[GHC] #9161: Pattern synonyms interact badly with data kinds

2 Jun 2014

      #9161: Pattern synonyms interact badly with data kinds
-------------------------------------------------+-------------------------
       Reporter:  Iceland_jack                   |             Owner:
           Type:  bug                            |            Status:  new
       Priority:  lowest                         |         Milestone:
      Component:  Compiler                       |           Version:
       Keywords:  renamer, pattern synonyms,     |  7.8.2
  data kinds                                     |  Operating System:
   Architecture:  Unknown/Multiple               |  Linux
     Difficulty:  Unknown                        |   Type of failure:
     Blocked By:                                 |  None/Unknown
Related Tickets:                                 |         Test Case:
                                                 |          Blocking:
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 If we define a pattern `A` and use it as a type:

 {{{
 {-# LANGUAGE PatternSynonyms, DataKinds #-}

 pattern A = ()

 b :: A
 b = undefined
 }}}

 we get the following error:

 {{{
 Prelude> :load tmp.XDGqzccOAV.hs
 [1 of 1] Compiling Main             ( /tmp/tmp.XDGqzccOAV.hs, interpreted
 )

 /tmp/tmp.XDGqzccOAV.hs:5:9:
     GHC internal error: ‘A’ is not in scope during type checking, but it
 passed the renamer
     tcl_env of environment: []
     In the type signature for ‘blah’: blah :: A
 Failed, modules loaded: none.
 }}}

 Also occurs with different arities of `A` (the arity of the pattern and
 type can differ) and defining it as an operator (enabling the
 `TypeOperators` pragma).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9161
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