Re: Overloading attributes

[Shark8 <onewingedshark@gmail.com>]

On Nov 26, 5:13 pm, Matt Borchers <mattborch...@gmail.com> wrote:
> Why is it that one cannot overload the 'Image and 'Value attribute
> similar to how we can overload 'Input, 'Output, 'Read, and 'Write?  I
> think it would be nice to be able to do the following:
>
> type FOO is ...;
>
> function Image( f : FOO ) return String;
> for FOO'Image use Image;
>
> function To_Foo( s : String ) return FOO;
> for FOO'Value use To_Foo;
>
> Is there a reason that this is not allowed?  Why limit 'Image and
> 'Value to only scalar types.
> Why expose a routine to "stringify" an object through the package
> specification when a language construct already exists that could do
> this?
>
> I can also imagine how overloading 'Pred and 'Succ would be kinda nice
> for link list traversals (and possibly other linked structures):
>
> type A_FOO is access FOO;  --FOO has a field that points to next
> record via pointer
>
> function Succ( f : A_FOO ) return A_FOO;
> for A_FOO'Succ use Succ;
> --similar for 'Pred if it is a doubly linked list
>
> x : A_FOO;
> x := x'Succ;
>
> Why expose a Next routine through the package specification when a
> language construct already exists that can do this?
>
> How about other Attributes like 'Pos, 'Val, 'First, 'Last, etc?  Could
> they be overloaded on user-defined types?

You can write an function to replace 'Image, it's a rather a good
question you bring up because it can also be stated:
"why are 'Read & 'Write required to have an attribute definition
clause,
 but 'Image is not (in fact it is forbidden)?"