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Newsgroups: comp.lang.ada
Date: Wed, 6 Jun 2018 07:18:19 -0700 (PDT)
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Subject: Re: Generic formal type with 'Image
From: ytomino <aghia05@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:52956
Date: 2018-06-06T07:18:19-07:00
List-Id: <comp.lang.ada>

On Wednesday, June 6, 2018 at 10:03:24 PM UTC+9, Alejandro R. Mosteo wrote:
> I'm pretty sure the answer is "no", but just in case:
> 
> Is there a formal for a generic that serves for any type that has a 
> predefined 'Image?
> 
> The purpose is to avoid:
> 
> generic
>     type Printable is ... -- What should go here?
>     with function Image (P : Printable) return String is <>;
> package
> 
> and then have to pass the 'Image attribute as the Image function in all 
> instantiations.
> 
> The closest thing I can think of is (<>) but that won't do for floating 
> point types.
> 
> I understand this is an unusually narrow case (I need a generic for many 
> numeric types, both discrete and floating, and this would save me some 
> typing -- that I have already spent here anyway.)
> 
> With these issues I feel a kind of overlap/missed connection between 
> attributes and interfaces.
> 
> Thanks,
> Alex.

If AI12-0020-1 (http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai12s/ai12-0020-1.txt?rev=1.7&raw=N) is adopted, 'Image will be able to be used in anywhere.

As the available but unrecommended solution, there is GNAT's Type_Class attribute.

 generic
    type Printable is private;
 package P is
    procedure Print (Item : Printable);
 end P;

 package body P is

    procedure Print (Item : Printable) is
    begin
       case Printable'Type_Class is
          when Type_Class_Floating_Point =>
             -- We have to convert from the formal private type to some float type, with some dirty hack.
             -- This is the case of using streams:
             Printable'Write (temporary-stream, Item);
             Set_Index (temporary-stream, 1);
             case Printable_Type'Stream_Size is
                when Float'Stream_Size =>
                   Float'Read (temporary-stream, Float_Var);
                when Long_Float'Stream_Size =>
                   Long_Float'Read (temporary-stream, Long_Float_Var);
                ...

Or, as disappointing answer, we have to write normally.

 generic
    type Printable is private;
    with function Image (P : Printable) return String is <>;
    with function "+" (Left, Right : Printable) return Printable is <>;
    -- Maybe you need "-", "*", "/", "<", "<=", Zero, One, etc
 package