[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 <miguelimo38 at yandex.ru 
> <mailto:miguelimo38 at 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 at gmail.com <mailto:courtenage at gmail.com>
>         <mailto:courtenage at gmail.com <mailto:courtenage at gmail.com>>
> 
> 
> 
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> 
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