On Thu, Apr 04, 2002, Hal Daume III wrote:
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:
I believe that ghc translates the signature above to foo :: forall q . Foo q => Double -> q (I don't understand why GHC does this... it seems to have more potential for confusion) This should more clearly show that foo is required to take a Double and give, in return, anything in class Foo that is requested, which it certainly does not (it always returns a Double).
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.
Actually, this is not really universal quantification, it is existential quantification. If you actually wrote a datatype that did universal quantification, it wouldn't work either: data T = T (forall q . Foo q => q) (I think this can also be written data T = T (Foo q => q) but I don't remember for sure). David