[GHC] #7066: isInstance does not work for compound types

[GHC]

#7066: isInstance does not work for compound types
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    Reporter:  edsko             |       Owner:  simonpj         
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Template Haskell  |     Version:  7.4.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Changes (by simonpj):

  * owner:  => simonpj
  * difficulty:  => Unknown


Comment:

 That's true, and currently so by design.  What if you did want to ask "is
 there an instance for pairs?"  You could try making up fresh type
 variables 'x' and 'y', say, but then with the behaviour you were expecting
 it would try to find an instance for `(Show x)`, fail, and hence return an
 empty list.

 It should be better documented, I agree, but I'm not sure what alternative
 behaviour would make sense.

 I need review and commit Reiner's patches from #5469, but then I'll add
 the docs.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7066#comment:1>
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