demo a struct that can be used with Rc or Arc pointer types

hkt_tree.rs

use std::ops::Deref;
use std::rc::Rc;
use std::sync::Arc;

trait PtrFunc<A> {type Ptr:Sized+Clone+Deref+From<A>;}

struct RcPtr;
impl<A> PtrFunc<A> for RcPtr {type Ptr = Rc<A>;}
struct ArcPtr;
impl<A> PtrFunc<A> for ArcPtr {type Ptr = Arc<A>;}

struct TreeNode<T, P> where
    P : PtrFunc<TreeNode<T,P>>,
{
    val: T,
    right: Option<P::Ptr>,
    left: Option<P::Ptr>,
}

type RcTree = TreeNode<i32,RcPtr>;
type ArcTree = TreeNode<i32,ArcPtr>;

fn main() {
    let rt: RcTree = TreeNode{
        val:2,
        right:Some(Rc::new(TreeNode{
            val:5,
            right:None,
            left:None,
        })),
        left:Some(Rc::new(TreeNode{
            val:1,
            right:None,
            left:None,
        })),
    };
    let at: ArcTree = TreeNode{
        val:4,
        right:Some(Arc::new(TreeNode{
            val:1,
            right:None,
            left:None,
        })),
        left:Some(Arc::new(TreeNode{
            val:6,
            right:None,
            left:None,
        })),
    };
    println!("The top values are");
    println!("{} and {}", rt.val, at.val);
    println!("The right values are");
    println!("{} and {}", rt.right.unwrap().val, at.right.unwrap().val);
    println!("The left values are");
    println!("{} and {}", rt.left.unwrap().val, at.left.unwrap().val);
}