Re: 'withing' problem

[tmoran@acm.org]

> Using the Ada-like notation of your example, 'toString' dispatches on the
> type of parameter x. In fact, the call is written 'expr.toString' as in
> other OO languages, so (unlike the case in Ada) it is obvious what the call
> dispatches on.
  Actually, it's not the case that
   function toString(x : anything) return seq;
shows an ambiguity about dispatching in Ada, since it's not (legal) Ada.

> We define a base class 'anything' from which all other classes derive (like
> Java's 'Object'). This base class has an abstract method 'toString' (just
> like Java) having a return type 'seq of char' (where 'seq' is a templated
> type representing a sequence). But 'seq' is, like all other classes, derived
> from 'anything'.
   Does "seq" have any children?  Does it inherit things from "anything"?
Given "x : seq;", what does "toString(x);" produce?  More generally,
why is "seq" derived from "anything" rather than just standing on its own?

> So the declaration of 'anything' needs to see a forward declaration of 'seq'.
Actually it's the declaration of "toString" that needs a preceding declaration
of "seq".

How about

  type anything is tagged null record;
  type seq is new anything with record
    ...
  end record;
  function toString(x:anything'class) return seq;

and then use overloading to create

  function toString(x : first_child_of_anything'class) return seq;
  function toString(x : second_child_of_anything'class) return seq;
  function toString(x : first_of_first_child_of_anything'class) return seq;
etc.
Then calls look like
  a_seq := first_child.toString(x);
  a_seq := second_child.toString(y);
  a_seq := first_of_first_child.toString(x);