Rusty Monoid

rusty-monoid.rs

#![feature(try_trait_v2)]
use std::ops::Try;

trait Monoid: Default {
    fn mappend(self, rhs: Self) -> Self;
}

impl Monoid for i32 {
    fn mappend(self, rhs: Self) -> Self {
        self + rhs
    }
}

trait IteratorExt: Iterator {
    fn try_for_each_monoid<F, C, R>(&mut self, f: F) -> R
    where
        F: Fn(Self::Item) -> R,
        C: Monoid,
        R: Try<Output = C>,
    {
        let mut ret = C::default();
        for x in self {
            let c = f(x)?;
            ret = ret.mappend(c);
        }
        Try::from_output(ret)
    }
}

impl<I> IteratorExt for I where I: Iterator {}

fn main() {
    let x = (1..=5).try_for_each_monoid(|x| {
        if x == 3 {
            None
        } else {
            println!("x == {}", x);
            Some(x)
        }
    });
    println!("{x:?}");
}