[GHC] #10499: Constraints should associate over ~

7 Jun 2015

      #10499: Constraints should associate over ~
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              Reporter:  joelteon    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 Building the smallcheck package fails with this error:

 {{{
 Test/SmallCheck/Property.hs:187:10:
     Could not deduce (Monad n)
       arising from the superclasses of an instance declaration
     from the context: (Monad m, m ~ n)
       bound by the instance declaration
       at Test/SmallCheck/Property.hs:187:10-52
     Possible fix:
       add (Monad n) to the context of the instance declaration
     In the instance declaration for ‘Testable n (Property m)’
 }}}

 Wondering if this is a bug in 7.11's constraint solver or 7.10's, since
 7.10 and previous GHCs accept it and 7.11 doesn't.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10499
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