Re: [GHC] #11125: Typechecker can't infer StM m Bool ~ Bool from StM m a ~ a

23 Nov 2015

      #11125: Typechecker can't infer StM m Bool ~ Bool from StM m a ~ a
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        Reporter:  nikomi            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hvr):

 Btw, it may be useful to know the definition of the associated type `StM`:

 {{{#!hs
 class MonadBase b m => MonadBaseControl b m | m -> b where
     type StM m a :: *
     liftBaseWith :: (RunInBase m b -> b a) -> m a
     restoreM :: StM m a -> m a

 instance MonadBaseControl IO IO where
     type StM IO a = a
     liftBaseWith f = f id
     restoreM = return

 instance MonadBaseControl STM STM where
     type StM STM a = a
     liftBaseWith f = f id
     restoreM = return

 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11125#comment:3
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