Almost-poly-map.rs

use std::boxed::Box;
use std::borrow::Borrow;

#[derive(Debug)]
enum List<T> {
    Null,
    Cons(T, Box<List<T>>),
}

fn map<A, B, F: Fn(&A) -> B, L: Borrow<List<A>>>(f: F, lst: L) -> List<B> {
    match lst.borrow() {
        &List::Null => List::Null,
        &List::Cons(ref h, ref t) => {
            let new_head = f(h);
            let new_tail = map(f, &**t);
            List::Cons(new_head, Box::new(new_tail))
        }
    }
}

#[derive(Debug)]
enum Foo {Foo}

fn main() {
  let il: List<Foo> = List::Cons(Foo::Foo, Box::new(List::Null));
  println!("{:?}", il);
  
  let il = map(|i| i, il);
  println!("{:?}", il);
  
  let ill = map(|i| 4, &il);
  println!("{:?}", ill);
}