Re: Your wish list for Ada 202X

["Randy Brukardt" <randy@rrsoftware.com>]

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"björn lundin" <b.f.lundin@gmail.com> wrote in message 
news:63812656-3d79-4330-8b02-1836f3669b00@googlegroups.com...
Den onsdagen den 23:e april 2014 kl. 16:06:55 UTC+2 skrev Dmitry A. Kazakov:
...
>> OK, considering no place for the sign as in T'Image(V), the image of a
>> modular and of an integer type will be same. But what about floating 
>> point
>> types:
>
>I stated discrete_types.

That would be a very weird restriction, as subtype'Image is defined for all 
scalar types (including float and fixed types).

Obj'Image almost certainly would not be defined for literals, as a literal 
cannot be the prefix of an attribute and changing the attribute machinery 
would kill the cost/benefit ratio of any such change. One could allow values 
that are names (Float'(3.14159)'Image) but there's not problem determining 
how to handle that.

Also note that the name would be "Image". GNAT uses "Img" because 
language-defined attributes cannot be redefined, but they would have much 
preferred to call it "Image". No need nor sense to use a different name 
('First can have an object or subtype prefix, so nothing new about that.)

If it sounds like I've considered this extensively, that's because I have. 
:-) Another proposal that I worked on carefully was Obj'Succ (where Obj 
could be a complex expression, which is better than Obj := Obj + 1; as that 
has a duplication of Obj.)

...
>No. Not the only reason.
>Another reason is when you work on a legacy system,
>that was implemented with Ada83.
>The system then turns out to be stubbornly vital,
>so it won't go away for many years. But there are
>few or no tagged types.

If it's still required to stay in Ada 83, then adding Obj'Image to Ada 2020 
isn't going to help any. If it's able to be changed, then there are lot of 
things that ought to be re-engineered if substantial work is going to be 
done, and using Obj'Image is down in the noise.

BTW, your proposal is a lot like the way stream attributes work, and that 
has proven to be a massive can-of-worms. The entire mess about availability 
of stream attributes comes about because of the ability of different views 
to have a different ideaa of whether an attribute is legal or not. This is 
not a trivial idea (nor is the one in AI12-0020-1).

(Also note that providing 'Image implies providing a matching 'Value, just 
like providing 'Write implies providing 'Read. 'Value is a lot messier than 
'Image, in general.)

                          Randy.