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

[GHC]

#7066: isInstance does not work for compound types
---------------------------------+------------------------------------------
    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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Comment(by edsko):

 Oh, I'm sorry. I would expect

 {{{
 isInstance clss typs
 }}}

 to return True if and only if I can safely generate code (in my Template
 Haskell code) that relies on 'typs' being an instance of 'clss'. In the
 example above, I can ''not'' safely generate code that relies on a Show
 instance for (Int, A) because there is no Show instance for A. However, I
 ''can'' safely generate code that relies on a Show instance for (Int,
 Bool), say.

 Context: In Cloud Haskell when you call 'remotable' on a monomorphic
 function f :: T1 -> T2, I automatically generate a top-level definition

 {{{
 f__sdict :: Static (SerializableDict T1)
 }}}

 but if T1 is not in fact serializable then this will subsequently cause a
 type error. So I would like to check (using isInstance) if T1 is an
 instance of Serializable before generating this code.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7066#comment:4>
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