impl Trait in Rust

Anonymous types in signatures: any argument that fits, any return that satisfies.

fn total(values: impl IntoIterator<Item = i32>) -> i32 {
    values.into_iter().sum()
}

fn evens(limit: i32) -> impl Iterator<Item = i32> {
    (0..limit).filter(|n| n % 2 == 0)
}

fn adder(step: i32) -> impl Fn(i32) -> i32 {
    move |n| n + step
}

fn main() {
    println!("{}", total(vec![1, 2, 3]));
    println!("{:?}", evens(10).collect::<Vec<i32>>());
    let add_five = adder(5);
    println!("{}", add_five(37));
}

How it works

1. In argument position it is shorthand for a bound.
2. In return position it hides the concrete type.
3. It is how closures and iterators get returned.

Keywords and builtins used here

• Fn
• IntoIterator
• Iterator
• Vec
• adder
• evens
• fn
• i32
• impl
• let
• main
• move
• total

The run, in numbers

Lines

18

Characters to type

397

Tokens

154

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 88 seconds.

Type this snippet

Step 4 of 4 in Generics, step 7 of 19 in Traits & generics.

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