[Haskell-cafe] Subtype polymorphism in Haskell

[Miguel Mitrofanov]

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 at gmail.com <mailto:courtenage at gmail.com>
> 
> 
> 
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