr to be any address it wishes. Intuitively you can think of this as parameterizing the package example_pkg by the named address named_addr, and the package can then be instantiated later on by an importing package.

named_addr can also be declared as:

[package]
name = "example_pkg"
...
[addresses]
named_addr = "0xCAFE"

which states that the named address named_addr is exactly 0xCAFE and cannot be changed. This is useful so other importing packages can use this named address without needing to worry about the exact value assigned to it.

With these two different declaration methods, there are two ways that information about named addresses can flow in the package graph:

• The former ("unassigned named addresses") allows named address values to flow from the importation site to the declaration site.
• The latter ("assigned named addresses") allows named address values to flow from the declaration site upwards in the package graph to usage sites.

With these two methods for flowing named address information throughout the package graph the rules around scoping and renaming become important to understand.

Scope and Renaming of Named Addresses

A named address N in a package P is in scope if:

1. P declares a named address N; or
2. A package in one of P's transitive dependencies declares the named address N and there is a dependency path in the package graph between between P and the declaring package of N with no renaming of N.

Additionally, every named address in a package is exported. Because of this and the above scoping rules each package can be viewed as coming with a set of named addresses that will be brought into scope when the package is imported, e.g., if you import example_pkg, that import will also bring the named_addr named address into scope. Because of this, if P imports two packages P1 and P2 both of which declare a named address N an issue arises in P: which "N" is meant when N is referred to in P? The one from P1 or P2? To prevent this ambiguity around which package a named address is coming from, we enforce that the sets of scopes introduced by all dependencies in a package are disjoint, and provide a way to rename named addresses when the package that brings them into scope is imported.

Renaming a named address when importing can be done as follows in our P, P1, and P2 example above:

[package]
name = "P"
...
[dependencies]
P1 = { local = "some_path_to_P1", addr_subst = { "P1N" = "N" } }
P2 = { local = "some_path_to_P2"  }

With this renaming N refers to the N from P2 and P1N will refer to N coming from P1:

module N::A {
    public fun x(): address { @P1N }
}

It is important to note that renaming is not local: once a named address N has been renamed to N2 in a package P all packages that import P will not see N but only N2 unless N is reintroduced from outside of P. This is why rule (2) in the scoping rules at the start of this section specifies a "dependency path in the package graph between between P and the declaring package of N with no renaming of N."

Instantiating Named Addresses

Named addresses can be instantiated multiple times across the package graph as long as it is always with the same value. It is an error if the same named address (regardless of renaming) is instantiated with differing values across the package graph.

A Move package can only be compiled if all named addresses resolve to a value. This presents issues if the package wishes to expose an uninstantiated named address. This is what the [dev-addresses] section solves in part. This section can set values for named addresses, but cannot introduce any named addresses. Additionally, only the [dev-addresses] in the root package are included in dev mode. For example a root package with the following manifest would not compile outside of dev mode since named_addr would be uninstantiated:

[package]
name = "example_pkg"
...
[addresses]
named_addr = "_"

[dev-addresses]
named_addr = "0xC0FFEE"

Usage and Artifacts

The Move package system comes with a command line option as part of the CLI: sui move <command> <command_flags>. Unless a particular path is provided, all package commands will run in the current enclosing Move package. The full list of commands and flags for the Move CLI can be found by running sui move --help.

Artifacts

A package can be compiled using CLI commands. This will create a build directory containing build-related artifacts (including bytecode binaries, source maps, and documentation). The general layout of the build directory is as follows:

a_move_package
├── BuildInfo.yaml
├── bytecode_modules
│   ├── dependencies
│   │   ├── <dep_pkg_name>
│   │   │   └── *.mv
│   │   ...
│   │   └──  <dep_pkg_name>
│   │       └── *.mv
│   ...
│   └── *.mv
├── docs
│   ├── dependencies
│   │   ├── <dep_pkg_name>
│   │   │   └── *.md
│   │   ...
│   │   └──  <dep_pkg_name>
│   │       └── *.md
│   ...
│   └── *.md
├── source_maps
│   ├── dependencies
│   │   ├── <dep_pkg_name>
│   │   │   └── *.mvsm
│   │   ...
│   │   └──  <dep_pkg_name>
│   │       └── *.mvsm
│   ...
│   └── *.mvsm
└── sources
    ...
    └── *.move
    ├── dependencies
    │   ├── <dep_pkg_name>
    │   │   └── *.move
    │   ...
    │   └──  <dep_pkg_name>
    │       └── *.move
    ...
    └── *.move

Move.lock

The Move.lock file is generated at the root of the Move package when the package is built. The Move.lock file contains information about your package and its build configuration, and acts as a communication layer between the Move compiler and other tools, like chain-specific command line interfaces and third-party package managers.

Like the Move.toml file, the Move.lock file is a text-based TOML file. Unlike the package manifest however, the Move.lock file is not intended for you to edit directly. Processes on the toolchain, like the Move compiler, access and edit the file to read and append relevant information to it. You also must not move the file from the root, as it needs to be at the same level as the Move.toml manifest in the package.

If you are using source control for your package, it's recommended practice to check in the Move.lock file that corresponds with your desired built or published package. This ensures that every build of your package is an exact replica of the original, and that changes to the build will be apparent as changes to the Move.lock file.

The Move.lock file is a TOML file that currently contains the following fields.

Note: other fields may be added to the lock file either in the future, or by third-party package package managers as well.

The [move] Section

This section contains the core information needed in the lockfile:

• The version of the lockfile (needed for backwards compatibility checking, and versioning lockfile changes in the future).
• The hash of the Move.toml file that was used to generate this lock file.
• The hash of the Move.lock file of all dependencies. If no dependencies are present, this will be an empty string.
• The list of dependencies.

[move]
version = <string> # Lock file version, used for backwards compatibility checking.
manifest_digest = <hash> # Sha3-256 hash of the Move.toml file that was used to generate this lock file.
deps_digest = <hash> # Sha3-256 hash of the Move.lock file of all dependencies. If no dependencies are present, this will be an empty string.
dependencies = { (name = <string>)* } # List of dependencies. Not present if there are no dependencies.

The [move.package] Sections

After the Move compiler resolves each of the dependencies for the package it writes the location of the dependency to the Move.lock file. If a dependency failed to resolve, the compiler will not write the Move.lock file and the build fails. If all dependencies resolve, the Move.lock file contains the locations (local and remote) of all of the package's transitive dependencies. These will be stored in the Move.lock file in the following format:

# ...

[[move.package]]
name = "A"
source = { git = "https://github.com/b/c.git", subdir = "e/f", rev = "a1b2c3" }

[[move.package]]
name = "B"
source = { local = "../local-dep" }

The [move.toolchain-version] Section

As mentioned above, additional fields may be added to the lock file by external tools. For example, the Sui package manager adds toolchain version information to the lock file that can then be used for on-chain source verification:

# ...

[move.toolchain-version]
compiler-version = <string> # The version of the Move compiler used to build the package, e.g. "1.21.0"
edition = <string> # The edition of the Move language used to build the package, e.g. "2024.alpha"
flavor = <string> # The flavor of the Move compiler used to build the package, e.g. "sui"

Unit Tests

Unit testing for Move uses three annotations in the Move source language:

• #[test] marks a function as a test;
• #[expected_failure] marks that a test is expected to fail;
• #[test_only] marks a module or module member (use, function, struct, or constant) as code to be included for testing only.

These annotations can be placed on any appropriate form with any visibility. Whenever a module or module member is annotated as #[test_only] or #[test], it will not be included in the compiled bytecode unless it is compiled for testing.

Test Annotations

The #[test] annotation can only be placed on a function with no parameters. This annotation marks the function as a test to be run by the unit testing harness.

#[test] // OK
fun this_is_a_test() { ... }

#[test] // Will fail to compile since the test takes an argument
fun this_is_not_correct(arg: u64) { ... }

A test can also be annotated as an #[expected_failure]. This annotation marks that the test is expected to raise an error. There are a number of options that can be used with the #[expected_failure] annotation to ensure only a failure with the specified condition is marked as passing, these options are detailed in Expected Failures. Only functions that have the #[test] annotation can also be annotated as an #[expected_failure].

Some simple examples of using the #[expected_failure] annotation are shown below:

#[test]
#[expected_failure]
public fun this_test_will_abort_and_pass() { abort 1 }

#[test]
#[expected_failure]
public fun test_will_error_and_pass() { 1/0; }

#[test] // Will pass since test fails with the expected abort code constant.
#[expected_failure(abort_code = ENotFound)] // ENotFound is a constant defined in the module
public fun test_will_error_and_pass_abort_code() { abort ENotFound }

#[test] // Will fail since test fails with a different error than expected.
#[expected_failure(abort_code = my_module::ENotFound)]
public fun test_will_error_and_fail() { 1/0; }

#[test, expected_failure] // Can have multiple in one attribute. This test will pass.
public fun this_other_test_will_abort_and_pass() { abort 1 }

Note: #[test] and #[test_only] functions can also call entry functions, regardless of their visibility.

Expected Failures

There are a number of different ways that you can use the #[expected_failure] annotation to specify different types of error conditions. These are:

1. #[expected_failure(abort_code = <constant>)]

This will pass if the test aborts with the specified constant value in the module that defines the constant and fail otherwise. This is the recommended way of testing for expected test failures.

Note: You can reference constants outside of the current module or package in expected_failure annotations.

module pkg_addr::other_module {
    const ENotFound: u64 = 1;
    public fun will_abort() {
        abort ENotFound
    }
}

module pkg_addr::my_module {
    use pkg_addr::other_module;
    const ENotFound: u64 = 1;

    #[test]
    #[expected_failure(abort_code = ENotFound)]
    fun test_will_abort_and_pass() { abort ENotFound }

    #[test]
    #[expected_failure(abort_code = other_module::ENotFound)]
    fun test_will_abort_and_pass() { other_module::will_abort() }

    // FAIL: Will not pass since we are expecting the constant from the wrong module.
    #[test]
    #[expected_failure(abort_code = ENotFound)]
    fun test_will_abort_and_pass() { other_module::will_abort() }
}

2. #[expected_failure(arithmetic_error, location = <location>)]

This specifies that the test is expected to fail with an arithmetic error (e.g., integer overflow, division by zero, etc) at the specified location. The <location> must be a valid path to a module location, e.g., Self, or my_package::my_module.

module pkg_addr::other_module {
    public fun will_arith_error() { 1/0; }
}

module pkg_addr::my_module {
    use pkg_addr::other_module;
    #[test]
    #[expected_failure(arithmetic_error, location = Self)]
    fun test_will_arith_error_and_pass1() { 1/0; }

    #[test]
    #[expected_failure(arithmetic_error, location = pkg_addr::other_module)]
    fun test_will_arith_error_and_pass2() { other_module::will_arith_error() }

    // FAIL: Will fail since the location we expect it the fail at is different from where the test actually failed.
    #[test]
    #[expected_failure(arithmetic_error, location = Self)]
    fun test_will_arith_error_and_fail() { other_module::will_arith_error() }
}

3. #[expected_failure(out_of_gas, location = <location>)]

This specifies that the test is expected to fail with an out of gas error at the specified location. The <location> must be a valid path to a module location, e.g., Self, or my_package::my_module.

module pkg_addr::other_module {
    public fun will_oog() { loop {} }
}

module pkg_addr::my_module {
    use pkg_addr::other_module;
    #[test]
    #[expected_failure(out_of_gas, location = Self)]
    fun test_will_oog_and_pass1() { loop {} }

    #[test]
    #[expected_failure(arithmetic_error, location = pkg_addr::other_module)]
    fun test_will_oog_and_pass2() { other_module::will_oog() }

    // FAIL: Will fail since the location we expect it the fail at is different from where
    // the test actually failed.
    #[test]
    #[expected_failure(out_of_gas, location = Self)]
    fun test_will_oog_and_fail() { other_module::will_oog() }
}

4. #[expected_failure(vector_error, minor_status = <u64_opt>, location = <location>)]

This specifies that the test is expected to fail with a vector error at the specified location with the given minor_status (if provided). The <location> must be a valid path to a module module location, e.g., Self, or my_package::my_module. The <u64_opt> is an optional parameter that specifies the minor status of the vector error. If it is not specified, the test will pass if the test fails with any minor status. If it is specified, the test will only pass if the test fails with a vector error with the specified minor status.

module pkg_addr::other_module {
    public fun vector_borrow_empty() {
        &vector<u64>[][1];
    }
}

module pkg_addr::my_module {
    #[test]
    #[expected_failure(vector_error, location = Self)]
    fun vector_abort_same_module() {
        vector::borrow(&vector<u64>[], 1);
    }

    #[test]
    #[expected_failure(vector_error, location = pkg_addr::other_module)]
    fun vector_abort_same_module() {
        other_module::vector_borrow_empty();
    }

    // Can specify minor statues (i.e., vector-specific error codes) to expect.
    #[test]
    #[expected_failure(vector_error, minor_status = 1, location = Self)]
    fun native_abort_good_right_code() {
        vector::borrow(&vector<u64>[], 1);
    }

    // FAIL: correct error, but wrong location.
    #[test]
    #[expected_failure(vector_error, location = pkg_addr::other_module)]
    fun vector_abort_same_module() {
        other_module::vector_borrow_empty();
    }

    // FAIL: correct error and location but the minor status differs so this test will fail.
    #[test]
    #[expected_failure(vector_error, minor_status = 0, location = Self)]
    fun vector_abort_wrong_minor_code() {
        vector::borrow(&vector<u64>[], 1);
    }
}

5. #[expected_failure]

This will pass if the test aborts with any error code. You should be incredibly careful using this to annotate expected tests failures, and always prefer one of the ways described above instead. Examples of these types of annotations are:

#[test]
#[expected_failure]
fun test_will_abort_and_pass1() { abort 1 }

#[test]
#[expected_failure]
fun test_will_arith_error_and_pass2() { 1/0; }

Test Only Annotations

A module and any of its members can be declared as test only. If an item is annotated as #[test_only] the item will only be included in the compiled Move bytecode when compiled in test mode. Additionally, when compiled outside of test mode, any non-test uses of a #[test_only] module will raise an error during compilation.

Note: functions that are annotated with #[test_only] will only be available to be called from test code, but they themselves are not tests and will not be run as tests by the unit testing framework.

#[test_only] // test only attributes can be attached to modules
module abc { ... }

#[test_only] // test only attributes can be attached to constants
const MY_ADDR: address = @0x1;

#[test_only] // .. to uses
use pkg_addr::some_other_module;

#[test_only] // .. to structs
public struct SomeStruct { ... }

#[test_only] // .. and functions. Can only be called from test code, but this is _not_ a test!
fun test_only_function(...) { ... }

Running Unit Tests

Unit tests for a Move package can be run with the sui move test command.

When running tests, every test will either PASS, FAIL, or TIMEOUT. If a test case fails, the location of the failure along with the function name that caused the failure will be reported if possible. You can see an example of this below.

A test will be marked as timing out if it exceeds the maximum number of instructions that can be executed for any single test. This bound can be changed using the options below. Additionally, while the result of a test is always deterministic, tests are run in parallel by default, so the ordering of test results in a test run is non-deterministic unless running with only one thread, which can be configured via an option.

These aforementioned options are two among many that can fine-tune testing and help debug failing tests. To see all available options, and a description of what each one does, pass the --help flag to the sui move test command:

$ sui move test --help

Example

A simple module using some of the unit testing features is shown in the following example:

First create an empty package and change directory into it:

$ sui move new test_example; cd test_example

Next add the following module under the sources directory:

// filename: sources/my_module.move
module test_example::my_module {

    public struct Wrapper(u64)

    const ECoinIsZero: u64 = 0;

    public fun make_sure_non_zero_coin(coin: Wrapper): Wrapper {
        assert!(coin.0 > 0, ECoinIsZero);
        coin
    }

    #[test]
    fun make_sure_non_zero_coin_passes() {
        let coin = Wrapper(1);
        let Wrapper(_) = make_sure_non_zero_coin(coin);
    }

    #[test]
    // Or #[expected_failure] if we don't care about the abort code
    #[expected_failure(abort_code = ECoinIsZero)]
    fun make_sure_zero_coin_fails() {
        let coin = Wrapper(0);
        let Wrapper(_) = make_sure_non_zero_coin(coin);
    }

    #[test_only] // test only helper function
    fun make_coin_zero(coin: &mut Wrapper) {
        coin.0 = 0;
    }

    #[test]
    #[expected_failure(abort_code = ECoinIsZero)]
    fun make_sure_zero_coin_fails2() {
        let mut coin = Wrapper(10);
        coin.make_coin_zero();
        let Wrapper(_) = make_sure_non_zero_coin(coin);
    }
}

Running Tests

You can then run these tests with the move test command:

$ sui move test
INCLUDING DEPENDENCY Sui
INCLUDING DEPENDENCY MoveStdlib
BUILDING test_example
Running Move unit tests
[ PASS    ] 0x0::my_module::make_sure_non_zero_coin_passes
[ PASS    ] 0x0::my_module::make_sure_zero_coin_fails
[ PASS    ] 0x0::my_module::make_sure_zero_coin_fails2
Test result: OK. Total tests: 3; passed: 3; failed: 0

Using Test Flags

Passing specific tests to run

You can run a specific test, or a set of tests with sui move test <str>. This will only run tests whose fully qualified name contains <str>. For example if we wanted to only run tests with "non_zero" in their name:

$ sui move test non_zero
INCLUDING DEPENDENCY Sui
INCLUDING DEPENDENCY MoveStdlib
BUILDING test_example
Running Move unit tests
[ PASS    ] 0x0::my_module::make_sure_non_zero_coin_passes
Test result: OK. Total tests: 1; passed: 1; failed: 0

-i <bound> or --gas_used <bound>

This bounds the amount of gas that can be consumed for any one test to <bound>:

$ sui move test -i 0
INCLUDING DEPENDENCY Sui
INCLUDING DEPENDENCY MoveStdlib
BUILDING test_example
Running Move unit tests
[ TIMEOUT ] 0x0::my_module::make_sure_non_zero_coin_passes
[ FAIL    ] 0x0::my_module::make_sure_zero_coin_fails
[ FAIL    ] 0x0::my_module::make_sure_zero_coin_fails2

Test failures:

Failures in 0x0::my_module:

┌── make_sure_non_zero_coin_passes ──────
│ Test timed out
└──────────────────


┌── make_sure_zero_coin_fails ──────
│ error[E11001]: test failure
│    ┌─ ./sources/my_module.move:22:27
│    │
│ 21 │     fun make_sure_zero_coin_fails() {
│    │         ------------------------- In this function in 0x0::my_module
│ 22 │         let coin = MyCoin(0);
│    │                           ^ Test did not error as expected. Expected test to abort with code 0 <SNIP>
│
│
└──────────────────


┌── make_sure_zero_coin_fails2 ──────
│ error[E11001]: test failure
│    ┌─ ./sources/my_module.move:34:31
│    │
│ 33 │     fun make_sure_zero_coin_fails2() {
│    │         -------------------------- In this function in 0x0::my_module
│ 34 │         let mut coin = MyCoin(10);
│    │                               ^^ Test did not error as expected. Expected test to abort with code 0 <SNIP>
│
│
└──────────────────

Test result: FAILED. Total tests: 3; passed: 0; failed: 3

-s or --statistics

With these flags you can gather statistics about the tests run and report the runtime and gas used for each test. You can additionally add csv (sui move test -s csv) to get the gas usage in a csv output format. For example, if we wanted to see the statistics for the tests in the example above:

$ sui move test -s
INCLUDING DEPENDENCY Sui
INCLUDING DEPENDENCY MoveStdlib
BUILDING test_example
Running Move unit tests
[ PASS    ] 0x0::my_module::make_sure_non_zero_coin_passes
[ PASS    ] 0x0::my_module::make_sure_zero_coin_fails
[ PASS    ] 0x0::my_module::make_sure_zero_coin_fails2

Test Statistics:

┌────────────────────────────────────────────────┬────────────┬───────────────────────────┐
│                   Test Name                    │    Time    │         Gas Used          │
├────────────────────────────────────────────────┼────────────┼───────────────────────────┤
│ 0x0::my_module::make_sure_non_zero_coin_passes │   0.001    │             1             │
├────────────────────────────────────────────────┼────────────┼───────────────────────────┤
│ 0x0::my_module::make_sure_zero_coin_fails      │   0.001    │             1             │
├────────────────────────────────────────────────┼────────────┼───────────────────────────┤
│ 0x0::my_module::make_sure_zero_coin_fails2     │   0.001    │             1             │
└────────────────────────────────────────────────┴────────────┴───────────────────────────┘

Test result: OK. Total tests: 3; passed: 3; failed: 0

See Sui's Coding Conventions for Move

DEPRECATED: Friends

NOTE: this feature has been superseded by public(package).

The friend syntax was used to declare modules that are trusted by the current module. A trusted module is allowed to call any function defined in the current module that have the public(friend) visibility. For details on function visibilities, refer to the Visibility section in Functions.

Friend declaration

A module can declare other modules as friends via friend declaration statements, in the format of

• friend <address::name> — friend declaration using fully qualified module name like the example below, or

  module 0x42::a {
      friend 0x42::b;
  }
  

• friend <module-name-alias> — friend declaration using a module name alias, where the module alias is introduced via the use statement.

  module 0x42::a {
      use 0x42::b;
      friend b;
  }
  

A module may have multiple friend declarations, and the union of all the friend modules forms the friend list. In the example below, both 0x42::B and 0x42::C are considered as friends of 0x42::A.

module 0x42::a {

    friend 0x42::b;
    friend 0x42::c;
}

Unlike use statements, friend can only be declared in the module scope and not in the expression block scope. friend declarations may be located anywhere a top-level construct (e.g., use, function, struct, etc.) is allowed. However, for readability, it is advised to place friend declarations near the beginning of the module definition.

Friend declaration rules

Friend declarations are subject to the following rules:

• A module cannot declare itself as a friend.

  module 0x42::m { friend Self; // ERROR! }
  //                      ^^^^ Cannot declare the module itself as a friend
  
  module 0x43::m { friend 0x43::M; // ERROR! }
  //                      ^^^^^^^ Cannot declare the module itself as a friend
  

• Friend modules must be known by the compiler

  module 0x42::m { friend 0x42::nonexistent; // ERROR! }
  //                      ^^^^^^^^^^^^^^^^^ Unbound module '0x42::nonexistent'
  

• Friend modules must be within the same account address.

  module 0x42::m {}
  
  module 0x42::n { friend 0x42::m; // ERROR! }
  //                      ^^^^^^^ Cannot declare modules out of the current address as a friend
  

• Friends relationships cannot create cyclic module dependencies.

  Cycles are not allowed in the friend relationships, e.g., the relation 0x2::a friends 0x2::b friends 0x2::c friends 0x2::a is not allowed. More generally, declaring a friend module adds a dependency upon the current module to the friend module (because the purpose is for the friend to call functions in the current module). If that friend module is already used, either directly or transitively, a cycle of dependencies would be created.

  module 0x2::a {
      use 0x2::c;
      friend 0x2::b;
  
      public fun a() {
          c::c()
      }
  }
  
  module 0x2::b {
      friend 0x2::c; // ERROR!
  //         ^^^^^^ This friend relationship creates a dependency cycle: '0x2::b' is a friend of '0x2::a' uses '0x2::c' is a friend of '0x2::b'
  }
  
  module 0x2::c {
      public fun c() {}
  }
  

• The friend list for a module cannot contain duplicates.

  module 0x42::a {}
  
  module 0x42::m {
      use 0x42::a as aliased_a;
      friend 0x42::A;
      friend aliased_a; // ERROR!
  //         ^^^^^^^^^ Duplicate friend declaration '0x42::a'. Friend declarations in a module must be unique
  }