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)
}