Generics

Declaration

// Generic types are declared using the <T> annotation

struct GenericType<T> {
    pub item: T
}

enum QualityChecked<T> {
    Excellent(T),
    Good(T),
    // enum fields can be generics too
    Mediocre { product: T }
}

## Instantiation

// explicit type declaration let some_value: Option = Some(13);

// implicit type declaration let some_other_value = Some(66);

Multiple type parameters

Generics types can have more than one type parameters, eg. Result is defined like this:

pub enum Result<T, E> {
    Ok(T),
    Err(E),
}

Bounded generic types

// Only accept T and U generic types that also implement Debug
fn print_objects<T: Debug, U: Debug>(a: T, b: U) {
    println!("A: {:?} B: {:?}", a, b);
}

print_objects(13, 44);
// or annotated explicitly
print_objects::<usize, u16>(13, 44);

The bounds must cover all uses of the type. Addition is done by the std::ops::Add trait, which has input and output parameters itself. where T: std::ops::Add<u32,Output=U> states that it’s possible to Add T to u32, and this addition has to produce type U.

fn try_add_one<T, U>(input_value: T) -> Result<U, String> 
    where T: std::ops::Add<u32,Output=U> 
{
    return Ok(input_value + 1);
}

Sized bound is implied by default. ?Sized bound allows unsized types as well.

Generic functions

Generic functions allow some or all of their arguments to be parameterised.

fn convert_values<T, U>(input_value: T) -> Result<U, String> {
  // Try and convert the value.
  // Actual code will require bounds on the types T, U to be able to do something with them.
}

If the compiler can’t infer the type parameter then it can be supplied manually upon call:

let result: Result<u32, String> = convert_value::<f64, u32>(13.5);