(,) is an example of a type that has a
instance Bifunctor (,) where bimap f g (x, y) = (f x, g y)
bimap takes a pair of functions and applies them to the tuple's respective components.
bimap (+ 2) (++ "nie") (3, "john") --> (5,"johnnie") bimap ceiling length (3.5 :: Double, "john" :: String) --> (4,4)
Either's instance of
Bifunctor selects one of the two functions to apply depending on whether the value is
instance Bifunctor Either where bimap f g (Left x) = Left (f x) bimap f g (Right y) = Right (g y)