GHC 8.0 swaps the order of provided vs required contexts in a pattern synonym signature. (7.10 was advertised as experimental). Thus:
pattern AddP :: () => (Num a, Eq a) => Exp a -> Exp a -> Exp a Then it's fine Simon | -----Original Message----- | From: Glasgow-haskell-users [mailto:glasgow-haskell-users- | boun...@haskell.org] On Behalf Of Emil Axelsson | Sent: 26 May 2016 16:27 | To: glasgow-haskell-users <glasgow-haskell-users@haskell.org> | Subject: Pattern synonyms and GADTs in GHC 8.0.1 | | I have a problem where a pattern synonym doesn't provide the expected | type refinement in GHC 8.0.1. | | > {-# LANGUAGE GADTs #-} | > {-# LANGUAGE PatternSynonyms #-} | > | > data Exp a | > where | > Num :: (Eq a, Num a) => a -> Exp a | > Add :: (Eq a, Num a) => Exp a -> Exp a -> Exp a | > | > pattern NumP a = Num a | > | > pattern AddP :: (Num a, Eq a) => Exp a -> Exp a -> Exp a pattern | AddP | > a b = Add a b | > | > simplifyP :: Exp a -> Exp a | > simplifyP (AddP a (NumP 0)) = a | > simplifyP a = a | | This gives the error | | • No instance for (Eq a) arising from a pattern | Possible fix: | add (Eq a) to the context of | the type signature for: | simplifyP :: Exp a -> Exp a | • In the pattern: AddP a (NumP 0) | In an equation for ‘simplifyP’: simplifyP (AddP a (NumP 0)) = a | | If I remove the type signature for `AddP`, the code goes through. | Unfortunately, in my real code I need the type signature in order to | resolve overloading. | | GHC 7.10 didn't have this problem. | | Is this a bug? | | / Emil | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.h | askell.org%2fcgi-bin%2fmailman%2flistinfo%2fglasgow-haskell- | users&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7ccb54f042472240 | 7ed99608d3857a317a%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=xv8M7C | IC4zyT4Zgq3HnXiGzUA1Z0tltZpE%2fIYhYP8KQ%3d _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
