Functor => Pointed => Applicative => Monad

[Stephen Tetley]

Hello wren

Convention has Bifunctor as this class:

class Bifunctor f where
  bimap :: (a -> x) -> (b -> y) -> f a b -> f x y

Personally I'd like to add these two so they can be optimized if required:
   mapFst :: (a -> x) -> f a b -> f x b
   mapSnd :: (b -> y) -> f a b -> f a y

Though, I'm sanguine about the names.

On pairs mapSnd is fmap, however I'd still like mapSnd in addition to
fmap - if I'm working with pairs I'd want the symmetric "vocabulary"
of pairs not a pidgin of two vocabularies.

Bifunctor is a class I'd really like to see in Base. When working with
pairs, I much prefer the notation from Bifunctor to the notations from
Control.Categrory & Arrows.

Best wishes

Stephen