main.rs

#![feature(conservative_impl_trait)]

use std::marker::PhantomData;
use std::collections::HashMap;

fn main() {

    
    let sum_arr =
        state::<i32, i32, i32, _>(0,
     // ^^^^^^^^^^^^^^^^^^^^^^^^^ error[E0277]: the trait bound `for<'a> impl Arr<(_, _), _>: Arr<(&'a {integer}, _), ({integer}, _)>` is not satisfied
                                  arr::<(&i32, i32), (i32, i32), _>(|(s, x)| (s + x, s + x)));

    println!("{:?}", sum_arr.push(1));
    println!("{:?}", sum_arr.push(1));
    println!("{:?}", sum_arr.push(2));
    println!("{:?}", sum_arr.push(2));
    println!("{:?}", sum_arr.push(3));
    println!("{:?}", sum_arr.push(3));
}

pub trait Arr<In, Out> {
    fn push(&mut self, input: In) -> Out;
}

struct FnMutArr<In, Out, F>
    where F: FnMut(In) -> Out
{
    _marker: PhantomData<(In, Out)>,
    f: F,
}

impl<In, Out, F> Arr<In, Out> for FnMutArr<In, Out, F>
    where F: FnMut(In) -> Out
{
    fn push(&mut self, input: In) -> Out {
        (self.f)(input)
    }
}

pub fn arr<In, Out, F: FnMut(In) -> Out>(f: F) -> impl Arr<In, Out> {
    FnMutArr {
        _marker: PhantomData,
        f: f,
    }
}

pub fn state<State, In, Out, A>(initial_state: State, mut a: A) -> impl Arr<In, Out>
    where A: for<'a> Arr<(&'a State, In), (State, Out)>
{
    let mut state = initial_state;
    arr(move |input| {
        let (new_state, out) = a.push((&state, input));
        state = new_state;
        out
    })
}