Rust Higher Kinded Types (HKT) practical example

hkt.rs

trait Family {
    type Member<T>;
}

trait Functor<A> {
    type Of: Family<Member<A> = Self>;
    fn fmap<B>(self, f: impl FnMut(A) -> B) -> <Self::Of as Family>::Member<B>;
}

//

struct VecFamily;

impl Family for VecFamily {
    type Member<T> = Vec<T>;
}

//

impl<A> Functor<A> for Vec<A> {
    type Of = VecFamily;
    fn fmap<B>(self, f: impl FnMut(A) -> B) -> Vec<B> {
        self.into_iter().map(f).collect()
    }
}

fn main() {
    let x = vec![1, 2, 3];
    let x = x.fmap(|i| format!("{i}"));

    println!("{x:?}");
}

see https://hugopeters.me/posts/14/ for more examples