Re: Variant Record Component

[Jeffrey Carter <jrcarter@acm.org>]

John Harbaugh wrote:
> 
> My question has more to do with understanding what language-level tools
> that are available.  Ada is pretty slick at testing membership of a
> value within/without a subtype range.  I was wondering if there was
> something analogous for variant records.  I'm getting the impression
> that the answer is no.  Yes?

No. Subtype membership is defined for all types. For a variant record 
type, you can say

type Disc is (Alpha, Bravo, Charlie);

type Variant (D : Disc) is record
    case D is
    when Alpha =>
       ...
    ...
    end case;
end record;

subtype Variant_Alpha is Variant (D => Alpha);

V : Variant := Some_Function;

...

if V in Variant_Alpha then

This will work if you've done a good design, and each Disc value has its 
own variant part. If most of the variant parts have multiple 
discriminant values then this is less useful.

I once saw a project (Ada 83, pre 1995) that had problems because they'd 
done something like

type Disc is range 1 .. 100;

type Variant (D : Disc) is record
    case D is
    when 1 .. 10 =>
       ...
    when 11 .. 20 =>
       ...
    ...
    end case;
end record;

What they really had was a set of about 10 main classes, each of which 
had a specific ID for one of about 10 specific things. What they should 
have done was something like

type Disc is (One, Two, ..., Ten);
type ID_One is (One_One, One_Two, ..., One_Ten);
type ID_Two is (Two_One, Two_Two, ..., Two_Ten);
...
type ID_Ten is (Ten_One, Ten_Two, ..., Ten_Ten);

-- The project would have had more meaningful names,
-- but since it was a classified project I can't tell
-- you what they would have been.

type Variant (D : Disc) is record
    case D is
    when One =>
       One_ID : ID_One;
       ...
    when Two =>
       Two_ID : ID_Two;
       ...
    ...
    end case;
end record;

-- 
Jeff Carter
"C++ is like giving an AK-47 to a monk, shooting him
full of crack and letting him loose in a mall and
expecting him to balance your checking account
'when he has the time.'"
Drew Olbrich