Re: Why constant components are not permitted ?

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Fri, 25 Nov 2011 07:56:03 +0100, Niklas Holsti  
<niklas.holsti@tidorum.invalid> a écrit:

> On 11-11-24 19:14 , David Sauvage wrote:
>> Hello,
>>
>> given the following :
>>
>> type Object is record
>>     Tag : constant String := "key"; -- GNAT compilation failed :
>> constant components are not permitted
>> end record;
>>
>> In my case I find it useful to declare my Object Tag component as a
>> constant, but it is not permitted, I would be interested to learn
>> why ?
>
> It seems no earlier responder has understood why you want to have a  
> constant component, nor do I, so I'll ask directly: Why do you think a  
> constant component would be  useful?
The same question for me. A record, is not an interface, that's an  
implementation solution. One want to use a record to store some aggregated  
stuff, thus its name, “record”.

If a type T is implemented as a record, the interface should not expose  
the record (as with any implementation details, except if part of an  
expected specification). If T is to be associated with a constant, the  
constant could be implemented as a function getting a T as parameter, and  
returning that constant.

Unless the OP was to reinvent a wheel already provided by Ada, which is  
record discriminant. If that constant is a kind of characterization of  
instances of T, then a discriminant is the obvious choice.

-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: [Epigrams on Programming — Alan J. — P. Yale University]