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

[Fawzi Mohamed]

David House wrote:
> On 19/03/07, Fawzi Mohamed <fmohamed at mac.com> 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