On Tue, Apr 8, 2008 at 11:05 PM, PR Stanley
What is the difference between
data T0 f a = MkT0 a instance Eq (T0 f a) where ...
and
data T0 f a = MkT0 a instance Eq a => Eq (T0 f a) where ...
The second one says that "TO f a" is only an instance of "Eq" if "a" is,
while the first says that "TO f a" is an instance regardless of the type of its arguments.
More explanation please. :-)
Well, it's similar with functions. If we try: foo :: a -> a -> Bool foo x y = x < y We'll get an error: the usage of `<` requires `a` to be in Ord class, which we write down as type signature: foo :: (Ord a) => a -> a -> Bool The similar effect is for type classes. Here:
instance Eq a => Eq (T0 f a) where ... We can see that we can compare elements of type `a` for equality. It's kind of saying: "if you give me an element of type `a`, and that type is in Eq class, I can tell you how to tell equality of elements of type T0 f a".
Regards Christopher Skrzętnicki