The Extended Functor Family

class position terminology intuition application
Functor positive covariant full of a's applicative alternative
Contravariant negative contravariant consuming a's divisible decidable discrimination
Produnctor left negative
right positive
left contravariant
right covariant
pipe Lens

Types in a type signature can be in positive position or in negative position. A type on its own is in positive position. Function return types are in positive position, while parameters are in negative position.