[GHC] #3743: type checker fails to infer an implicit parameter constraint in the presence of existential types.

[GHC]

#3743: type checker fails to infer an implicit parameter constraint in the
presence of existential types.
---------------------------------+------------------------------------------
    Reporter:  int-e             |        Owner:              
        Type:  bug               |       Status:  new         
    Priority:  normal            |    Milestone:  6.14.1      
   Component:  Compiler          |      Version:  6.13        
    Keywords:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Testcase:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------

Comment(by simonpj):

 Interesting example.  Somehow I missed this bug when you first submitted
 it.

 Any type inference under local constraints is tricky.  See our paper "Let
 should not be generalised".  In the type system of that paper the program
 would be rejected on the grounds that we can't solve the implication
 {{{
   forall a. Foo a => (?x::())
 }}}
 But in this case there's an obvious simple constraint we could infer.
 I'll discuss with Dimitrios, and add it to my list for the new constraint
 solver.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3743#comment:2>
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