On Sun, 2010-11-28 at 22:59 +0800, Jafet wrote:
> > {-# LANGUAGE TypeSynonymInstances #-}
> > type Identity a = a
> > instance Applicative Identity where
> > -- something like
> > pure a = a
> > f <*> a = f a
>
> But GHC does not accept type synonym instances unless they are fully
> applied.
>
> Is it sound for such an instance to exist? If so, how might it be
> defined? > data Tag a = Tag > > instance Applicative Tag where > pure _ = Tag > Tag <*> Tag = Tag > > cast :: Tag a -> Tag b > cast Tag = Tag 1. pure id <*> Tag = Tag 2. I'm too lazy to prove it 3. pure f <*> pure x = Tag <*> Tag = Tag = pure (f x) 4. u <*> pure y = u <*> Tag = u = Tag <*> u = pure ($ y) <*> u > x = pure undefined > y = x :: Tag () Is y defined? pure!Tag undefined = Tag pure!Identity undefined = undefined Regards
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