[Haskell-cafe] Have you seen this functor/contrafunctor combo?

[Sjoerd Visscher]

If there would be a package where this could be in it would be contravariant[1], but it isn't.

> > newtype Q' p f a = Q' (p a -> f a)

This compiles:

> instance (Contravariant p, Functor m, MonadFix m) => Applicative (Q' p m) where
>   pure a = Q' (pure (return a))
>   Q' fs <*> Q' as = Q' $ \r -> do
>     rec 
>       f <- fs (contramap ($ a) r)
>       a <- as (contramap (f $) r)
>     return $ f a

[1] http://hackage.haskell.org/package/contravariant

--
Sjoerd Visscher
https://github.com/sjoerdvisscher/blog