The composition of functions in Rust.

compose.rs

pub fn compose<'l, X, Y, Z>(
    left: &'l dyn Fn(Y) -> Z
) -> Box<dyn
    Fn(&'l dyn Fn(X) -> Y) -> Box<dyn
        Fn(X) -> Z
        + 'l
    >
    + 'l
> {
    Box::new(move |right| {
        Box::new(move |x| {
            left(right(x))
        })
    })
}

#[test]
fn twice_then_add_one() {
    let add_one = |x: u32| x + 1;
    let twice = |x: u32| x * 2;
    let res = compose(&add_one)(&twice)(4);
    assert_eq!(res, 9);
}