bifunctors-4.1.1: Bifunctors

MaintainerEdward Kmett <>
Safe HaskellSafe-Inferred






class Bifunctor p whereSource

Minimal definition either bimap or first and second

Formally, the class Bifunctor represents a bifunctor from Hask -> Hask.

Intuitively it is a bifunctor where both the first and second arguments are covariant.

You can define a Bifunctor by either defining bimap or by defining both first and second.

If you supply bimap, you should ensure that:

bimap id idid

If you supply first and second, ensure:

 first idid
 second idid

If you supply both, you should also ensure:

bimap f g ≡ first f . second g

These ensure by parametricity:

 bimap  (f . g) (h . i) ≡ bimap f h . bimap g i
 first  (f . g) ≡ first  f . first  g
 second (f . g) ≡ second f . second g



Map over both arguments at the same time.

bimap f g ≡ first f . second g


Map covariantly over the first argument.

first f ≡ bimap f id


Map covariantly over the second argument.

secondbimap id