Advanced Types

Creating Type Synonyms with Type Aliases

Sway provides the ability to declare a type alias to give an existing type another name. For this we use the type keyword. For example, we can create the alias Kilometers to u64 like so:

type Kilometers = u64;

Now, the alias Kilometers is a synonym for u64. Note that Kilometers is not a separate new type. Values that have the type Kilometers will be treated the same as values of type u64:

    let x: u64 = 5;
    let y: Kilometers = 5;
    assert(x + y == 10);

Because Kilometers and u64 are the same type, we can add values of both types and we can pass Kilometers values to functions that take u64 parameters. However, using this method, we don’t get the type checking benefits that we get from introducing a separate new type called Kilometers. In other words, if we mix up Kilometers and i32 values somewhere, the compiler will not give us an error.

The main use case for type synonyms is to reduce repetition. For example, we might have a lengthy array type like this:

[MyStruct<u64, b256>; 5]

Writing this lengthy type in function signatures and as type annotations all over the code can be tiresome and error prone. Imagine having a project full of code like this:

fn foo_long(array: [MyStruct<u64, b256>; 5]) -> [MyStruct<u64, b256>; 5] {

A type alias makes this code more manageable by reducing the repetition. Below, we’ve introduced an alias named MyArray for the verbose type and can replace all uses of the type with the shorter alias MyArray:

type MyArray = [MyStruct<u64, b256>; 5];

fn foo_shorter(array: MyArray) -> MyArray {

This code is much easier to read and write! Choosing a meaningful name for a type alias can help communicate your intent as well.