[erlang-questions] Erlang modelling question

[Damien Morton]

Might have been a different conversation, but I think you were right 
that the extensibility problem is the same as in the expression problem.
> Damien Morton <dmorton@REDACTED> wrote:
>   
>> So your problem is one of code management - Darius, who frequents this 
>> list sometimes, pointed out that its an instance of the expression 
>> problem. see http://www.daimi.au.dk/~madst/tool/papers/expression.txt
>>     
>
> That was a different conversation; I haven't really followed this one.
>
> FWIW, if the goal is to code mutable objects with inheritance, the
> most straightforward way seems to be with a process for each level of
> inheritance for each object: Java code like
>
> class Shape {
>   int x, y;
>   Shape(int x, int y) { this.x = x; this.y = y }
>   void move(int dx, int dy) { x += dx; y += dy; }
>   int getX() { return x; }
> }
>
> class Square extends Shape {
>   int size;
>   Square(int x, int y, int size) { Shape(x, y); this.size = size; }
>   int getSize() { return size; }
> }
>
> might look like
>
> make_shape(X, Y) ->
>     spawn(fun () -> shape(X, Y) end).
> shape(X, Y) ->
>     receive 
>         {Self, {move, DX, DY}} -> shape(X + DX, Y + DY);
>         {Self, {get_x, Cont}} -> Cont ! {Self, X}, shape(X, Y)
>     end.
>
> make_square(X, Y, Size) ->
>     Shape = make_shape(X, Y),
>     spawn(fun () -> square(Shape, Size) end).
> square(Shape, Size) ->
>     receive
>         {Self, {get_size, Cont}} -> Cont ! Size, square(Shape, Size);
>         Other -> Shape ! Other, square(Shape, Size)
>     end.
>
> ask(Object, Message) ->
>     Object ! {Object, Message}.
>
> (The Self argument is there to allow "superclass methods" to "call
> subclass methods", which I forgot to include an example of here.)
>
> This differs from the original post mapping classes to modules.
>
> Darius
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