[Haskell-cafe] What's the difference?

[PR Stanley]

Hi
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 ...

I've only seen the "=>" operator used for declaring extended classes 
but never with class instances.
By the way, what is the correct terms for the "=>" and the "->"?

Cheers
Paul