Re: [GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")

21 Jul 2014

      ...
@diatchki please do not base your opinion on the examples above - they
 are a little old and of course, they do not obey some basic principles.
The idea with `-XDysfunctionalDependencies` is just to lift both the
 Paterson Conditions and the Coverage Condition - something
 `-XUndecidableInstances` claims  to do (according to documentation), but
 does not (as simonpj noticed above). When using this extension you can
 just give some interesting hints to typechecker and compile programs like
#8634: Relax functional dependency coherence check ("liberal coverage condition")
-------------------------------------+-------------------------------------
              Reporter:  danilo2     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:  7.10.1
              Priority:  high        |           Version:  7.7
             Component:  Compiler    |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D69    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #1241,      |
  #2247, #8356, #9103, #9227         |
-------------------------------------+-------------------------------------

Comment (by diatchki):

 This program has the exact same problem as the one in the ticket above: it
 violates the functional dependency.  Having a "functional dependency"
 simply means that you are telling GHC that you want to work with a
 "functional relation (in the specified parameters)", and it should give
 you an error if you made a mistake.  Here is how you can rewrite your
 program without `Dysfunctional Dependencies`:

 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleInstances #-}

 data X = X

 class C a b where
   ctest :: a -> b

 class D a f b | a -> b where
   dtest :: a -> f b

 instance Monad m => D X m Int where
   dtest _ = return 5

 instance (Monad m) => C X (m Int) where
   ctest = dtest

 main = print (ctest X :: [Int]) -- [5]
 }}}

 Replying to [comment:19 danilo2]:
 the one I've posted on https://phabricator.haskell.org/D69 :
...
{{{#!haskell
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DysfunctionalDependencies #-}
class CTest a b | a -> b where
   ctest :: a -> b
data X = X
instance Monad m => CTest X (m Int) where
ctest _ = return 5
main = print (ctest X :: [Int]) -- [5]
}}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:31
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