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From: "John R. Strohm" <strohm@airmail.net>
Newsgroups: comp.lang.ada
Subject: Re: Ideas for Ada 200X
Date: Sat, 31 May 2003 16:38:19 -0500
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"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wccbrxic21e.fsf@shell01.TheWorld.com...
> Bill Findlay <yaldnifw@blueyonder.co.uk> writes:
>
> > An "idem" feature, meaning, "the same as the destination" and used thus:
> >
> > 1.  I := (idem + 1);  -- or perhaps: (<> + 1); or maybe: (all + 1);
> >
> > 2.  I := (1 - idem);  -- N.B. not possible using any ?:= operator
> >
> > 3.  X := 0.5*(idem + a/idem);
>
> I like it.  I'm not sure I like the word "idem", but I like the concept.
> It serves the same purpose as pronouns in English (not that English is
> a good example of a "safe" language).

OK, so what are the implications for parsing expressions?

For example:

What does
  I := idem + idem;
mean?

How about

  do_some_proc(foo, baz, idem);

Does "idem" refer back to foo, baz, or do_some_proc?

Should we allow, or not allow,

  J := idem;

Why or why not?  What are the semantics?

In particular, consider:

DEVICE_DATA_REGISTER : BYTE;
for DEVICE_DATA_REGISTER use at 16#4000_00F0#;
   ...
DEVICE_DATA_REGISTER := 16#40#;
DEVICE_DATA_REGISTER := idem;
DEVICE_DATA_REGISTER := idem + 1;

How many times should DEVICE_DATA_REGISTER be *read* by the above code
fragment?  Why?