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From: Warren <ve3wwg@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Dynamic Variant Record Creation
Date: Tue, 16 Mar 2010 20:01:20 +0000 (UTC)
Organization: A noiseless patient Spider
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Date: 2010-03-16T20:01:20+00:00
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Adam Beneschan expounded in
news:c3fa9a0f-99c7-4fff-9310-7e4d769065db@s25g2000prd.googlegroups.com: 

> On Mar 16, 10:11�am, Warren <ve3...@gmail.com> wrote:
>> I am trying to solve a lexer Ada design problem, using a
>> variant record (Token_Unit) definition:
>>
>> � � type Token_Type is (
>> � � � � LEX_000,
>> � � � � LEX_BINARY,
>> � � � � ...etc...
>> � � � � LEX_EOF
>> � � );
>>
>> � � type Token_Unit(Token : Token_Type := LEX_EOF) is
>> � � � � record
...
>> � � � � end record;
>>
>> � � procedure Get_Token(Object : in out Lexer; Token : out Token_Unit
> ) is
>>
>> � � � � procedure Emit_Token(T : Token_Type) is
>> � � � � � � T : Token_Type := Token_Type'Val(Character'Pos(
> Ch));
>> � � � � begin
>> � � � � � � Token := ( Token => T );
>> � � � � end;
>>
>> � � � � ... � � � �
>>
>> � � end;
>>
>> In the above, I need to create a Token_Unit from � � � � � �
>> a single character (there is no "id" in this case).
> 
> And how is the compiler supposed to know that there is no "id"?
> 
> When you use a record aggregate, you need to specify values for every
> component of the record.  If the record has a variant part, then the
> compiler won't know what components have to be specified unless it
> knows what the discriminant is going to be;

Of course. But it is not conceptually inconceivable to 
check this at runtime and realize that the value I handed 
out will work with what was provided. If not, then
raise an exception.

The point here is that there is a wide number of discriminant values
that map to the same layout.

Now I don't expect anyone to change the rules in Ada to accomodate
my request. I am simply laying out a "problem" and looking for a
suitable "solution".

>> But the issue is this: � �
>>
>> *.adb:339:33: value for discriminant "token" must be static
>> *.adb:339:33: "T" is not static constant or named number (RM 4.9(5))
> 
> You can't get what you want with an aggregate, but Georg's solution
> will give you some of what you want, at least.  However, this will
> leave all the components (besides the discriminants) uninitialized,
> and you'll need to assign values into them.

I don't currently see "Georg's solution" post, but perhaps it will
arrive soon.

> 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.

As I originally prototyped it, I just shoe-horned it into one 
value with nasty C-like conversions. But I am looking for a cleaner 
"ada way" to do this.

> ...  I'll consider making a proposal
> for this, depending on how loud a groan Randy, Bob, etc., make when
> they read this idea...  :)
> 
>                                      -- Adam

I wasn't looking for compiler-implementation/language amendments.
Just looking for good advice on how others would takle this problem.
Perhaps the full case statement is the right way, but I gotta say
that it puts a damper on readability.

Warren