classes

[Juan Carlos Arévalo Baeza]

On Fri, 31 Aug 2001 22:24:06 +0300, Cagdas Ozgenc wrote:

>>class Ord a where =A0 =A0(<) :: a ->=A0a ->=A0Bool
>>
>>data Set a =3D Ord a =3D>=A0[...]
>
>The only thing that bothers me here is the Asymmetry. The=
 syntax
>treats the first, and the second argument differently.

   Not really, except for the order not being commutative (same=
 as in C++).

>Although
>there is equal emphasis on both "a"s in an "<" operation. C++=
 solves
>this problem by taking them out of the class.
>
>bool operator <=A0(const Ord&=A0a, const Ord&=A0b);
>
>where arguments are treated with equal rights. Maybe I am just=
 too
>concerned.

   I don't know if I follow your exact concern. In any case, I=
 just thought that you can always do in Haskell something like=
 this:

equals :: (Eq a, Eq b) =3D> a -> b -> Bool
a =B4equals=B4 b =3D [...] -- I don't know how to implement this

   which is something like what you want, except that the=
 definition that you put on that function MUST serve for all=
 appropriate types. In Haskell, you can't add overloads to a=
 function unless that function belongs to a class. Then you can=
 make overloads by making different instances of that class. That=
 has trade-offs over the C++ system, like:

- Code is more organized the Haskell way. Easier on the compiler=
 tools.
- Programs and modules are more predictable in Haskell, because=
 what they define cannot be 'modified'.
- C++ allows dynamic inheritance, which is best in some=
 situations. In Haskell you have to hack it in.
- C++ has more flexible static overloading.

   What can I say? My favorite programming language hasn't been=
 invented yet. But I'm sure it'll borrow concepts from both,=
 Haskel and C++.

   I like Patrick's little talk about concepts a lot better than=
 anything I said, BTW. It's like the GCC extension of signatures,=
 in a way.
 
   Salutaciones,
                        JCAB
email: jcab@roningames.com
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