Re: [Haskell-cafe] Product Profunctor and Contravariant

20 Nov 2013

      On Sun, Sep 29, 2013 at 10:44:15AM +0100, Tom Ellis wrote:
...
Does anyone recognise these typeclasses:
import Data.Profunctor (Profunctor)
    import Data.Functor.Contravariant (Contravariant)
class Profunctor p => ProductProfunctor p where
      empty :: p () ()
      (***!) :: p a b -> p a' b' -> p (a, a') (b, b')
class Contravariant f => ProductContravariant f where
      point :: f ()
      (***<) :: f a -> f b -> f (a, b)
It now 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'

So my question becomes: are there standard definitions of these somewhere?

Thanks,

Tom