[GHC] #9747: Odd failure to deduce a constraint

30 Oct 2014


      #9747: Odd failure to deduce a constraint
-------------------------------------+-------------------------------------
       Reporter:  acowley            |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:  TypeFamilies       |        Operating System:
  ConstraintKinds                    |  Unknown/Multiple
   Architecture:  Unknown/Multiple   |         Type of failure:
     Difficulty:  Unknown            |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 Using a type family to build a compound constraint that includes several
 constraints produced by applying another type family results in a type
 checker error like,

 {{{
 Could not deduce (ConFun l a, ConFun l b)
       arising from a use of ‘bar’
     from the context (ConFun l a, ConFun l b)
       bound by the type signature for
                  baz :: (ConFun l a, ConFun l b) => Proxy l -> HList '[a,
 b] -> ()
 }}}

 An example program is given at,
 https://gist.github.com/acowley/a001bd092599b5914bad

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9747
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler