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

19 May 2015

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

Comment (by simonpj):

 This is my fault, fallout from my removal of "silent superclasses".  I
 understand what is happening here. It is clearly a bug.  I think that if
 you replace
 {{{
 instance (Monad m, m ~ n) => Testable n (Property m) where
 }}}
 by putting `Monad n` in the context, thus
 {{{
 instance (Monad n, m ~ n) => Testable n (Property m) where
 }}}
 it'll probably work.  It'll be a couple of weeks before I can actually fix
 it.

 Thanks very much for the report.

 Simon

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10423#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler