swap package:assoc

Symmetric Bifunctors.

swap . swap = id

If p is a Bifunctor the following property is assumed to hold:

swap . bimap f g = bimap g f . swap

Swap isn't a subclass of Bifunctor, as for example

>>> newtype Bipredicate a b = Bipredicate (a -> b -> Bool)

is not a Bifunctor but has Swap instance

>>> instance Swap Bipredicate where swap (Bipredicate p) = Bipredicate (flip p)