[GHC] #1241: Functional dependencies not checked.

[GHC]

#1241: Functional dependencies not checked.
-------------------------------------+--------------------------------------
 Reporter:  guest                    |          Owner:         
     Type:  bug                      |         Status:  new    
 Priority:  normal                   |      Milestone:  _|_    
Component:  Compiler (Type checker)  |        Version:  6.6    
 Severity:  normal                   |     Resolution:         
 Keywords:                           |     Difficulty:  Unknown
 Testcase:                           |   Architecture:  Unknown
       Os:  Unknown                  |  
-------------------------------------+--------------------------------------
Comment (by claus):

 actually, while FDs are used to instantiate type variables, that never
 seems to happen for explicit type declarations. and the type parameters of
 an instance head become part of the explicit type declarations for the
 class methods.

 also, even after we filter out the instantiations of the instance that do
 not conform to the FD, we still don't know precisely what b is, and yet it
 isn't universally quantified either.

 so, while interpretation 2 might be self-consistent, and possibly useful,
 and apparently closest to what GHC is doing, it differs from the standard
 interpretation of the constructs it would apply to, and should probably
 get its own syntactic clues to indicate that difference.

 {{{
 > class F a b | a -> b where f :: (a,b)
 > instance F Int b
 }}}

 as i can no longer offer an interpretation in which this instance would be
 valid (at least not without some extra syntax to indicate the special role
 of b in interpretation 2), i agree that it is not. and unless GHC's
 interpretation is based on something other than interpretation 2, its
 behaviour ought to change. it might be helpful to report the problem not
 just as "instance conflicts with FD", as Hugs does, but as "instance is
 more general than FD permits".

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1241>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler