Am Samstag, 7. März 2009 21:48 schrieb Maurício:
(...)
When you have
data Test = Test (forall w. (C1 w, C2 w, ..., Cn w) => w)
and
function (Test w) = classmethod w,
there is no way to decide which instance to use, hence the type variable is ambiguous. (...)
But, then, how can I reach the data inside a polymorphic component? Or, better, what can I do with it? If I say:
function (Test w) = classmethod (w :: specificType)
then I have to suppose that w is always of 'specificType', and this may not be true.
If w :: forall a. (Class a) => a, then w is (can be) of all specific types which are instances of Class. Perhaps what you wanted was an existential type: {-# LANGUAGE ExistentialQuantification #-} data ETest = forall w. WidgetClass w => ETest w ? Or a GADT: data GTest where GTest :: forall a. WidgetClass a => a -> GTest ?
Thanks, Maurício
Cheers, Daniel