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From: "Chad R. Meiners" <crmeiners@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Ideas for Ada 200X "left hand side" repeater.
Date: Wed, 18 Jun 2003 19:50:27 -0400
Organization: Michigan State University
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Date: 2003-06-18T19:50:27-04:00
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"John R. Strohm" <strohm@airmail.net> wrote in message
news:bcqp5o$coi@library2.airnews.net...
>
> 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.

I would definitely agree given that more than one variable needs to be
declared, exception handling is needed or more than one assignment needs to
be made.  However for one 'rename' with one assignment statement, I have
always found a block statement annoying.

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

Actually the concept is more like
<id> is <variable_name> in <assignment_statement>

> Ada has EVERYWHERE ELSE eschewed single-statement scope limits, preferring
> explicitly-bracketed scope instead.

Scope is ended by the semicolon like many other constructs in Ada.

>In C, one writes

Irrelevant, the form explicitly forbids multiple statements hence the
<id> is <variable_name> in <assignment_statement>
                                                           ^
                                                     an assignment statment
NOT a statement

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

There is no bracketing.  The id is only for the single statement by form.

> 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;

Related multiple statements do deserve a block.

> 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;

First let us fix the example

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

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

or

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)),
                    Highlight => 255 - Pixel.Highlight
                  );

The idea was to simplify certain assignment statements; the syntax should
not be used as a replacement of blocks.  It does not make sense to use
multiple renamed assignment statement for the same variable name.  The block
statement will win out in that case.

I was suggesting an alternative syntax for the

Var := {} + 1;

suggestion in the hopes that if someone wanted to suggest such an addition
to the language that it would encourage a readable syntax in that
suggestion.