Re: Ideas for Ada 200X

[Bill Findlay <yaldnifw@blueyonder.co.uk>]

As I see it there are several arguments in favour of the 'idem' proposal:

1. It lets the programmer indicate that the occurrences of the LHS in the
RHS are *necessarily* the same, and not contingently so, which makes the
code more self-documenting.

2. It removes a source of error in transcribing the LHS multiple times.

3. It allows more concisely readable code when the LHS is lengthy.

4. It provides functionality that it only partially available by means of
much clumsier renaming declarations, because not all objects can be renamed.

5. It requires the compiler to evaluate the lvalue of the LHS once and reuse
that lvalue as often as needed to evaluate the RHS. This has three potential
benefits: shorter code, faster execution, and once-only invocation of any
side effects.

6. It might make it somewhat easier for the compiler to generate
update-in-place object code, where the target architecture allows that and
where it offers a performance advantage.

7. It provides all the utility of C's multitude of combined assignment
operators with one small, compatible change to the syntax of operands.
(I specify C, rather than C++, to avoid getting into a argument about
overloading assignment).

8. It allows the expression of useful forms that C's combined assignment
operators cannot achieve (e.g. X := 1 - idem;).

-- 
Bill-Findlay chez blue-yonder.co.uk ("-" => "")

BTW I would like to emphasize that I am not wedded to the concrete syntax of
'idem', '<>', 'all' or whatever - I regard that as a minor matter to be
determined, and hope that aesthetic preferences do not cloud the wider
issue.