{-# LANGUAGE TemplateHaskell #-}
module Data.Profunctor.Product (module Data.Profunctor.Product.Class,
module Data.Profunctor.Product.Newtype,
module Data.Profunctor.Product) where
import Prelude hiding (id)
import Data.Profunctor (Profunctor, dimap, lmap, WrappedArrow)
import qualified Data.Profunctor as Profunctor
import Data.Functor.Contravariant (Contravariant, contramap)
import Control.Category (id)
import Control.Arrow (Arrow, (***), (<<<), arr, (&&&))
import Control.Applicative (Applicative, liftA2, pure)
import Data.Monoid (Monoid, mempty, (<>))
import Data.Profunctor.Product.Newtype
import Data.Profunctor.Product.Class
import Data.Profunctor.Product.Flatten
import Data.Profunctor.Product.Tuples
import Data.Profunctor.Product.Tuples.TH (pTns, maxTupleSize, pNs)
-- ProductProfunctor and ProductContravariant are potentially
-- redundant type classes. It seems to me that these are equivalent
-- to Profunctor with Applicative, and Contravariant with Monoid
-- respectively:
--
-- import Data.Profunctor
-- import Control.Applicative hiding (empty)
-- import Data.Functor.Contravariant
-- import Data.Monoid
--
-- empty :: (Applicative (p ())) => p () ()
-- empty = pure ()
--
-- (***!) :: (Applicative (p (a, a')), Profunctor p) =>
-- p a b -> p a' b' -> p (a, a') (b, b')
-- p ***! p' = (,) <$> lmap fst p <*> lmap snd p'
--
-- point :: Monoid (f ()) => f ()
-- point = mempty
--
-- (***<) :: (Monoid (f (a, b)), Contravariant f) =>
-- f a -> f b -> f (a, b)
-- p ***< p' = contramap fst p <> contramap snd p'
--
--
-- The only thing that makes me think that they are not *completely*
-- redundant is that (***!) and (***<) have to be defined
-- polymorphically in the type arguments, whereas if we took the
-- Profunctor+Applicative or Contravariant+Monoid approach we do not
-- have a guarantee that these operations are polymorphic.
--
-- Previously I wanted to replace ProductProfunctor and
-- ProductContravariant entirely. This proved difficult as it is not
-- possible to expand the class constraints to require Applicative and
-- Monoid respectively. We can't enforce a constraint 'Applicative (p
-- a)' where 'a' does not appear in the head. This seems closely
-- related to the above issue of adhoc implementations.
--
-- There is a potential method of working around this issue using the
-- 'constraints' package:
-- stackoverflow.com/questions/12718268/polymorphic-constraint/12718620
--
-- Still, at least we now have default implementations of the class
-- methods, which makes things simpler.
-- This appears to be just 'Data.Functor.Contravariant.Divisible'
class Contravariant f => ProductContravariant f where
point :: f ()
(***<) :: f a -> f b -> f (a, b)
-- | This is exactly the same as @Applicative@'s @\<*\>@, but for a
-- 'ProductProfunctor'.
(****) :: ProductProfunctor p => p a (b -> c) -> p a b -> p a c
(****) f x = Profunctor.dimap dup (uncurry ($)) (f ***! x)
where dup y = (y, y)
-- | This is exactly 'Profunctor.rmap', given a name which highlights
-- the similarity to @Applicative@'s @\<$\>@.
(***$) :: ProductProfunctor p => (b -> c) -> p a b -> p a c
(***$) = Profunctor.rmap
defaultEmpty :: Applicative (p ()) => p () ()
defaultEmpty = pure ()
defaultProfunctorProduct :: (Applicative (p (a, a')), Profunctor p)
=> p a b -> p a' b' -> p (a, a') (b, b')
defaultProfunctorProduct p p' = liftA2 (,) (lmap fst p) (lmap snd p')
defaultPoint :: Monoid (p ()) => p ()
defaultPoint = mempty
defaultContravariantProduct :: (Contravariant f, Monoid (f (a, b)))
=> f a -> f b -> f (a, b)
defaultContravariantProduct p p' = contramap fst p <> contramap snd p'
newtype PPOfContravariant f a b = PPOfContravariant (f a)
unPPOfContravariant :: PPOfContravariant c a a -> c a
unPPOfContravariant (PPOfContravariant pp) = pp
instance Contravariant f => Profunctor (PPOfContravariant f) where
dimap f _ (PPOfContravariant p) = PPOfContravariant (contramap f p)
instance ProductContravariant f => ProductProfunctor (PPOfContravariant f) where
empty = PPOfContravariant point
PPOfContravariant f ***! PPOfContravariant f' = PPOfContravariant (f ***< f')
instance ProductProfunctor (->) where
empty = id
(***!) = (***)
instance Arrow arr => ProductProfunctor (WrappedArrow arr) where
empty = id
(***!) = (***)
data AndArrow arr z a b = AndArrow { runAndArrow :: arr z b }
instance Arrow arr => Profunctor (AndArrow arr z) where
dimap _ f (AndArrow g) = AndArrow (arr f <<< g)
instance Arrow arr => ProductProfunctor (AndArrow arr z) where
empty = AndArrow (arr (const ()))
(AndArrow f) ***! (AndArrow f') = AndArrow (f &&& f')
-- { Sum
class Profunctor p => SumProfunctor p where
-- Morally we should have 'zero :: p Void Void' but I don't think
-- that would actually be useful
(+++!) :: p a b -> p a' b' -> p (Either a a') (Either b b')
instance SumProfunctor (->) where
f +++! g = either (Left . f) (Right . g)
list :: (ProductProfunctor p, SumProfunctor p) => p a b -> p [a] [b]
list p = Profunctor.dimap fromList toList (empty +++! (p ***! list p))
where toList :: Either () (a, [a]) -> [a]
toList = either (const []) (uncurry (:))
fromList :: [a] -> Either () (a, [a])
fromList [] = Left ()
fromList (a:as) = Right (a, as)
-- SumContravariant would be 'Data.Functor.Contravariant.Decidable'
-- (without the requirement to also be Divisible).
-- }
pTns [0..maxTupleSize]
pNs [0..maxTupleSize]