Re: Dynamic Variant Record Creation

[Warren <ve3wwg@gmail.com>]

Robert A Duff expounded in news:wccljdsnh48.fsf@shell01.TheWorld.com:

> Warren <ve3wwg@gmail.com> writes:
>>...I am simply laying out a "problem" and looking for a
>> suitable "solution".
> 
> Sure.  It's a real problem.  I've run into it myself.

The other way for me to solve this is simply provide
a discrimanant that only identifies the few variants. But 
to keep to 32-bits, I'd have to find a way to specify the
discriminant as 3-bits, which I don't think is possible.

I'm thinking of bitwise layouts like (random example):

for DeviceDetails use
  record at mod 2;
    status at 0 range 0 .. 7;
    rd_stat at 1 range 0 .. 3;
    wr_stat at 1 range 4 .. 7;
  end record;

except that the discriminant also be described somehow
as 3-bits.

> I don't see it either.  My guess is it involves creating an object
> constrained to the right discriminant, without using an
> aggregate.

I see it now.. yes, same idea.

>> 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.
> 
> One solution is to write the aggregates out at each place.

No, that won't work because after I have an "identifier", I
look it up in keyword (and function name) tables. Once it is 
recognized as a keyword, LEX_IDENT is replaced with the 
returned specific keyword id. This simplifies the parser pass
later.

The character case, is an "others =>" case. Once completed,
there will not be many aside from the usual single operators,
brackets, comma and colon etc.  There's just enough there to be
a nuisance.

> You said you wanted efficiency.  Well, this:
> 
>             T : Token_Type := Token_Type'Val(Character'Pos(Ch));
> 
> is unlikely to be efficient.  The case statement will be better
> in that regard.

If true, I'd like to know why. I can't see that in the compiled
code being much other than a move short.  If I get time tonight,
I'll investigate it.

> Another solution is to make the record non-variant.
> Store Func and Id in the same component, and use
> whatever conversions you need.  Wrap this in accessor
> functions.  

Ya, or I could use a 2005 union perhaps. I'm just trying
to keep overhead down and safety "up". This is an experimental
rewrite of a C project of mine.

Warren