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