[GHC] #10753: Type checker fails to recognize equality

7 Aug 2015

      #10753: Type checker fails to recognize equality
-------------------------------------+-------------------------------------
              Reporter:  ljli        |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
  (Type checker)                     |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE KindSignatures #-}
 module Test where

 import qualified Control.Monad.State as S

 class HasConns (m :: * -> *) where
     type Conn m

 foo :: (HasConns m, Monad m) => S.StateT (Conn m) m ()
 foo =
     do _ <- S.get
        return ()
 }}}

 This fails to compile with GHC 7.10.2:

 {{{
     Could not deduce (S.MonadState (Conn m) (S.StateT (Conn m) m))
       arising from a use of ‘S.get’
     from the context (HasConns m, Monad m)
       bound by the type signature for
                  foo :: (HasConns m, Monad m) => S.StateT (Conn m) m ()
 }}}

 It compiles with GHC 7.8.4, though.
 Adding "Conn m ~ Conn m" to the type context of foo lets it compile again.
 This seems not right to me.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10753>
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