[GHC] #7910: ConstraintKinds and reifyInstances

14 May 2013


      #7910: ConstraintKinds and reifyInstances
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Reporter:  elliottt                     |          Owner:                                  
    Type:  bug                          |         Status:  new                             
Priority:  normal                       |      Component:  Compiler                        
 Version:  7.6.3                        |       Keywords:  ConstraintKinds, TemplateHaskell
      Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple                
 Failure:  Incorrect result at runtime  |      Blockedby:                                  
Blocking:                               |        Related:                                  
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 reifyInstances doesn't appear to know how to deal with a constraint that
 is just an alias. For example, the following prints (True,False):


 {{{
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE TemplateHaskell #-}

 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax

 class C a
 instance C Int

 type D a = C a

 main = print $(
   do isCInst <- isInstance ''C [ConT ''Int]
      isDInst <- isInstance ''D [ConT ''Int]
      lift (isCInst,isDInst))

 }}}

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7910
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