Rust Tree Fold

tree.rs

use Tree::{Leaf, Node};

enum Tree<A> {
    Leaf,
    Node(Box<Tree<A>>, A, Box<Tree<A>>)
}

fn tree_fold<A, B, F>(f: &F, a: B, t: &Tree<A>) -> B where F: Fn(B, &A, B) -> B, B: Clone {
    match *t {
        Leaf => a, 
        Node(ref l, ref x, ref r) => {
            let l = tree_fold(f, a.clone(), l);
            let r = tree_fold(f, a.clone(), r);
            f(l, x, r)
        }
    }
}

fn main() {
    let l = Node(Box::new(Leaf), 1, Box::new(Leaf));
    let r = Node(Box::new(Leaf), 3, Box::new(Leaf));
    let t = Node(Box::new(l),    2, Box::new(r));
    println!("{}", tree_fold(&|l, &x, r| l + x + r, 0, &t));
}