Re: [Haskell-cafe] what I learnt from my first serious haskell programm

David House wrote:

On 19/03/07, Fawzi Mohamed wrote:

...

This is is very ugly in my opinion, because for me a type class should represent something more than just a way to overload, is something is not a number then it should not have the class Num.

Num is a collection of types whose members can be added and subtracted and so on. As numbers are the most common example of this, one could say the members of Num _act like numbers_, rather than are numbers in themselves.

Really typeclasses are all about overloading. For example, Eq is the collection of types that the equality predicate (==) applies to. I don't see this as ugly; quite the contrary, in that if you know a type instantiates Eq you can use (==) without worrying about using a type-specific equality predicate. E.g. it's nice to see the same (==) everywhere rather than seeing (say) (Int.==), (Bool.==) and so on.

Maybe I did not express me clearly enough, I think that classes are useful (and the language that I was speaking of, aldor, has them), but it is not nice that the only way to have an overloaded function is to define a type class, or that you need to fully implement a class just to overload one function of it. Vectors don't act like numbers, a vector space is not a field, even if they have some common operations. I find it misleading to define something a number when it does not satisfy all the properties of numbers. The numerical prelude might fix this, but still I think that class and overloading should be distinct concepts. Fawzi