Data.Profunctor

For a good explanation of profunctors in Haskell see Dan Piponi's article:

http://blog.sigfpe.com/2011/07/profunctors-in-haskell.html

For more information on strength and costrength, see:

http://comonad.com/reader/2008/deriving-strength-from-laziness/

Profunctors

class Profunctor p where

Formally, the class Profunctor represents a profunctor from Hask -> Hask.

Intuitively it is a bifunctor where the first argument is contravariant and the second argument is covariant.

You can define a Profunctor by either defining dimap or by defining both lmap and rmap.

If you supply dimap, you should ensure that:

dimap id id ≡ id

If you supply lmap and rmap, ensure:

 lmap id ≡ id
 rmap id ≡ id

If you supply both, you should also ensure:

dimap f g ≡ lmap f . rmap g

These ensure by parametricity:

 dimap (f . g) (h . i) ≡ dimap g h . dimap f i
 lmap (f . g) ≡ lmap g . lmap f
 rmap (f . g) ≡ rmap f . rmap g

Methods

dimap :: (a -> b) -> (c -> d) -> p b c -> p a d

Map over both arguments at the same time.

dimap f g ≡ lmap f . rmap g

lmap :: (a -> b) -> p b c -> p a c

Map the first argument contravariantly.

lmap f ≡ dimap f id

rmap :: (b -> c) -> p a b -> p a c

Map the second argument covariantly.

rmap ≡ dimap id

Instances

Profunctor (->)

Monad m => Profunctor (Kleisli m)

Functor w => Profunctor (Cokleisli w)

Profunctor (Tagged *)

Profunctor (Forget r)

Arrow p => Profunctor (WrappedArrow p)

Functor f => Profunctor (DownStar f)

Functor f => Profunctor (UpStar f)

Profunctorial Strength

class Profunctor p => Strong p where

Generalizing upstar of a strong Functor

Minimal complete definition: first' or second'

Note: Every Functor in Haskell is strong.

Methods

first' :: p a b -> p (a, c) (b, c)

second' :: p a b -> p (c, a) (c, b)

Instances

Strong (->)

Monad m => Strong (Kleisli m)

Strong (Forget r)

Arrow p => Strong (WrappedArrow p)

Functor m => Strong (UpStar m)

class Profunctor p => Choice p where

The generalization of DownStar of a "costrong" Functor

Minimal complete definition: left' or right'

Note: We use traverse and extract as approximate costrength as needed.

Methods

left' :: p a b -> p (Either a c) (Either b c)

right' :: p a b -> p (Either c a) (Either c b)

Instances

Choice (->)

Monad m => Choice (Kleisli m)

Comonad w => Choice (Cokleisli w)

extract approximates costrength

Choice (Tagged *)

Monoid r => Choice (Forget r)

ArrowChoice p => Choice (WrappedArrow p)

Traversable w => Choice (DownStar w)

sequence approximates costrength

Applicative f => Choice (UpStar f)

Common Profunctors

newtype UpStar f d c

Lift a Functor into a Profunctor (forwards).

Constructors

UpStar

Fields

runUpStar :: d -> f c

Instances

Functor f => Profunctor (UpStar f)

Applicative f => Choice (UpStar f)

Functor m => Strong (UpStar m)

Functor f => Functor (UpStar f a)

newtype DownStar f d c

Lift a Functor into a Profunctor (backwards).

Constructors

DownStar

Fields

runDownStar :: f d -> c

Instances

Functor f => Profunctor (DownStar f)

Traversable w => Choice (DownStar w)

sequence approximates costrength

Functor (DownStar f a)

newtype WrappedArrow p a b

Wrap an arrow for use as a Profunctor.

Constructors

WrapArrow

Fields

unwrapArrow :: p a b

Instances

Arrow p => Arrow (WrappedArrow p)

ArrowZero p => ArrowZero (WrappedArrow p)

ArrowChoice p => ArrowChoice (WrappedArrow p)

ArrowApply p => ArrowApply (WrappedArrow p)

ArrowLoop p => ArrowLoop (WrappedArrow p)

Category p => Category (WrappedArrow p)

Arrow p => Profunctor (WrappedArrow p)

ArrowChoice p => Choice (WrappedArrow p)

Arrow p => Strong (WrappedArrow p)

newtype Forget r a b

Constructors

Forget

Fields

runForget :: a -> r

Instances

Profunctor (Forget r)

Monoid r => Choice (Forget r)

Strong (Forget r)

Functor (Forget r a)

Foldable (Forget r a)

Traversable (Forget r a)