monoidfun.rs

trait Monoid {
    static fn mempty() -> self;
    fn mappend(other: self) -> self;
}

impl int: Monoid {
    static fn mempty() -> int { 0 }
    fn mappend(other: int) -> int { self + other}
}

// Generalise with foldable later
fn mconcat<M: Monoid>(elems: ~[M]) -> M {
    let mut a: M = Monoid::mempty();
    let mut i: uint = 0;
    while i < elems.len() {
        a=a.mappend(elems[i]);
        i+=1;
    }
    return a;
}

// This works
fn sum(elems: ~[int]) -> int {
    let mut a: int = 0;
    let mut i: uint = 0;
    while i < elems.len() {
        a += elems[i];
        i+=1;
    }
    return a;
}

fn main() {
    let x: int = Monoid::mempty();
    io::println(x.mappend(5).mappend(4).to_str());

    let y: ~[int] = ~[5,6,7,8];
    // io::println(mconcat(y).to_str());
    io::println(sum(y).to_str());

    io::println("Hello, world")
}