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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Dynamic Variant Record Creation
Date: Tue, 16 Mar 2010 18:39:18 -0500
Organization: Jacob Sparre Andersen
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Date: 2010-03-16T18:39:18-05:00
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"Adam Beneschan" <adam@irvine.com> wrote in message 
news:c3fa9a0f-99c7-4fff-9310-7e4d769065db@s25g2000prd.googlegroups.com...
...
>By the way, now that Ada 2005 has the <> construct for aggregates,
>it's just occurred to me that maybe 4.3.1(17) can be relaxed a bit, to
>make it legal to specify a record aggregate with a nonstatic
>discriminant for a variant record, *if* the only component
>associations for aggregates are components that are not in variant
>parts *and* there is an others=><> in the aggregate (or something
>along those lines).  I don't know whether it's worthwhile, though.
>(It wouldn't help too much in this exact example, since there are no
>non-variant components, but if it were expanded to include, say, a
>source file name, line number, and column number for each Token_Unit,
>then there would be some benefit.)  I'll consider making a proposal
>for this, depending on how loud a groan Randy, Bob, etc., make when
>they read this idea...  :)

GROAAANNN!!!  :-)

I'd like to see a solution to this problem, but I don't think this is it. 
The problem is that the compiler wouldn't know what components to generate, 
so it would effectively have to generate a giant case statement:

X := (Token => T, others => <>) would become:

case T is
   when LEX_ID => X := (Token => LEX_ID, String_Id => <>);
   when '!' => X := (Token => '!');
   ...
end case;

The compiler could combine similar variants, I guess, but it often doesn't 
help much. And it looks like a very complex mess.

                                   Randy.