For my benefit (I am not the language-design mavens some of you are)
would someone give me a code example where using a label inside a
block instruction was useful, necessary, or even reasonable. Aside
from violating many fundamentals of the Rexx Philosophy, I can't
understand what algorithmic problem would be solved by allowing such
labels.
-Chip-
On 10/23/2024 9:31 AM, Josep Maria Blasco wrote:
Hi all,
There are some ongoing changes to the ooRexx interpreter that will
strongly affect the language definition, in such a way that the
5.1.0 release may end up implementing a version of the language that
does no longer allow us to hold true what is asserted in the landing
page for the project:
"Home of the Open Object Rexx Project. ooRexx is the open source
version of IBM's Object REXX Interpreter. *It is upwardly
compatible with classic REXX and will execute classic REXX
programs unchanged*. The project is managed by the Rexx Language
Association".
In the preceding paragraph, I have highlighted the part that will
become problematic if the ongoing changes consolidate. Namely,
* Any program containing labels inside block instructions will
immediately stop working (with syntax error 47.002 for DO/LOOP,
47.003 for IF, and 47.004 for SELECT).
* Any program containing labels before the initial EXPOSE or USE
LOCAL method instructions will fail (with 99.910 for USE LOCAL
and 99.907 for EXPOSE).
Please note that _these programs will stop working even if they
never branch_ (i.e., SIGNAL or CALL) _to any of these labels_.
Normal ("classic Rexx") semantics for such labels is to treat them
as null clauses, except for tracing purposes: when TRACE Labels is
in effect, the language processor
Traces [...] labels passed during program execution. This is
especially useful with debug mode, when the language processor
pauses after each invocation or call (rexxref 2.29.1).
If the ongoing changes consolidate into the 5.1.0 release, our claim
of compatibility with classic Rexx will no longer be valid.
My impression is that these changes should be reverted, but I
understand that there has been a considerable amount of effort put
by the developers in implementing these modifications, and therefore
such a reversal should not be undertaken slightly.
Please allow me to elaborate on the background behind these changes,
to widen our perspective about the subject.
*Statement of the problem*
A label is a clause. Following TRL2 (and TRL1, in that respect),
"more than one label may precede /any instruction/" (emphasis mine).
Some interpreters seem to allow labels preceding /any clause/. To
appreciate the difference between the two concepts, please consider
the following small program:
Trace L
A: If 1 = 1
B: Then
C: Say "Hi"
Object Rexx (6.00, ArcaOS) chokes on B:, but allows A: and C: (THEN
is not an instruction by itself); Regina Rexx happily processes A:,
B: and C: (and traces them, when asked); the current version of
ooRexx refuses to run the above program, even if we eliminate the B:
label (it produces a 47.3, 'Labels are not allowed within an IF
block; found "C"').
The ANSI standard defines labels inside a block instruction as
"trace-only", and reserves errors 16.2 and 16.3 for the cases when a
CALL or SIGNAL instruction tries to target one of these labels.
The Errata for the Rexx standard explicitly corrects 6.3.2.14 and
6.3.2.19, stating "This disallows labels before the THEN keyword".
Now the question is the following:
*
*
*¿What variant of the language should ooRexx implement?*
There was some discussion in the developers list (starting at
https://sourceforge.net/p/oorexx/mailman/message/58813104/) about
whether labels inside block instructions should be allowed to be
called/branched to. The consensus was that this should not be
allowed, putting ooRexx in line with the ANSI standard in this
respect. I agree with that.
There was also a discussion about whether labels should be allowed
when/ they cannot be branched to/. The example used was relatively
ambiguous, since it used a label before a THEN keyword:
label: THEN
¿Why do I say that this is an ambiguous example? Because one might
object to disallowing such a label, a) because THEN is not an
instruction, or b) because THEN is part of an IF. Depending on how
we understand the example, we will have two different versions of
the language.
*The main point is this*
One may have good reasons to want to disallow labels before THEN
and, at the same time, think that /instructions/ inside other
instructions (i.e., /not/ clauses which are not instructions by
themselves) deserve to have labels, even if they are, as the ANSI
standard says, trace-only.
*My take is the following*
Labels before THEN, ELSE, WHEN, OTHERWISE or END should not be
allowed. All other labels should be allowed, including before EXPOSE
and USE LOCAL. SIGNALing or CALLing a label before EXPOSE or USE
LOCAL, or a label inside an IF/DO/LOOP/SELECT should produce an error.
*What do you all think?*
This is important. We are about to change the definition of the
language, making it potentially incompatible with many existing
programs.
Kind regards,
Josep Maria
_______________________________________________
Oorexx-devel mailing list
Oorexx-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/oorexx-devel
_______________________________________________
Oorexx-devel mailing list
Oorexx-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/oorexx-devel
