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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Simon Courtenage wrote:

This is for a project to port an open-source C++ library to haskell.

Great! We'd love to give you whatever support you need for your efforts.

 My initial plan is to more or less preserve the way the library works in the first draft of the port and see how far we can get like that

That's fine, as long as you truly mean the way it works, and not the way the code is structured. Haskell is a post-OO language. Its abstractions are very different than class structures in C++. There is no direct translation - any given C++ class structure could correspond to many totally different kinds of Haskell programs, depending on what the program is trying to do. If you are trying to find a method to transliterate a strongly OO-style C++ program more or less word for word into Haskell in a way that the class structure of the C++ will still be apparent in the result, you are likely in for a frustrating experience. You will spend a lot more time than you expected, and the results will be very unsatisfying. Many others have ended up that way. On the other hand, if you are willing to be a little more flexible in your thinking, you'll probably find the task much easier than you thought, enjoy it, and reap many benefits from the process that you never imagined. In any case, please keep us in the loop, we'd like to hear how it's going. And, uh... would you be willing to share a few more details about what it is that you're trying to port? ;) Thanks, Yitz