Prototype implementation of Monads in Rust

monad.rs

use std::fmt::Display;

trait Monadic {}
#[derive(Debug)]
struct Monad<M:Monadic,T>(M,T);

#[derive(Debug)]
struct Opt(bool);
impl Monadic for Opt {}
impl<T:Default> Monad<Opt,T> {
    fn ret(t:T) -> Self {
        Monad(Opt(true),t)
    }
    fn bind<F,U:Default>(self,f:F) -> Monad<Opt,U> where
        F: FnOnce(T) -> Monad<Opt,U>
    {
    	let Monad(Opt(is_some),val) = self;
        if is_some { f(val) } else { Monad(Opt(false),Default::default()) }
    }
    fn none() -> Self { Monad(Opt(false),Default::default())}
}

#[derive(Debug)]
struct Log(String);
impl Monadic for Log {}
impl<T:Display> Monad<Log,T> {
    fn ret(t:T) -> Self {
        Monad(Log(format!("{}",t)),t)
    }
    fn bind<F,U>(self,f:F) -> Monad<Log,U> where
        F: FnOnce(T) -> Monad<Log,U>
    {
    	let Monad(Log(old_log),val) = self;
        let Monad(Log(new_log),val) = f(val);
        Monad(Log(format!("{},{}",old_log,new_log)),val)
    }
}




fn main() {
    let just3 = Monad::<Opt,_>::ret(3);
    println!("{:?}",just3);
    let just4 = just3.bind(|x|Monad::<Opt,_>::ret(x+1));
    println!("{:?}",just4);
    let nope = Monad::<Opt,usize>::none();
    let nope2 = nope.bind(|x|Monad::<Opt,_>::ret(x+1));
    println!("{:?}",nope2);
    let nope3 = just4.bind(|_|Monad::<_,usize>::none());
    println!("{:?}",nope3);
    let nope = Monad::<Opt,usize>::none();
    let nope4 = nope.bind(|x|Monad::<Opt,_>::ret(format!("{}",x)));
    println!("{:?}",nope4);
    let just5 = Monad::<Opt,_>::ret(5);
    let str5 = just5.bind(|x|Monad::<Opt,_>::ret(format!("{}",x)));
    println!("{:?}",str5);

    let logone = Monad::<Log,_>::ret(1);
    let logtwo = logone.bind(|x|Monad::<Log,_>::ret(x+1));
    let loghuh = logtwo.bind(|_|Monad::<Log,_>::ret("huh"));
    let logpi = loghuh.bind(|_|Monad::<Log,_>::ret(3.14159));
    let logtau = logpi.bind(|p|Monad::<Log,_>::ret(p*2.0));
    let logdub = logtau.bind(|_|Monad::<Log,_>::ret("once").bind(|_|Monad::<Log,_>::ret("twice")));
    print!("{:?}", logdub);

}