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.
