Re: for S'Image use Func??

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

Le Thu, 06 May 2010 19:10:20 +0200, Warren <ve3wwg@gmail.com> a écrit:

> I have tried to google for this and have not yet found
> a suitable answer, so I'll troll, er, poll for an
> answer here..
>
> Is there the ability to substitute your own S'Image
> function? For example, a basic interpreter might
> define:
>
>  type LNumber_Type is range 0..99_999;
>
> Can I declare..
>
>  for LNumber_Type'Image use LNumber_To_String;
>
> If so, then the question is what the signature of
> the S'Image function looks like-- is it:
As far as I could tell, your intuition is good, as there is indeed such  
things in Ada, called either representation clauses or operational  
clauses. See [ARM 2005 13.3] for more about it.

An excerpt to be kind:

[ARM 2005 13.3 (2)]
attribute_definition_clause ::=
       for local_name'attribute_designator use expression;
     | for local_name'attribute_designator use name;


[ARM 2005 13.3 (4)]
For an attribute_definition_clause that specifies an attribute that  
denotes a subprogram, the expected profile for the name is the profile  
required for the attribute.

However, it later says:

[ARM 2005 13.3 (5/1)]
An attribute_designator is allowed in an attribute_definition_clause only  
if this International Standard explicitly allows it

Comes with a tiny example:


[AARM 2005 13.3 (6a)]
Ramification: This implies, for example, that if one writes:
for T'Read use R;
R has to be a procedure with two parameters with the appropriate subtypes  
and modes as shown in

Finally, it should be checked if Image is explicitely allowed as an  
attribute designator for an operational clause (I may check later to give  
you a more formal answer to this one question).

I use representation clauses from time to time, but I've never used this  
kind of one, for the reason I give you right after now.

>  function LNumber_To_String(LNO : LNumber_Type) return String;
>
> Finally, there is actually a third question- more along
> the lines of "Should this language feature be used
> in this manner?", or is it preferable to just code your
> own along the lines of (which is what I presently use):
>
>  function To_String(LNO : LNumber_Type) return String;
>
> Inquiring minds need to know,
>
> Warren
I would say, it is more handy to define a function, because a function  
would be able to hold the exact formatting parameter your application  
requires, it would be able to hold the exact optional defaults for those  
parameters and it would better integrates with the overall general design  
of an Ada application, that, “withing” package and using renames clause.  
Using attribute, you are require to always use thye type name as a prefix  
for such things as ’Image. With function, you may “withed” the package  
defining this function, and make it part of the local scope using  
something like “function F (...) ... renames My_Package.F ...”.

More handy IMHO.

All of this providing I've understood what you were requesting for.

-- 
No-no, this isn't an oops ...or I hope (TM) - Don't blame me... I'm just  
not lucky