instances for First and Last

[matthewleon]

Prompted by a discussion about strict instances that would work with iterate: #16

The First instance should fit that bill. Given that Maybe is in the deps, it feels like it belongs here.

[JordanMartinez]

I'm not sure what the definition was for Unfoldable previously and if it changed at all between then and now. However, I don't think these instances as is will work. Consider the following instance:

instance unfoldableFirst :: Unfoldable First where
  unfoldr :: forall a b. (b -> Maybe (Tuple a b)) -> b -> First a
  unfoldr step x = First (maybe Nothing (Just <<< fst) (step x))

If we write (unfoldr (const Nothing) 1) :: First String, the value returned would be First Nothing :: First (Maybe String). The compiler correctly catches this unification issue with a compiler error.

We could get around this limitation if a had a Monoid instance. However, Unfoldable as it's currently defined does not provide a slot to require that anywhere (e.g. instance (Monoid a) => Unfoldable (First a) would no longer match the required kind signature despite being a valid instance otherwise).

So, I'm going to close this.