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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Interesting article on ARG work
Date: Mon, 9 Apr 2018 21:47:59 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:51421
Date: 2018-04-09T21:47:59+02:00
List-Id: <comp.lang.ada>

On 2018-04-09 18:45, Niklas Holsti wrote:

> You seem to be suggesting a specific way to use "@" to include/exclude 
> entire statements (not text lines) from the effective code.

Right:

- statements in the context of control flow;
- declarations in the context of declarations;
- Boolean expression within an expression (replaced by False when removed).

Conditional declaration are invisible outside conditional code:

    declare
       @ X : Integer;
    begin
       X := 1; -- Illegal, X is not visible

Unconditionally declared objects are immutable within conditional code.

    declare
       X : Integer;
    begin
       @ X := 1; -- Illegal, X is read-only

Maybe this rule should be relaxed a bit.

> Could you 
> please make your suggestion explicit? For example, with regard to the 
> need (or perhaps not) to have at least a "null;" statement in each branch.

All rules regarding null, return apply:

    if Foo then
       @ null;
       -- Illegal, statement is expected
    end if;

    function Foo return Integer is
    begin
       null;
       @ return 123;
    end Foo; -- Illegal, no return statement

But this is legal:

    function Foo return Integer is
    begin
       @ return 123;
       return 122;
    end Foo;

BTW, I don't like @ much. Maybe there should be a better syntax, 
especially to support multiple conditional spaces, e.g. for different 
things to debug. A sort of brackets would work better:

    pragma Debug (Pool) do
       -- Debugging code for Poll
    end Debug;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de