Re: Concatenate enumeration

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org> writes:

> Pablo wrote:
>> type Enumerate_Type is
>>   (
>>    NONE,
>>    READY
>>   );
>> for Enumerate_Type use
>>   (
>>    NONE  => 0,
>>    READY=> 1
>>   );
>> for Enumerate_Type'Size use 1;
>> and I want to create a new type Enumerate2_Type which could be the
>> form
>> type Enumerate_Type is
>>   (
>>    NONE,
>>    READY,
>>    OFF
>>   );
>> for Enumerate_Type use
>>   (
>>    NONE  => 0,
>>    READY=> 1,
>>    OFF     => 2
>>   );
>> for Enumerate_Type'Size use 2;

Note that all of the rep clauses above are unnecessary: the language
defines the default rep of enums to be (0, 1, ...),
and defines the 'Size to be 1 and 2 for the above.

If you want to confirm the 'Size, I'd do:

    pragma Assert (Enumerate_Type'Size = 1);

for example.

>> How to I do this without having to explicit clone the first one?
>
> You can't. Enumeration types are not extensible.

Extensible enumeration types were proposed for Ada 9X,
but were dropped.

> You can do something similar in the reverse direction:
>
> type Big is (None, Ready, Off);
>
> subtype Small is Big range None .. Ready;
>
> or
>
> type Small is new Big range None .. Ready;

Right.  And Big'Size = 2, and Small'Size = 1, by default.

- Bob