[GHC] #13018: TH-spliced pattern synonym declaration fails to typecheck

[GHC]

#13018: TH-spliced pattern synonym declaration fails to typecheck
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Thank you Simon, that's very helpful. I forgot about the need for two
 `forall`s, and it turns out that as a workaround, you can fix the issue by
 specifying the `forall`s manually. As evidence, this compiles:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
 module M where

 data T a where
   MkT :: Eq b => b -> T a

 $([d| pattern P :: forall a. forall b. b -> T a
       pattern P x <- MkT x
     |])
 }}}

 So the only issue to investigate is why this:

 {{{#!hs
 $([d| pattern P :: b -> T a
       pattern P x <- MkT x
     |])
 }}}

 fails to infer the `forall`s correctly.

 Replying to [comment:5 simonpj]:
 > I wonder if we could just genertate TH syntax with an implicit forall?

 TH is already coming up with an implicit `forall`, just in an incorrect
 way. Note that this:

 {{{#!hs
 $([d| pattern P :: b -> T a
       pattern P x <- MkT x
     |])
 }}}

 gets turned into this (as reported by `-ddump-splices`):

 {{{#!hs
     [d| pattern P_a1iH :: b_a1iI -> T a_a1iJ
         pattern P_a1iH x_a1iK <- MkT x_a1iK |]
   ======>
     pattern P_a4tM :: forall b_a4tN a_a4tO. b_a4tN -> T a_a4tO
     pattern P_a4tM x_a4tP <- MkT x_a4tP
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13018#comment:6>
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