#816: Weird fundep behavior (with -fallow-undecidable-instances)
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Reporter: nibro | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.8
Component: Compiler (Type checker) | Version: 6.4.2
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Os: Unknown | Testcase: tc216
Architecture: Unknown |
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Comment (by simonpj):
I doubt I'll ever fix this, because we have a better way. With indexed
type families we'll write
{{{
class Foo x where
type Y x
foo :: x -> Y x
class Bar x y where
bar :: x -> y -> Int
instance (Foo x y, Bar y z) => Bar x z where
bar x z = bar (foo x) z
}}}
So now the inference problem becomes
{{{
GIVEN: (Foo x, Bar (Y x) z)
WANTED: (Foo x, Bar (Y x) z)
}}}
which is trivially solvable.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/816>
GHC <http://www.haskell.org/ghc/>
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