-----Original Message----- From: Jules Bean [mailto:jules@jellybean.co.uk] Sent: 10 January 2008 14:22 To: Nicholls, Mark Cc: Bulat Ziganshin; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] confusion about 'instance'....
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?
The packet labelled "type class"....you're from .co.uk....you should understand my English idioms. :-)
Classes are not types.
Yep.
Classes are groups of types. Sets of types. Classifications of types.
I had them down as an n-ary relation on types....someone's said something somewhere that's made me question that...but I think I misinterpreted them....so I may default back to n-ary relation.
For any type, you can ask the quesiton "is this type a member of this class, or not?"
yep
Without wishing to split hairs too finely, I find it a useful
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
intuition the
left of the => as "some extra constraints on the type".
Hmmm...I'm not sure that helps....it may just make me more confused.
So you might say this has the type "a -> b -> a", providing that a is
a
Num and b is an Eq.
Jules