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

6 Jun 2012


      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