Re: [GHC] #10778: GHC doesn't infer all constrains

18 Aug 2015

      #10778: GHC doesn't infer all constrains
-------------------------------------+-------------------------------------
        Reporter:  danilo2           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Description changed by danilo2:

Old description:
...
Hello! Let's consider following signatures:
{{{
class (IndexOf a cont ~ idx, ElementByIdx idx cont ~ a, Measurable cont)
=> Container cont idx a
class Container cont idx a => Appendable cont idx a
class HasContainer a cont | a -> cont
class PtrFrom p i | p -> i
class Monad m => RefBuilder3 a m ref | a m -> ref where
    mkRef3 :: a -> m (ref a)
}}}
Now when I define following instance:
{{{
instance (PtrFrom idx i, Appendable cont idx a, HasContainer g cont,
Monad m)
      => RefBuilder3 a (GraphBuilderT g m) (Ptr i) where
    mkRef3 = undefined
}}}
I get an error:
{{{
Illegal instance declaration for
      ‘RefBuilder3 a (GraphBuilderT g m) (Ptr i)’
      The liberal coverage condition fails in class ‘RefBuilder3’
        for functional dependency: ‘a m -> ref’
      Reason: lhs types ‘a’, ‘GraphBuilderT g m’
        do not jointly determine rhs type ‘Ptr i’
    In the instance declaration for
      ‘RefBuilder3 a (GraphBuilderT g m) (Ptr i)’
}}}
But when I add the constraint to the instance head:
{{{
instance (PtrFrom idx i, Appendable cont idx a, HasContainer g cont,
Monad m, IndexOf a cont ~ idx)
      => RefBuilder3 a (GraphBuilderT g m) (Ptr i) where
    mkRef3 a = fmap ptrFrom . withGraph . append $ a
}}}
It compiles fine. What's interesting, the constraint should be inferred
by GHC, because we've got following funds here:
{{{
g -> cont     -- from HasContainer
cont a -> idx -- from Appendable -> Container
idx -> i      -- from PtrFrom
}}}
Is this a GHC bug or am I missing something? If thats a bug, I will try
to clean the code and make a minimal example.
New description:

 Hello! Let's consider following signatures:
 {{{
 class (IndexOf a cont ~ idx, ElementByIdx idx cont ~ a, Measurable cont)
 => Container cont idx a
 class Container cont idx a => Appendable cont idx a
 class HasContainer a cont | a -> cont
 class PtrFrom p i | p -> i
 class Monad m => RefBuilder3 a m ref | a m -> ref where
     mkRef3 :: a -> m (ref a)

 }}}

 Now when I define following instance:

 {{{
 instance (PtrFrom idx i, Appendable cont idx a, HasContainer g cont, Monad
 m)
       => RefBuilder3 a (GraphBuilderT g m) (Ptr i) where
     mkRef3 = undefined
 }}}

 I get an error:

 {{{
 Illegal instance declaration for
       ‘RefBuilder3 a (GraphBuilderT g m) (Ptr i)’
       The liberal coverage condition fails in class ‘RefBuilder3’
         for functional dependency: ‘a m -> ref’
       Reason: lhs types ‘a’, ‘GraphBuilderT g m’
         do not jointly determine rhs type ‘Ptr i’
     In the instance declaration for
       ‘RefBuilder3 a (GraphBuilderT g m) (Ptr i)’
 }}}

 But when I add the constraint to the instance head:

 {{{
 instance (PtrFrom idx i, Appendable cont idx a, HasContainer g cont, Monad
 m, IndexOf a cont ~ idx)
       => RefBuilder3 a (GraphBuilderT g m) (Ptr i) where
     mkRef3 a = fmap ptrFrom . withGraph . append $ a
 }}}

 It compiles fine. What's interesting, the constraint should be inferred by
 GHC, because we've got following fundeps here:
 {{{
 g -> cont     -- from HasContainer
 cont a -> idx -- from Appendable -> Container
 idx -> i      -- from PtrFrom
 }}}

 Is this a GHC bug or am I missing something? If thats a bug, I will try to
 clean the code and make a minimal example.

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10778#comment:2
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