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.
