Re: accesing internal codes used for an enumeration

[Marius Amado Alves <alves@systran.lu>]

> I am trying to implement a lexicon (in the database sense) as an
> enumeration with a representation clause.
> 
> Example lexicon:
> 
>   Name <-> Code
>   -------------
>   Ping      123
>   Pong      456
> 
> Implementation:
> 
>   type Names is (Ping, Pong);
>   for Names use (Ping => 123, Pong => 456);
> 
> I found out the hardway that attribute Pos does not return the internal
> code.  The ARM says this at 13.4(11), and tells you to use
> Unchecked_Conversion instead--but does not instruct you on the types of
> the conversion.  Where might one learn that?  Thanks.

Replying to myself.  Spent some time on it, so perhaps it is also a result
of some value to you.  It is tested.

So we have:

  type Names is (Ping, Pong);

Now a representation type i.e. a type for the internal codes must be set
up e.g.

  type Codes is Integer range 0..999;

Then the representation clause

  for Names'Size use Codes'Size;

provides for clean instantiantions of the conversion functions:

  function Code is Unchecked_Conversion(
    Source => Names,
    Target => Codes);

  function Name is Unchecked_Conversion(
    Source => Codes,
    Target => Names);
 
(Integer could be used for Target above, but in that case the compiler
may--most correctly--warn about differing sizes.)

The representation clause above also ensures at compile time (I think)
that the actual internal codes are good:

  for Names use (Ping => 123, Pong => 456);

Or, just to be sure:

  for Names use (Ping => Codes(123), Pong => Codes(456));

So now we can go from Names to Codes e.g.

  Ping_Code := Code(Ping); -- Ping_Code is 123

and vice-versa e.g.

  Ping_Name := Name(123); -- Ping_Name is of course PING

And voila, a lexicon with no arrays, sparse or otherwise :-)  Throw in an
efficiency comparison with the latter idiom and it's Ada Letters stuff:
any volunteer for coauthoring this part? ;-)

-- 
Marius Amado Alves
 alves@systran.lu
Project Tradaut-Pt
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