Functor package:relude

Convenient functions to work with Functor.

A type f is a Functor if it provides a function fmap which, given any types a and b lets you apply any function from (a -> b) to turn an f a into an f b, preserving the structure of f. Furthermore f needs to adhere to the following:

• Identity fmap id == id
• Composition fmap (f . g) == fmap f . fmap g

Note, that the second law follows from the free theorem of the type fmap and the first law, so you need only check that the former condition holds. See these articles by School of Haskell or David Luposchainsky for an explanation.

Useful combinators for bifunctors inside functors. This set of functions is useful when you want to work with types like these ones:

foo :: IO (Either a b)
bar :: IO (a, b)

baz :: Maybe (Either a b)
qux :: Maybe (a, b)

doo :: (a, a)
dee :: Either a a