Re: [Haskell-cafe] Subtype polymorphism in Haskell

[Miguel Mitrofanov]

Actually, I liked Tillmann Rendel's idea much better than my own one:

data A = A {do_x :: Int -> Int -> Int}
b = A {do_x = \x y -> ...}
c = A {do_x = \x y -> ...}


Simon Courtenage wrote:

My thanks to everyone who replied with their helpful comments. You are right that I forgot to add the public inheritance on the C++ classes (that's what happens when you write code in an email without passing it through a compiler first).

I like the idea below, which is easy to understand. It seems to me, though, that there's a lot more to the use of typeclasses than is covered in some of the haskell textbooks, and if anyone knows of a good in-depth treatment of this, I would be grateful for a pointer.

Thanks again to everyone who responded,

Simon

On Mon, Jul 5, 2010 at 2:28 PM, Miguel Mitrofanov <miguelim...@yandex.ru <mailto:miguelim...@yandex.ru>> wrote:

    My guess is that it's
    class B : public A
    and
    class C : public A

    In this case it seems perfect to use type classes:

    class A t where do_x :: t -> Integer -> Integer -> Integer
    data B = ...
    instance A B where do_x b x y = ...
    data C = ...
    instance A C where do_x c x y = ...

    If you want some general "A" object, you can use existentials (or,
    better yet, GADTs):

    data A_general = forall t. A t => A_general t

    or

    data A_GADT where A_GADT :: A t => t -> A_GADT

    so that

    int foo (A v) {... v.do_x(1,2)...}

    becomes

    foo :: A_GADT -> Integer
    foo (A_GADT v) = ... do_x v 1 2 ...

    Simon Courtenage wrote:

        Hi,

        I am porting a C++ program to Haskell.  My current task is to
        take a class hierarchy and produce something equivalent in
        Haskell, but I don't seem to be able to get a grip on how type
        classes and instances contribute to the solution.  Can anyone
        enlighten me?

        Roughly, the class hierarchy in C++ is of the form

        class A {
        public:
          virtual int do_x(int,int) = 0;
        };

        class B {
        public:
          int do_x(int x,int y) { ...}
        };

        class C {
        public:
          int do_x(int x,int y) { ...}
        };

        Any help would be greatly appreciated.

        Thanks

        Simon
        courten...@gmail.com <mailto:courten...@gmail.com>
        <mailto:courten...@gmail.com <mailto:courten...@gmail.com>>



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