[GHC] #10778: GHC doesn't infer all constrains
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Tue Aug 18 10:35:50 UTC 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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