Patrick Browne
I think the problem is there is just one constant p1 in the class and there needs to be multiple constants in the implementation (one for each person). It seems difficult to specify this using type classes So, some data declaration as you suggest will probably be needed.
Yes. I'm going to assume you're a beginner here (always a dangerous assumption on this list, no offense if I just misinterpret what you're trying to do). The (one?) difference between a Haskell class and a typical OO class, is that an instance of a class in Haskell is a *type*, but an instance of a class in, say, C++, is an object (which we like to call a *value*, to dissociate ourselves from That Other Crowd :-). Let's ignore the 'name' for now. class Person p where id :: p means that a *type* p is in the Person class if you can identify a value of type p that is the person-id for that type. So Int is a person-type if you say instance Person Int where id = 42 and (id::Int) will return 42 in subsequent code. Similarly, you can make other Person-types by instantiating them and defining id¹ for them. A function can take a Person-type as a parameter, for instance foo :: Person p => p -> Bool foo x = bla blah (id x) blah Alternatively, data Person = P Int on the other hand declares a new type called "Person", with a data constructor (tag) P, and containing just an Int. This means you can make multiple values of this type, so P 4, P 42, and P 911 are all Persons your program can juggle around in your program. A function can take these persons: foo :: Person -> Int -> Bool foo (P x) y = bla bla apply int functions on x and y Hope that was helpful? -k ¹ Dumb name, as this is the identity function. Sorry. -- If I haven't seen further, it is by standing in the footprints of giants