explicitly quantified classes in functions

4 Apr 2002

      Why can I not define the following (in ghc):
...
class Foo p where
instance Foo Double where
foo :: Double -> (forall q . Foo q => q)
foo p = p
...
From my humble (lack of) knowledge, there seems to be nothing wrong here,
but ghc (5.03) complains about unifying q with Double.  I *can* write:
...
class Foo p where
instance Foo Double where
data T = forall q . Foo q => T q
foo :: Double -> T
foo p = T p
which is very similar, except that the explicit universal quantification
is happening in in the datatype and not the function type.

why is the former disallowed?

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume