Pattern synonyms and GADTs in GHC 8.0.1

[Emil Axelsson]

Ah, excellent! Thank you!

However, it seems that `:t` gives the wrong type:

   *Main> :t AddP
   AddP :: (Num a, Eq a) => Exp a -> Exp a -> Exp a

This type is reported whether or not I include the (correct) signature 
for `AddP`.

`:i` is correct though:

   *Main> :i AddP
   pattern AddP :: () => (Num a, Eq a) => Exp a -> Exp a -> Exp a

/ Emil

Den 2016-05-26 kl. 18:46, skrev Simon Peyton Jones:
> 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-
> |  bounces at haskell.org] On Behalf Of Emil Axelsson
> |  Sent: 26 May 2016 16:27
> |  To: glasgow-haskell-users <glasgow-haskell-users at 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
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