Simple chain outline

chain2.rs

use core::from_str::*;

fn stringify<T: ToStr>(x: T) ⟶  ~str {
    x.to_str()
}

fn unstringify<T: FromStr> (s: ~str) ⟶  Option<T> {
    FromStr::from_str(s)
}   

fn add_one(n: int) ⟶  int {
    n + 1
}   

fn times_two(n: int) ⟶  int {
    n * 2
}   

fn chain<A,B,C>(f1: ~fn(a: A) ⟶  B,f2:  ~fn(b: B) ⟶  C) ⟶  ~fn(A) ⟶  C {
    |x: A| { 
        f2(f1(x))
    }   
}   

fn main() {
    let plus_one_times_two = chain(add_one,times_two);
    let times_two_plus_one = chain(times_two,add_one);
    let to_str_and_back: ~fn(x: int) -> Option<int>  = chain(stringify,unstringify);
    
    let fifty = match to_str_and_back(50) {
        Some(n) ⟹  {n}
        None ⟹  {0}
    };

    let res1 = plus_one_times_two(50);
    let res2 = times_two_plus_one(50);
    
    println(fmt!("to_str_and_back(50) = %d", fifty));
    println(fmt!("plus_one_times_two(50) = %d",res1));
    println(fmt!("times_two_plus_one(50) = %d",res2));
}   


// Output
// to_str_and_back(50) = 50
// plus_one_times_two(50) = 102
// times_two_plus_one(50) = 101