[GHC] #7019: Wrong error message when using forall in constraints

[GHC]

#7019: Wrong error message when using forall in constraints
---------------------------------+------------------------------------------
    Reporter:  sjoerd_visscher   |       Owner:  simonpj         
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.4.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by sjoerd_visscher):

 Apparently it is allowed to use forall in constraints (I just saw that
 [http://stackoverflow.com/questions/11258536/how-to-put-constraints-on-
 the-associated-data here]). I used the type synonym as a workaround, but
 that's not needed, so here's a simplified case:

 {{{
 {-# LANGUAGE ConstraintKinds, RankNTypes, UndecidableInstances #-}

 import Data.Monoid

 newtype Free a = Free { runFree :: forall r. Monoid r => (a -> r) -> r }

 instance (forall a. Monoid (Free a)) => Monad (Free) where
   return a = Free ($ a)
   (>>=) = runFree
 }}}

 gives the error

 {{{
     Could not deduce (Monoid (Free b)) arising from a use of `runFree'
     from the context (forall a. Monoid (Free a))
 }}}

 I replaced the constaint variable with Monoid, to check that that's not
 the problem. So this new version of course has a simple workaround, but
 that workaround is not available in the constraint variable version.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019#comment:2>
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