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 <sjo...@w3future.com> wrote: > 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 > > > > > >
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