[Haskell-cafe] Subtype polymorphism in Haskell

[Tillmann Rendel]

Simon Courtenage wrote:
> 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.  

They probably do not contribute at all.

> 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) { ...}
> };

I guess B and C are subclasses of A?


If you do not need complicated subtyping schemes, you could consider on 
of the following two options: You could emphasize the structure of the 
class hierarchy, or you could emphasize the object abstraction.

The structure of the class hierarchy could be represented by an 
algebraic data type

   data A = B | C

with the do_x function implementing by pattern matching on an A value

   do_x :: A -> Int -> Int -> Int
   do_x B = ...
   do_x C = ...

If your classes have fields, they could be added as fields to the data 
constructors B and C.


Alternatively, the object abstraction could be represented by encoding 
objects as records of first-class functions.

   data A = A {
     do_x :: Int -> Int -> Int
   }

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

If your classes have fields, they could be added as arguments to 
constructor functions b and c.

   Tillmann