Future with map and monadic bind in Rust.

Future.rs

use std::comm::{Receiver, channel};
use std::io;
use std::mem::replace;
use std::task::spawn;

struct Future<'a, A> {
  state: FutureState<'a, A>
}

enum FutureState<'a, A> {
  Pending(proc():'a -> A),
  Evaluating, // This state is only here to satify the compiler in get_ref
  Forced(A),
}

/*
 * This is based on the rust std::sync::Future
 * https://github.com/rust-lang/rust/blob/master/src/libstd/sync/future.rs
 * with the addition of map and and_then.
 * Note, I am very new to rust and much of this is non-idiomatic (and probably dumb and/or broken, but it runs).
 *
 * This future is only designed to either be asynchronous and evaluate in another task,
 * or lift a literal value into the future.
 *
 * Note this Future is lazy and mapping and and_then procs are called as needed to evaluate
 * get_ref calls. If there are side-effects in those functions, all leaf Futures should have
 * their get_ref fn called for the logic to be correct.
 *
 * For me, it is still unclear where the right parts of the design space are: there are
 * interesting trade-offs between using references and lifetimes vs boxes vs cloning.
 * Here we avoid copying/cloning or GC/ref-counting in map/and_then.
 */
impl<'a, A> Future<'a, A> {
    /*
     * This is the function to lift something into the future (monad)
     */
    pub fn from(val: A) -> Future<'a, A> {
      Future { state: Forced(val) }
    }
    /*
     * This can be used on non-mutable refs if the future is Forced
     */
    pub fn get_opt<'b>(&'b self) -> Option<&'b A> {
      match self.state {
        Forced(ref a) => return Some(a),
        _ => return None
      }
    }
    pub fn get_ref<'b>(&'b mut self) -> &'b A {
      match self.state {
        Forced(ref a) => return a,
        Pending(_) => {
          /*
           * This is copying the sync::Future since
           * we can't take p and evaluate it in one go, and
           * we can't call p if we are just borrowing self (as far as I can see)
           */
          match replace(&mut self.state, Evaluating) {
            Forced(_) | Evaluating => panic!("Unreachable"),
            Pending(p) => {
              self.state = Forced(p());
              return self.get_ref()
            }
          }
        }
        Evaluating => panic!("Unreachable")
      }
    }
    /*
     * Should this be a proc or a closure? We only call it once.
     */
    pub fn map<B>(&mut self, mapfn: proc(&A) -> B) -> Future<'a, B> {
      match self.state {
        Forced(ref a) => Future { state: Forced(mapfn(a)) },
        Pending(_) => Future {
          state: Pending(proc() {
            mapfn(self.get_ref())
          })
        },
        Evaluating => panic!("Unreachable")
      }
    }

    /*
     * This is the monadic bind function
     *
     * Should this be a proc or a closure? We only call it once.
     */
    pub fn and_then<'b, B>(&mut self, bindfn: proc(&A) -> Future<'b, B>) -> Future<'b, B> {
      match self.state {
        Forced(ref a) => bindfn(a),
        Pending(_) => Future {
          state: Pending(proc() {
            let a = self.get_ref();
            let mut fb = bindfn(a);
            // Force the future, but ignore the reference, use move semantics below
            let _ = fb.get_ref();
            match fb.state {
              Forced(b) => return b,
              _ => panic!("get_ref should have forced us")
            }
          })
        },
        Evaluating => panic!("Unreachable")
      }
    }
}

impl<'a, A:Send> Future<'a, A> {
    pub fn async(f: proc(): Send -> A) -> Future<'a, A> {
      let (tx, rx) = channel();
      spawn(proc() {
        tx.send(f())
      });
      Future { state: Pending(proc() { rx.recv() }) }
    }
}

fn main() {
    println!("please enter a line:");
    let input = io::stdin().read_line().ok().expect("Failed to read a line");

    println!("You entered: {}", input)
    println!("string had len = {}",
      Future::from(input.clone())
        .map(proc(s) { return s.len() })
        .get_ref()
    )

    println!("string had len (and_then) = {}",
      Future::from(input)
        .and_then(proc(s) {
          let sc = s.clone();
          Future::async(proc() { return sc.len() } )
        })
        .get_ref()
    )
}