Type Aliasing: give a new name to an existing type

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

type alias example

 
// `NanoSecond`, `Inch`, and `U64` are new names for `u64`.
type NanoSecond = u64;
type Inch = u64;
type U64 = u64;

fn main() {
    // `NanoSecond` = `Inch` = `U64` = `u64`.
    let nanoseconds: NanoSecond = 5 as U64;
    let inches: Inch = 2 as U64;

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

Attributes