[GHC] #10423: Can't infer Monad n from (Monad m, m ~ n)

17 May 2015

      #10423: Can't infer Monad n from (Monad m, m ~ n)
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              Reporter:  Feuerbach   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 This looks like a regression in GHC HEAD.

 {{{
 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)’
 }}}

 The code is here:
 https://github.com/feuerbach/smallcheck/blob/v1.1.1/Test/SmallCheck/Property...

 Originally reported here:
 https://github.com/feuerbach/smallcheck/issues/28

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10423
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