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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Strings with discriminated records
Date: Sun, 27 May 2018 16:50:55 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:52727
Date: 2018-05-27T16:50:55+02:00
List-Id: <comp.lang.ada>

On 2018-05-27 16:34, NiGHTS wrote:
> On Sunday, May 27, 2018 at 4:39:36 AM UTC-4, Dmitry A. Kazakov wrote:
>> On 2018-05-27 03:42, NiGHTS wrote:
>>> On Saturday, May 26, 2018 at 7:42:22 PM UTC-4, Shark8 wrote:
>>>> You can do this:
>>>>
>>>> Type Message(Length : Positive) is record
>>>>     Text  :  String(1..Length);
>>>>     -- Other components.
>>>> end record;
>>>
>>> More interesting to me would be to pass a string literal as a discriminant. Using your example, something like this...
>>>
>>> Type Message (Length : Positive; Text_Value : String) is record
>>>     Text  :  String(1..Length) := Text_Value;
>>>     -- Other components.
>>> end record;
>>>
>>> This gives me an error: Discriminants must must have a discrete or access type
>>
>> There is no constructors and no user-defined aggregates, which you are
>> actually asking for, but sometimes a constructing function does the job:
>>
>>      function Create (Value : String) return Message is
>>      begin
>>         return (Length => Value'Length, Text => Value);
>>      end Create;
>>
>> -- 
>> Regards,
>> Dmitry A. Kazakov
>> http://www.dmitry-kazakov.de
> 
> This is actually a good solution to me. I use this kind of technique often but not in the context of strings, so I find this an elegant way to solve my problem. Here is the final code for anyone who want to see it.
> 
> Type Message (Length : Positive) is record
>      Text  :  String (1 .. Length);
> end record;
>          
> function Create (Value : String) return Message is
> begin
>      return (Length => Value'Length, Text => Value);
> end Create;
>          
> M : Message := Create ("Hello World");

Some reduce it to an operator

    function "+" (Value : String) return Message is
    begin
       return (Length => Value'Length, Text => Value);
    end "+";

    M : Message := +"Hello World";

as a poor-man solution to supertyping. I.e. if it were possible to make 
Message a subtype of String (by providing conversions), you could simply 
write:

    M : Message := "Hello World";

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de