Add a loop-iterator for option (suggest better names)

[bblum]

I found myself wanting this a lot during ICFP (#2928):

fn sequence<T>(+x: option<T>, blk: fn(+T) -> option<T>) {
    if x.is_some() {
        sequence(blk(option::unwrap(x)), blk)
    }   
}

impl iteration<T: copy> for option<T> {
    fn sequence(blk: fn(+T) -> option<T>) {
        if self.is_some() {
            blk(self.get()).sequence(blk)
        }   
    }   
}

(It would have to be written non-tail-recursively, for non-optimised builds to be able to infinite loop with it, but the tail recursive way is so much prettier.)

As it is, option::iter is just a special case of option::map where the return type of the block is unit. No need for it to be a different function.

[nikomatsakis]

The naming of option::iter is consistent with e.g. vec::iter. I think what you are describing here is actually chain.

[nikomatsakis]

Oh, I see, your version of iterate reaches a fixed point, so no, it's not chain, but I think it's also not what option::iter ought to do...

[bblum]

Yeah, I was looking at chain, too. Chain only recurses once, though - the point of this is to enable writing loops.

I wanted to call it chain more than I want to call it iterate; do suggest better names if you can think of any.

[nikomatsakis]

apply_until_none?

[bblum]

It's not as snappy... maybe sequence? My original thought was while_some (and I wondered if while_none would ever be useful).

I'd say change chain to bind or link, but that would break existing code because they have compatible type signatures but different semantics...