Haskell map on Right value of Either

I have a type Either a [b] and a function f :: (b -> c)

How can I use f on a value of type Either a [b] to get Either a [c] ?


  • Use fmap function on it.

    Either is an instance of the Functor typeclass with the first type fixed:

    data Either a b = Left a | Right b
    instance Functor (Either a)
      fmap _ (Left x)  = Left x
      famp f (Right y) = Right (f y)

    and the fmap type for Either a therefore would be:

    fmap :: (b -> c) -> Either a b -> Either a c

    Appliction of fmap to Either leads to: Left data constructor of type a remains unchanged, and the Right data constructor of type b gets the function f applied to get to the type c.