Re: Strings with discriminated records

[NiGHTS <nights@unku.us>]

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");