Rust by Example

5.3 Alias

The type statement can be used to give a new name to an existing type. Types must have CamelCase names, or the compiler will raise a warning. The exception to this rule are the primitive types: usize, f32, etc.

// `NanoSecond` is a new name for `u64`.
type NanoSecond = u64;
type Inch = u64;

// Use an attribute to silence warning.
type u64_t = u64;
// TODO ^ Try removing the attribute

fn main() {
    // `NanoSecond` = `Inch` = `u64_t` = `u64`.
    let nanoseconds: NanoSecond = 5 as u64_t;
    let inches: Inch = 2 as u64_t;

    // Note that type aliases *don't* provide any extra type safety, because
    // aliases are *not* new types
    println!("{} nanoseconds + {} inches = {} unit?",
             nanoseconds + inches);

The main use of aliases is to reduce boilerplate; for example the IoResult<T> type is an alias for the Result<T, IoError> type.

See also: