Re: Dynamic Variant Record Creation

[Adam Beneschan <adam@irvine.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,
>         LEX_SECTION,
>         '!',
>         '"',
>         '#',
>         '$',
>         ...etc..
>         '^',
>         '_',
>         '`',
>         LEX_SFUN3,
>         LEX_SFUN2,
>         LEX_SFUN1,
>         LEX_SFUNN,
>         LEX_FUN3,
>         LEX_FUN2,
>         LEX_FUN1,
>         LEX_FUNN,
>         ...etc...
>         LEX_EOF
>     );
>
>     type Token_Unit(Token : Token_Type := LEX_EOF) is
>         record
>             case Token is
>                 when LEX_SFUN3 | LEX_SFUN2 | LEX_SFUN1 | LEX_SFUNN
>                    | LEX_FUN3  | LEX_FUN2  | LEX_FUN1  | LEX_FUNN =>
>                     Func :      Func_Type;
>                 when LEX_IDENT | LEX_SIDENT | LEX_LNUMBER | LEX_NUMBER
>                    | LEX_HEX   | LEX_STRLIT | LEX_INLREM  | LEX_FLOAT
>                    | LEX_REM =>
>                     ID :        String_ID;                                                                                                                                                            
>                 when others =>
>                     null;
>             end case;
>         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; and it doesn't know what
the discriminants are going to be unless the value of the discriminant
is static.  That's why you get the error you're getting:

> 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.

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...  :)

                                     -- Adam