GitLab

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.

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture