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
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