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p5 on the JVM

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Ben Evans
August 21, 2009 00:23
p5 on the JVM
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As quite a few of you are aware, a few of us have been working on approaches
to putting perl5 on top of the JVM.

We've decided to call it quits, but I didn't want to just disappear without
explaining a little.

First off, it was a very enjoyable exercise (and very informative for those
of us working on it - just sorry we couldn't provide more useful artifacts
for people not on the project).

Secondly, we would strongly recommend that anyone who feels tempted to give
this another go read some of our meagre outputs, and perhaps contact one of

Thirdly, in many ways, I was surprised by how much could be done. Code
generation is eminently possible, and while far from perfect, the JVM really
is changing to fit the needs of more dynamic languages, (eg the forthcoming
version 3 of the JVM spec is now totally decoupled from the Java language -
the only linkage they are supposed to share is the .class format) and a lot
of smart people are working hard to make that happen.

The problem with Perl is parsing. In particular:

Nullability - - I'm not sure I've
fully grasped the full details of Jeffrey's argument as it related to
Halting yet, but the basic point about nullability as discussed by him (and
originally, I believe, Randal) stands as a big parsing problem.

This combination of the language features - "does not require function
prototypes" and "does not require brackets round function-call arguments" is
not a happy one, from the point of view of static analysis. In my view, Perl
further complicates this with the form of LIST which does not (always)
require brackets round it, and possibly the comma operator's behaviour, but
that's somewhat off topic.

Lack of clean separation between parsing phases. This is more a problem for
the automated parser-generator tools, but it does hugely increase the amount
of work required to build an alternative implementation of Perl 5. If we
can't really meaningfully use the existing (and extremely powerful) tools,
then we're back to hand porting the parser components from C to Java (or
bytecode). That's a huge barrier to entry. It's not insurmountable, the Ruby
people did it, but it was a lot of work for them, and their community was,
shall we say, somewhat more amenable to there being multiple

Existing syntax missteps - Indirect object syntax, lack of clarity of BLOCK
vs SUBREF vs closure and grep/map syntax all deserve a mention here. Now, I
realise that all of these are too deeply ingrained to be fixed, so I'm not
proposing that we do. I'm just pointing out how these language features,
some of which have much broader use cases than the problem they were
intended to solve, interact to basically make non-heuristic parsing and all
forms of static analysis (that I can think of) basically impossible.

So, those are all the reasons we stopped - too much would have to change in
order to make a different implementation possible, and we felt that there
would be too much ill-feeling generated if we were to seriously propose it.

I guess the next question is, does it matter?

To me, I think it does. You may, of course, disagree, but here's why I think

My interests have, for quite a while now, been in languages which are safe
enough for day to day use by developers of average ability, but with
features that should provoke curiousity and which contain a very large
amount of expressive power if one knows how to use it. For many years, that
language was Perl for me, and I find it sad that just as a new generation of
developers discover functional aspects, and as improvements in chip design
force us to reconsider parallelism abstractions - and I firmly believe that
the thread paradigm is fundamentally *not* safe enough for the avergae
developer to be using on a day-day basis - that Perl's relevance in that
arena should be fading, due to being trapped in its' implementation.

So there we are. Thanks to everyone who helped out - James Laver, Ovid,
Shevek and co.


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