Uses and Aliases

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::vector as V;

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::vector as V; would let you use V instead of std::vector

use std::vector;
use std::vector as V;

fun new_vecs(): (vector<u8>, vector<u8>, vector<u8>) {
    let v1 = std::vector::empty();
    let v2 = vector::empty();
    let v3 = V::empty();
    (v1, v2, v3)
}

If you want to import a specific module member (such as a function, struct, or constant). 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::empty;
use std::vector::empty as empty_vec;

This would let you use the function std::vector::empty without full qualification. Instead, you could use empty and empty_vec respectively. Again, use std::vector::empty; is equivalent to use std::vector::empty as empty;

use std::vector::empty;
use std::vector::empty as empty_vec;

fun new_vecs(): (vector<u8>, vector<u8>, vector<u8>) {
    let v1 = std::vector::empty();
    let v2 = empty();
    let v3 = empty_vec();
    (v1, v2, v3)
}

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, length as len, pop_back};

fun swap_last_two<T>(v: &mut vector<T>) {
    assert!(len(v) >= 2, 42);
    let last = pop_back(v);
    let second_to_last = pop_back(v);
    push_back(v, last);
    push_back(v, second_to_last)
}

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::vector::{Self, empty};

For clarity, all the following are equivalent:

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

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

use std::vector::{
    Self,
    Self as V,
    length,
    length as len,
};

fun pop_twice<T>(v: &mut vector<T>): (T, T) {
    // all options available given the `use` above
    assert!(vector::length(v) > 1, 42);
    assert!(V::length(v) > 1, 42);
    assert!(length(v) > 1, 42);
    assert!(len(v) > 1, 42);

    (vector::pop_back(v), vector::pop_back(v))
}

Inside a module

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

address 0x42 {
module example {
    use std::vector;

    fun example(): vector<u8> {
        let v = empty();
        vector::push_back(&mut v, 0);
        vector::push_back(&mut v, 10);
        v
    }

    use std::vector::empty;
}
}

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

address 0x42 {
module example {

    fun example(): vector<u8> {
        use std::vector::{empty, push_back};

        let v = empty();
        push_back(&mut v, 0);
        push_back(&mut v, 10);
        v
    }
}
}

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

address 0x42 {
module example {

    fun example(): vector<u8> {
        let result = {
            use std::vector::{empty, push_back};
            let v = empty();
            push_back(&mut v, 0);
            push_back(&mut v, 10);
            v
        };
        result
    }

}
}

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

fun example(): vector<u8> {
    let result = {
        use std::vector::{empty, push_back};
        let v = empty();
        push_back(&mut v, 0);
        push_back(&mut v, 10);
        v
    };
    let v2 = empty(); // ERROR!
//           ^^^^^ unbound function 'empty'
    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 x = 0;
    use std::vector; // ERROR!
    let v = vector::empty();
}

Naming rules

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

address 0x42 {
module data {
    struct S {}
    const FLAG: bool = false;
    fun foo() {}
}
module example {
    use 0x42::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

address 0x42 {
module example {

    use std::vector::{empty as foo, length as foo}; // ERROR!
    //                                        ^^^ duplicate 'foo'

    use std::vector::empty as bar;

    use std::vector::length as bar; // ERROR!
    //                         ^^^ duplicate 'bar'

}
}

And, they cannot overlap with any of the module's other members

address 0x42 {
module data {
    struct S {}
}
module example {
    use 0x42::data::S;

    struct S { value: u64 } // ERROR!
    //     ^ conflicts with alias 'S' above
}
}

Inside 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;

address 0x42 {
module example {

    struct WrappedVector { vec: vector<u64> }

    fun empty(): WrappedVector {
        WrappedVector { vec: std::vector::empty() }
    }

    fun example1(): (WrappedVector, WrappedVector) {
        let vec = {
            use std::vector::{empty, push_back};
            // 'empty' now refers to std::vector::empty

            let v = empty();
            push_back(&mut v, 0);
            push_back(&mut v, 1);
            push_back(&mut v, 10);
            v
        };
        // 'empty' now refers to Self::empty

        (empty(), WrappedVector { vec })
    }

    fun example2(): (WrappedVector, WrappedVector) {
        use std::vector::{empty, push_back};
        let w: WrappedVector = {
            use 0x42::example::empty;
            empty()
        };
        push_back(&mut w.vec, 0);
        push_back(&mut w.vec, 1);
        push_back(&mut w.vec, 10);

        let vec = empty();
        push_back(&mut vec, 0);
        push_back(&mut vec, 1);
        push_back(&mut vec, 10);

        (w, WrappedVector { vec })
    }
}
}

Unused Use or Alias

An unused use will result in an error

address 0x42 {
module example {
    use std::vector::{empty, push_back}; // ERROR!
    //                       ^^^^^^^^^ unused alias 'push_back'

    fun example(): vector<u8> {
        empty()
    }
}
}