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From: "John R. Strohm" <strohm@airmail.net>
Newsgroups: comp.lang.ada
Subject: Re: Ideas for Ada 200X "left hand side" repeater.
Date: Wed, 18 Jun 2003 17:13:25 -0500
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"Chad R. Meiners" <crmeiners@hotmail.com> wrote in message
news:bcnvrr$mdg$1@msunews.cl.msu.edu...
>
> "Bill Findlay" <yaldnifw@blueyonder.co.uk> wrote in message
> news:BB14D345.35C8%yaldnifw@blueyonder.co.uk...
> >>
> > > What about using 'is' to express identity? e.g.:
> > >
> > >  Pixel is Screen.Buffer(i) in Pixel := Pixel + 1;
>
> I think I like this.
>
> > > It's not exactly concise, though. Nor is any 'implicit renaming'
> solution.
>
> Perhaps simplified is a better description because it allows the written
> format to enhance readability.
>
> Pixel is Screen.Buffer(i) in
>     Pixel := Pixel + 1;
>
> It also separates the dual concerns of naming complex expression and
having
> clear statements while keeping the names in sight of each other. Perhaps
the
> above sample is too simple how about
>
> Pixel is Screen.Buffer(i) in
>     Pixel := To_Red(Get_Green(Pixel)) + To_Green(Get_Blue(Pixel)) +
> To_Blue(Get_Red(Pixel));
>
> versus
>
> declare
>     Pixel : Buffer_Element renames Screen.Buffer(i);
>     Red   : constant Scale := Get_Red(Pixel);
>     Green : constant Scale := Get_Green(Pixel);
>     Blue  : constant Scale := Get_Blue(Pixel);
> begin
>     Pixel := To_Red(Green) +  To_Green(Blue) + To_Blue(Red);
> end;
>
> or
>
> Screen.Buffer(i) := To_Red(Get_Green(Screen.Buffer(i))) +
>                                To_Green(Get_Blue(Screen.Buffer(i))) +
>                                To_Blue(Get_Red(Screen.Buffer(i)));

Maybe I'm just weird, but I happen to think that
    declare
      Pixel : Buffer_Element renames Screen.Buffer(i);
    begin
      Pixel :=
        To_Red(Get_Green(Pixel)) +
        To_Green(Get_Blue(Pixel)) +
        To_Blue(Get_Red(Pixel));
    end;
is about right.

Further, more to the point, your concept
  <id> is <lhs-expr> in <statement>
is unbracketed.  This is explicitly contrary to Ada practice.

Ada has EVERYWHERE ELSE eschewed single-statement scope limits, preferring
explicitly-bracketed scope instead.  In C, one writes

    if (condition) statement;
    while (condition) statement;
    do statement while (condition);

In PASCAL, one writes:

    if condition then statement else statement;
    while condition do statement;

(In PASCAL, repeat-until is the exception.)

In both of these languages, one frequently gets trampled by the bugs when
one needs to add a second statement to the activity.

By contrast, Ada in all cases intentionally brackets the statement bodies:

    if condition then
      statement;
      [statement; statement;]
    end if;

    while condition loop
      statement;
      [statement; statement;]
    end loop;

One would expect that any such construct as yours should use explicit
bracketing, rather than implicitly bracket to one statement.

Now consider:

    declare
      Pixel : Buffer_Element renames Screen.Buffer(i);
    begin
      Pixel.Color :=
        To_Red(Get_Green(Pixel.Color)) +
        To_Green(Get_Blue(Pixel.Color)) +
        To_Blue(Get_Red(Pixel.Color));
      Pixel.Highlight := 255 - Pixel.Highlight;
    end;

vs. your

      Pixel is Screen.Buffer(i) in
        Pixel.Color :=
          To_Red(Get_Green(Pixel.Color)) +
          To_Green(Get_Blue(Pixel.Color)) +
          To_Blue(Get_Red(Pixel.Color));

      Pixel is Screen.Buffer(i) in
        Pixel.Highlight := 255 - Pixel.Highlight;

This is precisely the kind of thing that Ada design specifically attempts to
avoid.