[GHC] #13307: Record pattern synonym fields have to be manually exported

20 Feb 2017

      #13307: Record pattern synonym fields have to be manually exported
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           Reporter:  ocharles       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following currently fails to compile:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 module A
   ( T(T)
   ) where

 data Impl = Impl Int

 newtype T = MkT Impl

 pattern T {x} = MkT (Impl x)

 {-# LANGUAGE RecordWildCards #-}
 module B where

 import A

 foo :: T -> Int
 foo T{x} = x
 }}}

 As far as GHC can see, in module `B`, `T` does not have a field `x`. The
 fix is to manually export `x` from `A`:

 {{{#!hs
 module A (T(T, x)) where
 }}}

 But this is tedious for records with a large amount of fields

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13307
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