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

[GHC]

#10778: GHC doesn't infer all constrains
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        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:
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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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