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Contravariant was added in GHC 8.6.

• https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Functor-Contravariant.html

Profunctor also looks like fundamental abstraction to be worth considering adding to base.

Having both Profunctor and Choice typeclasses in the base library will also allow to write microprism package similar to microlens. Prisms often turns to be very useful since they allow to work nicely with sum types.

type Prism s t a b = forall p f. (Choice p, Applicative f) => p a (f b) -> p s (f t)

Additional context for this ticket from Reddit:

• https://www.reddit.com/r/haskell/comments/8v53cb/announce_ghc_861alpha1_available/e1o1giy/

It was proposed on Reddit to use QuantifiedConstraints for Profunctor. I'm not quite fluent with QuantifiedConstraints, but I think this may looks like this (mostly copy-pasting code from profunctors package):

{-# LANGUAGE QuantifiedConstraints #-}

class (forall a . Functor (p a)) => Profunctor p where
  {-# MINIMAL dimap | (lmap, rmap) #-}

  dimap :: (a -> b) -> (c -> d) -> p b c -> p a d
  dimap f g = lmap f . rmap g

  lmap :: (a -> b) -> p b c -> p a c
  lmap f = dimap f id

  rmap :: (b -> c) -> p a b -> p a c
  rmap = dimap id

instance Profunctor (->) where
  dimap ab cd bc = cd . bc . ab
  lmap = flip (.)
  rmap = (.)

Trac metadata

Trac field	Value
Version	8.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	andrewthad, ekmett
Operating system
Architecture