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

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

Map over both arguments at the same time.

bimap f g ≡ first f . second g

Examples

>>> bimap toUpper (+1) ('j', 3)
('J',4)

>>> bimap toUpper (+1) (Left 'j')
Left 'J'

>>> bimap toUpper (+1) (Right 3)
Right 4