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, public(friend), or public(script).
public visibility
A public function can be called by any function defined in any module or script. 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
- the function defined in a script.
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
}
}
public(friend) visibility
The public(friend) visibility modifier is a more restricted form of the public modifier to give more control about where a function can be used. A public(friend) function can be called by:
- other functions defined in the same module, or
- functions defined in modules which are explicitly specified in the friend list (see Friends on how to specify the friend list).
Note that since we cannot declare a script to be a friend of a module, the functions defined in scripts can never call a public(friend) function.
address 0x42 {
module M {
friend 0x42::N; // friend declaration
public(friend) fun foo(): u64 { 0 }
fun calls_foo(): u64 { foo() } // valid
}
module N {
fun calls_M_foo(): u64 {
0x42::M::foo() // valid
}
}
module other {
fun calls_M_foo(): u64 {
0x42::M::foo() // ERROR!
// ^^^^^^^^^^^^ 'foo' can only be called from a 'friend' of module '0x42::M'
}
}
}
script {
fun calls_M_foo(): u64 {
0x42::M::foo() // ERROR!
// ^^^^^^^^^^^^ 'foo' can only be called from a 'friend' of module '0x42::M'
}
}
public(script) visibility
The public(script) modifier is designed to allow module functions to be safely and directly invoked much like scripts. A public(script) function can only be called from a script context, which is either:
- the function defined in a transaction script, or
- another
public(script)function.
Essentially, this rule implies that once the execution transitions to a non-script context via a call to any non-public(script) function, there is no turning back, i.e., there is no way to call a public(script) function again.
address 0x42 {
module M {
public(script) fun foo(): u64 { 0 }
fun calls_foo(): u64 { foo() } // ERROR!
// ^^^ 'foo' can only be called from a script context
}
module N {
fun calls_M_foo(): u64 {
0x42::M::foo() // ERROR!
// ^^^^^^^^^^^^ 'foo' can only be called from a script context
}
}
module other {
public(script) 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: 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
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 {
struct Balance has key { 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 {
struct Balance has key { 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 Std::Vector;
struct Balance has key { value: u64 }
struct Box<T> has key { items: vector<T> }
public fun store_two<Item1: store, Item2: store>(
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
module Std::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 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 () }