Diem Developers

Functions

Function syntax in Move is shared between module functions and script functions. Functions inside of modules are reusable, whereas script functions are only used once to invoke a transaction.

Declaration

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

fun <identifier><[type_parameters: constraint],*>([identifier: type],*): <return_type> <acquires [identifier],*> <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 from scripts.

address 0x42 {
module M {
    fun foo(): u64 { 0 }
    fun calls_foo(): u64 { foo() } // valid
}

module other {
    fun calls_M_foo(): u64 {
        0x42::M::foo() // ERROR!
//      ^^^^^^^^^^^^ 'foo' is internal to '0x42::M'
    }
}
}

script {
    fun calls_M_foo(): u64 {
        0x42::M::foo() // ERROR!
//      ^^^^^^^^^^^^ 'foo' is internal to '0x42::M'
    }
}

To allow access from other modules or from scripts, the function must be declared public

address 0x42 {
module M {
    public fun foo(): u64 { 0 }
    fun calls_foo(): u64 { foo() } // valid
}

module other {
    fun calls_M_foo(): u64 {
        0x42::M::foo() // valid
    }
}
}

script {
    fun calls_M_foo(): u64 {
        0x42::M::foo() // valid
    }
}

Name

Function names can start with letters a to z or 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 _bAZ19() {}

Type Parameters

After the name, functions can have type parameters

fun id<T>(x: T): T { x }
fun example<T1: copyable, 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

address 0x42 {
module Example {
  struct Counter { count: u64 }

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

}
}

Acquires

When a function accesses a resource using move_from, borrow_global, or borrow_global_mut, the function must indicate that it acquires that resource. This is then used by Move's type system to ensure the references into global storage are safe, specifically that there are no dangling references into global storage.

address 0x42 {
module Example {

    resource struct Balance { value: u64 }

    public fun add_balance(s: &signer, value: u64) {
        move_to(s, Balance { value })
    }

    public fun extract_balance(addr: address): u64 acquires Balance {
        let Balance { value } = move_from(addr); // acquires needed
        value
    }

}
}

acquires annotations must also be added for transitive calls within the module. Calls to these functions from another module do not need to annotated with these acquires because one module cannot access resources declared in another module--so the annotation is not needed to ensure reference safety.

address 0x42 {
module Example {

    resource struct Balance { value: u64 }

    public fun add_balance(s: &signer, value: u64) {
        move_to(s, Balance { value })
    }

    public fun extract_balance(addr: address): u64 acquires Balance {
        let Balance { value } = move_from(addr); // acquires needed
        value
    }

    public fun extract_and_add(sender: address, receiver: &signer) acquires Balance {
        let value = extract_balance(sender); // acquires needed here
        add_balance(receiver, value)
    }

}
}

address 0x42 {
module Other {
    fun extract_balance(addr: address): u64 {
        0x42::Example::extract_balance(addr) // no acquires needed
    }
}
}

A function can acquire as many resources as it needs to

address 0x42 {
module Example {

    use 0x1::Vector;

    resource struct Balance { value: u64 }
    resource struct Box<T> { items: vector<T> }

    public fun store_two<Item1, Item2>(
        addr: address,
        item1: Item1,
        item2: Item2,
    ) acquires Balance, Box {
        let balance = borrow_global_mut<Balance>(addr); // acquires needed
        balance.value = balance.value - 2;
        let box1 = borrow_global_mut<Box<Item1>>(addr); // acquires needed
        Vector::push_back(&mut box1.items, item1);
        let box2 = borrow_global_mut<Box<Item2>>(addr); // acquires needed
        Vector::push_back(&mut box2.items, item2);
    }
}
}

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() { }

script functions must have a return type of unit ()

script {
    fun do_nothing() {
    }
}

As mentioned in the tuples section, these tuple "values" are virtual and do not exist at runtime. So for a function that returns unit (), it will not be returning any value at all 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

address 0x1 {
module Vector {
    native public fun empty<Element>(): vector<Element>;
    ...
}
}

Calling

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

address 0x42 {
module Example {
    public fun zero(): u64 { 0 }
}
}

script {
    use 0x42::Example::{Self, zero};
    fun call_zero() {
        // With the `use` above all of these calls are equivalent
        0x42::Example::zero();
        Example::zero();
        zero();
    }
}

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

address 0x42 {
module 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 }
}
}

script {
    use 0x42::Example;
    fun call_all() {
        Example::takes_none();
        Example::takes_one(0);
        Example::takes_two(0, 1);
        Example::takes_three(0, 1, 2);
    }
}

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

address 0x42 {
module Example {
    public fun id<T>(x: T): T { x }
}
}

script {
    use 0x42::Example;
    fun call_all() {
        Example::id(0);
        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
}

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 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 0x1::Vector;
use 0x1::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 () }

Updated about a month ago


Functions


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