`type` aliases - The Rust Programming Language

# Type Aliases

The `type` keyword lets you declare an alias of another type:

    # #![allow(unused_variables)]
    #fn main() {
    type Name = String;
    #}

You can then use this type as if it were a real type:

    # #![allow(unused_variables)]
    #fn main() {
    type Name = String;

    let x: Name = "Hello".to_string();
    #}

Note, however, that this is an alias, not a new type entirely. In other words, because Rust is strongly typed, you’d expect a comparison between two different types to fail:

    let x: i32 = 5;
    let y: i64 = 5;

    if x == y {
       // ...
    }

this gives

    error: mismatched types:
     expected `i32`,
        found `i64`
    (expected i32,
        found i64) [E0308]
         if x == y {
                 ^

But, if we had an alias:

    # #![allow(unused_variables)]
    #fn main() {
    type Num = i32;

    let x: i32 = 5;
    let y: Num = 5;

    if x == y {
       // ...
    }
    #}

This compiles without error. Values of a `Num` type are the same as a value of type `i32`, in every way. You can use tuple struct to really get a new type.

You can also use type aliases with generics:

    # #![allow(unused_variables)]
    #fn main() {
    use std::result;

    enum ConcreteError {
        Foo,
        Bar,
    }

    type Result<T> = result::Result<T, ConcreteError>;
    #}

This creates a specialized version of the `Result` type, which always has a `ConcreteError` for the `E` part of `Result<T, E>`. This is commonly used in the standard library to create custom errors for each subsection. For example, io::Result.