:: (a -> c) -> (b -> d) -> Either a b -> Either c d package:either

The mapBoth function takes two functions and applies the first if iff the value takes the form Left _ and the second if the value takes the form Right _. Using Data.Bifunctor:

mapBoth = bimap

Using Control.Arrow:

mapBoth = (+++)

>>> mapBoth (*2) (*3) (Left 4)
Left 8

>>> mapBoth (*2) (*3) (Right 4)
Right 12