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

18 Aug 2015

      #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
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 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 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
 }}}

 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
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