Functors in Rust

main.rs

trait Functor<A> {
    type Res<B>;
    fn fmap<B, F: Fn(&A) -> B>(&self, f: F) -> Self::Res<B>;
}

impl<A> Functor<A> for Option<A> {
    type Res<B> = Option<B>;

    fn fmap<B, F: Fn(&A) -> B>(&self, f: F) -> Self::Res<B> {
        match self {
            Some(a) => Some(f(a)),
            None => None,
        }
    }
}

impl<A> Functor<A> for Vec<A> {
    type Res<B> = Vec<B>;

    fn fmap<B, F: Fn(&A) -> B>(&self, f: F) -> Self::Res<B> {
        self.iter().map(f).collect()
    }
}

fn double_string<F, G>(param: F) -> G
where
    F: Functor<usize, Res<String> = G>,
    G: Functor<String>,
{
    param.fmap(|n| (n * 2).to_string())
}

fn main() {
    println!("{:?}", double_string(Some(32)));
    println!("{:?}", double_string(vec![1, 2, 3, 4]));
}