[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:                    |  
---------------------------------+------------------------------------------

Comment(by simonpj):

 GHC does not support polymorphic constraints to the left of an arrow.  The
 !StackOverflow thread you point to contains this program:
 {{{
 {-# LANGUAGE FlexibleInstances    #-}
 {-# LANGUAGE TypeFamilies         #-}
 {-# LANGUAGE Rank2Types           #-}
 {-# LANGUAGE ConstraintKinds      #-}
 {-# LANGUAGE UndecidableInstances #-}

 module T7019a where

 class Context c where
   func1 :: c -> String

 class (forall b. Context (Associated a b))   -- URK!
    => Class a where
   data Associated a :: * -> *

 newtype ClassTest = ClassTest { runClassTest :: String }

 instance (forall b. Context (Associated ClassTest b))  -- URK!
       => Class ClassTest where
   data Associated ClassTest b = ClassTestAssoc b (b -> ClassTest)

 instance Context (Associated ClassTest b) where
   func1 (ClassTestAssoc b strFunc) = runClassTest $ strFunc b

 main = putStr . func1 $ ClassTestAssoc 37 (ClassTest . show)
 }}}
 I'm astonished that this program typechecks.  It certainly shouldn't.  We
 have not begun to think about constraints like `(forall b. (Associated
 ClassTest b))` I'm afraid.

 Actually, your Monoid example makes a lot more sense, but can't you just
 have a top-level instance
 {{{
 instance Monoid (Free a) where ...(not sure)....
 }}}
 in which case you are fine.

 Wanting foralls on the left of an arrow is not stupid.  See #5927, #2893,
 #2456.  But it's not going to happen soon, because I don't know how to
 specify or implement it.  (Which is not to say that someone else may not
 be able to crack it.)

 Meanwhile, these programs should be rejected, and I'll add the tests so
 that they are.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019#comment:3>
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