Re: Parameterised 'Image Attributes

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2023-08-20 09:53, G.B. wrote:

> Could there be a language defined type F whose purpose is to support
> the description of formats? Objects of type F would "configure"
> what 'Image does when computing a representation of a date, a number, ...

Not without multiple dispatch support and classes:

    Type x Format [ x Target ]

Otherwise you get an untyped mess as in C:

    printf ("%s", 123);

In the case of 'Image the dispatch is hard-wired. The compiler generates 
it according to one of built-in classes like 'integer type'. So yes it 
would be no problem to add parameters specific to each of the classes as 
well as common parameters like padding or alignment inside a field. But 
it will never ever happen.

You seem suggesting a class-wide parameter type instead:

    type Format_Type is tagged record
       Width     : Natural := 0;
       Alignment : Alignment_Type := Left;
       Padding   : Character := ' ';
    end record;
    type Integer_Format is new Format_Type with record
       Plus_Sign : Boolean := False;
       Base      : Base_Type := 10;
    end record;

    X'Image (Format => Format_Type'Class)

This still requires a change that will be outright rejected on highest 
philosophical grounds. (:-))

However with a Format_Type you do not need 'Image. You can simply use a 
binary operation, e.g.

    function "/" (Value : Integer; Format : Integer_Format)
       return String;

So would do

    Put_Line ("X=" & X / (Width=>10, Padding=>'0', Alignment=>Right));

instead of

    Put_Line ("X=" & X'Image (Width=>10, Padding=>'0', Alignment=>Right));

Of course it must be generic, which kills all fun.

Ergo

1. Compiler magic is necessary because the language type system is too 
weak to express things like formatting.

2. No proposal however useful and reasonable will survive ARG because of #1.

3. Use a library that does the stuff. E.g.

    http://www.dmitry-kazakov.de/ada/strings_edit.htm#Integer_Edit

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