[Haskell-cafe] confusion about 'instance'....

[Jules Bean]

Nicholls, Mark wrote:
> 
> My confusion is not between OO classes and Haskell classes, but exactly
> are the members of a Haskell type class...I'd naively believed them to
> be types (like it says on the packet!)...but now I'm not so sure.
> 


Which packet?

Classes are not types.

Classes are groups of types. Sets of types. Classifications of types.

For any type, you can ask the quesiton "is this type a member of this 
class, or not?"

Without wishing to split hairs too finely, I find it a useful intuition 
not to consider the class context "part of the type" somehow.

So, when you see this:

(Num a, Eq b) => a -> b -> a

Rather than thinking of that whole thing as a type, it helps to think of 
the part on the right of the => as the 'actual type' and the part on the 
left of the => as "some extra constraints on the type".

So you might say this has the type "a -> b -> a", providing that a is a 
Num and b is an Eq.

Jules