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 <miguelimo38@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
courtenage@gmail.com <mailto:courtenage@gmail.com>



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