Re: Imitation is the sincerest form of flattery

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On Tue, 01 Apr 2003 14:38:08 GMT, "Frank J. Lhota"
<NOSPAM.lhota.adarose@verizon.net> wrote:

>It is not entirely clear how multiple dispatch would work with a function
>such as Max. Consider this version of the function:
>
>    -- Assuming Integer is tagged and MD is allowed
>    function Max (Left, Right : Integer'Class) return Integer'Class is
>    begin
>        if Left < Right then

I assume that "<" has to be MD and declared as:

   function "<" (Left, Right : Integer) return Boolean;

Here is the first problem. It cannot be a primitive operation of
Boolean, which is also "tagged". So it should to be:

   function "<" (Left, Right : Integer) return Boolean'Class;

But this is potentially inefficient because Boolean'Class is a large
object as compared with Boolean. It has the type tag. Probably one
should allow an explicit specification of contravariant /
non-dispatching parameters / results. Sort of:

   function "<" (Left, Right : Integer) return Boolean'Base;

Here the result is contravariant/non-dispatching.

>            return Right;
>        else
>            return Left;
>        end if;
>    end Max;
>
>Now assume we have the following declarations:
>
>    type X_Integer is new Integer;

I assume that this is actually

   type X_Integer is new Integer with null record;

>    -- Specialized version of "<" for X_Integer ...
>    function "<" ( Left, Right : in X_Integer ) return Boolean;

At the freezing point of X_Integer the compiler has to cry out: you
have overridden

   function "<" (Left, Right : X_Integer) return Boolean'Base;

so you have to do it for

   function "<" (Left : Integer; Right : X_Integer)
      return Boolean'Base;

and

   function "<" (Left : X_Integer; Right : Integer)
      return Boolean'Base;

Otherwise, it is not clear how to deal with:

X : Integer;
Y : X_Integer;
...
X < Y

One should probably allow something like

   function "<" (Left : X_Integer; Right : Integer)
      return Boolean'Base renames Integer."<";

>    X : X_Integer;
>
>    type Y_Integer is new Integer;
>    -- Specialized version of "<" for Y_Integer ...
>    function "<" ( Left, Right : in Y_Integer ) return Boolean;
>    Y : Y_Integer;
>
>Within Max( X, Y ), which version of "<" is used?

With siblings it becomes even worse. A solution as above, to require
that all possible combinations of parameter types be overriden, is
hard to imagine. X_Integer and Y_Integer might be declared in
different compilation units, so the error could appear only when the
units are with-ed. Freezing points passed. Nothing can be done. Full
stop.

Alternatives are:

1. It dispatches to Integer.">". I.e. it dispatches to the most
specific common base type.

2. It causes Constraint_Error. No dispatch on siblings.

Both are unsatisfactory in my view. There are indeed problems with MD.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de