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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Your wish list for Ada 202X
Date: Thu, 24 Apr 2014 20:22:24 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2014-04-24T20:22:24-05:00
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"björn lundin" <b.f.lundin@gmail.com> wrote in message 
news:cdd237eb-7cb1-404f-b18e-695cef9f2f73@googlegroups.com...
Den onsdagen den 23:e april 2014 kl. 22:28:29 UTC+2 skrev Randy Brukardt:
...
>> 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).
>
>I do not see the connection. Is it legal to use T'Image(Value)
>then why would not Value'Image be valid?
>It seems (to me) that gnat did figure that out, with Value'Img

Sorry, I was talking about the idea of user-defined 'Image for record types.

That's currently not legal for T'Image(Obj), so it's not going to be legal 
for Obj'Image either.

To make it legal is somewhat a can of worms, because visibility gets 
entwined -- especially if any automatic composition is provides (something I 
would like to see, otherwise there is little need for it to be an 
attribute).

>> (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.)
>
>I can see that. But I suggested Img/'Image only. Not 'Value.

If one supplies user-defined 'Image, we'd pretty much have to support 
user-defined 'Value as well. (You wouldn't have to use it, of course). It's 
rare that only one way I/O is needed. Of course, this ups the effort for 
composition a lot. I don't think I could support user-defined 'Image without 
user-defined 'Value -- it seems asymetric to me.

                       Randy.