#1624: internal error caused by adding an instance to a type class with a
functional dependency and a default method
--------------------------------------+-------------------------------------
Reporter: int-e | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.7
Severity: normal | Keywords:
Difficulty: Unknown | Os: Linux
Testcase: | Architecture: x86
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I've got trouble with the following program:
{{{
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
class Foo a b | a -> b where
foo :: a -> b
foo = undefined
instance Foo a (Maybe a)
}}}
This results in a type checking error, and an internal error:
{{{
xx.hs:2:0:
Couldn't match expected type `b' (a rigid variable)
against inferred type `Maybe a'
`b' is bound by the class declaration for `Foo'
When using functional dependencies to combine
Foo a (Maybe a), arising from the instance declaration at x.hs:8:0
Foo a b,
arising from is bound by the class declaration for `Foo'
at x.hs:(2,0)-(4,18)
[snip]
x.hs:6:0:
GHC internal error: `Main.$dmfoo' is not in scope
In the expression: Main.$dmfoo
In the definition of `foo': foo = Main.$dmfoo
In the definition for method `foo'
}}}
Slightly less convincing variations on the scheme are
{{{
class Foo a b | a -> b
instance Foo a (Maybe a)
foo :: Foo a b => a -> b
foo = undefined
class Bar a b c
instance (Foo a b, Foo b c) => Bar a b c
}}}
Both the definition of {{{foo}}} and the instance declaration of {{{Bar}}}
give a type checking error "when combining functional dependencies".
Apparently {{{ghc}}} doesn't use functional dependencies to simplify the
contexts of user supplied type signatures; I think the above examples
indicate that it should.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1624>
GHC <http://www.haskell.org/ghc/>
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