7 Jun
2012
7 Jun
'12
11:21 a.m.
Oh, yeah. Thanks, Sjoerd.
I wonder if there's some way not to require Monad. Some sort of
ApplicativeFix instead. Hm.
-- Conal
On Wed, Jun 6, 2012 at 2:43 PM, 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