Creating a HashMap of closures in Rust

callbacks.rs

#![feature(unboxed_closures)]
#![feature(core)]
#![feature(io)]

use std::old_io::stdio::{stdin};
use std::collections::HashMap;

// This is our toy state example.
#[derive(Debug)]
struct State {
    x: i64,
    y: i64,
}

// This is a type alias for a _boxed function_. This requires a bit of
// picking apart, so bit-by-bit:
// - the reason we need a Box is because the size of a function is
//   not always guaranteed to be the same from function to function.
//   In order to put a thing in a HashMap, it /must/ have a known
//   size! So we put it behind a layer of indirection by adding
//   Box<...> around it. Boxes are always the same size.
// - The thing inside the Box has two bounds on it: one is that
//   it is 'static, which means it lives for the entire length of
//   the program. This is simple enough.
// - The other is the trait Fn<(&'a mut State,),Output=()>, which
//   itself has two parts: the first argument, which tells you what
//   its argument type is, and the second, which tells you what its
//   output type is. this means it's a function that takes a mutable
//   reference to a State as argument, and produces nothing as output.
type Callback<'a> = Box<(Fn<(&'a mut State,),Output=()> + 'static)>;

// This is just so we don't get newlines on the end of our input.
// Not all that elaborate here.
fn read_line() -> String {
    let mut s = stdin().read_line().unwrap();
    let l = s.len();
    s.truncate(l - 1);
    return s;
}

// This takes a function of a callback type and wraps it in a
// box, which allows us to gloss over the differences between different
// callbacks and put them in the same HashMap. If we didn't do this,
// Rust would complain that we're trying to put different functions into
// the same HashMap. By wrapping them like this, Rust allows us to
// treat them as though they are "the same thing" and keep them in the
// same structure.
fn mk_callback<'a, F>(f: F) -> Callback<'a>
    where F: Fn<(&'a mut State,),Output=()> + 'static {
        Box::new(f) as Callback
}

fn main() {
    // this state is mutable, and can be changed below...
    let mut state = State { x: 0, y: 0 };
    // but this hashmap can't be changed after it's created!
    let callbacks = {
        // we do this trick by creating it in a local scope as mutable
        let mut h = HashMap::new();
        // adding the relevant callbacks...
        h.insert("w", mk_callback(|st: &mut State| st.y -= 1 ));
        h.insert("a", mk_callback(|st: &mut State| st.x -= 1 ));
        h.insert("s", mk_callback(|st: &mut State| st.y += 1 ));
        h.insert("d", mk_callback(|st: &mut State| st.x += 1 ));
        // and then returning it to the outer scope, which means that
        // it is no longer mutable.
        h };
    // We loop forever (or until we break)
    loop {
        println!("Current state: {:?}", state);
        // this will read in a line without a newline afterwards
        let key = read_line();
        // break if it's Q, otherwise...
        if key == "q" {
            break;
        } else {
            // look it up in our callbacks hash, and if it's
            // present, call it!
            callbacks.get(key.as_slice()).map({ |fun| fun(&mut state) });
        }
    }
}

Thanks!

Awesome!

Can you show this for async? I am not sure if https://docs.rs/futures/latest/futures/future/type.BoxFuture.html helps