[GHC] #4091: Parse error on curried context of instance declaration

[GHC]

#4091: Parse error on curried context of instance declaration
---------------------------------+------------------------------------------
    Reporter:  basvandijk        |       Owner:              
        Type:  bug               |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.2            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 Assume we have the following classes:
 {{{
 class C1 a; class C2 a; class C3 a; class C4 a
 }}}

 GHC allows you to write a type with an uncurried context, as in:
 {{{
 foo :: (C1 a, C2 b, C3 c) => a -> b -> c
 }}}
 It also allows you to write a type with a curried context, as in:
 {{{
 foo :: C1 a => C2 b => C3 c => a -> b -> c
 }}}
 However it is not allowed to use this curried style of contexts in
 instance declarations, as in:
 {{{
 instance C1 a => C2 b => C3 c => C4 (SomeType a b c)
 }}}
 Is this intentional?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4091>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler