On Tue, Jan 20, 2009 at 2:51 PM, Mauricio
But how is this: data SomeNum = forall a. SN a different from: data SomeNum = SN (forall a. a)
At a glance they look the same to me — but only the first is accepted by ghc.
Following the link you pointed in the last message, I found this at 8.8.4.1:
data T a = T1 (forall b. b -> b -> b) a
If I understand properly, it can be activated with -XPolymorphicComponents. It's different from my example, but I would like to know what it does that this can't:
data T a = forall b. T1 (b->b->b) a
In the first example, T stores a function which works for every type. In the second example, T stores a function which works on a specific type. So with the first definition, you can do something like this: foo :: T1 a -> (Bool, Int) foo (T1 f _) = (f True False, f 1 2) But you can't really do anything useful with the second example.
(The last one I think I understand well after the previous message, although I see no use for this particular form, since after pattern match there's no other 'b' value to which we could aply that function field.)
Here's a more useful example of existential quantification:
data Stream a = forall b. Stream (b -> a) (b -> b) b
head :: Stream a -> a
head (Stream h t b) = h b
tail :: Stream a -> Stream a
tail (Stream h t b) = Stream h t (t b)
--
Dave Menendez