#3958: method definitions fail to refer to method-context classes
-------------------------------+--------------------------------------------
Reporter: visionete | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.12.1 | Keywords:
Os: Linux | Testcase:
Architecture: x86_64 (amd64) | Failure: GHC rejects valid program
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Sorry in advance if this is my lack of understanding...[[BR]]
The following code fails:[[BR]]
{{{
class Cfoo a where
f :: Int -> a -> Int
data Foo = F Int
instance Cfoo Foo where
f m (F n) = m + n
data Bar = B Int
class Cbar a where
g :: (Cfoo b) => a -> b
instance Cbar Bar where
g (B m) = F m
}}}
with error:[[BR]]
{{{
Couldn't match expected type `b' against inferred type `Foo'
`b' is a rigid type variable bound by
the type signature for `g' at [...]
In the expression: F m
In the definition of `g': g (B m) = F m
In the instance declaration for `Cbar Bar'
}}}
It's as if the "instance Cfoo Foo" statement is ignored when processing
"instance CBar Bar"
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3958>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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