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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: idem (Was: += in ada)
Date: Mon, 20 Oct 2003 10:28:57 +0200
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On Mon, 20 Oct 2003 09:41:39 +0200, Jacob Sparre Andersen
<sparre@crs4.it> wrote:

>Preben Randhol wrote:
>
>> The problem I have with idem is that it looks like just another
>> variable.
>
>Yes.  But it is a cool idea.  And with a suitable idem marker, I think
>it would make it possible to write even more readable code.  Also,
>"idem" doesn't look any more like another variable than the operators
>"and", "or" and "not".
>
>What I worry most about with idem is that it would make the expression
>syntax used in assignments different from that used in other places.

Probably there could be a generalization. Consider assignment as a
procedure:

procedure ":=" (Idem : in out Type; Right : Type);

Then in

X := Idem + 1;

Idem would be just the name of a formal parameter. This could be
applicable to all kinds of subroutines. Of course, a direct use of a
parameter name would lead to possible name clashes. So it would be
better to have a special notation to access the parameter names as
"=>" does. For example:

X := ":=".Idem + 1;

It looks extremely ugly, but illustrates the idea. So for a function
Mean one could write something as unreadable as:

X := 
   Mean
   (  Left => Some_Awfully_Complex_Expression,
      Right => Mean.Left * 3.0
   );

This would also impose additional requirements on parameter evaluation
order. Or else, allow excellent:

Mean (Left => Mean.Right, Right => Mean.Left);

All C++ programmers will immediately switch to Ada, to have this!
(:-))

--------
In one of previous incarnations of this ever lasting thread there was
made a better proposal for idem. We could allow loop labels for the
assignment statement:

[ <label> : ] <target> := <expression>;

So:

Idem : X := Idem + 1;

which would be an abbreviation of:

declare
   Idem : X_Type renames X;
begin
   Idem := Idem + 1;
end;

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