The Perl 6 Summary for the week ending 20030629
Welcome to the third of my US tour Perl 6 summaries. Once again I'm
pleased to report that the denizens of the Perl 6 mailing lists continue
to make the life of a touring summarizer an easy one by not posting all
that much to the lists. So, I can sit here in my room at the Shaker Inn
in Enfield and marvel at the traffic noise outside, wonder about the car
next door with the New Hampshire plates reading PERLFAN, and just
generally appreciate the loveliness of the room.
But, while I'm doing that, I should start with perl6-internals
Exceptions
At the end of last week, Dan outlined his thoughts on how exception
handling will work in Parrot. This week, people talked about it.
Discussion revolved around how much information should be attached to an
exception and how/whether we should support resumable exceptions.
http://xrl.us/j73
More on Continuation Passing
Last week I said that "I get the strong feeling that Leo Tötsch isn't
entirely happy with the new Continuation Passing Style". This week Leo
corrected me; I hadn't noticed that the speed issues had been addressed
by the latest changes to parrot (in fact the current CPS implementation
is faster than the old "invoke/ret" scheme.
Sean O'Rourke addressed Leo's problem with the Perl 6 Compiler tests
failing by saying that the compiler should really be ported to use CPS
rather than implementing a new variant of the Sub PMC that uses the old
scheme. Leo reckoned that such a port wasn't currently doable because
IMCC needed to be modified to use the CPS scheme, which would also
involve reworking the register allocator. Given Leo's prodigious rate of
implementation, this may have already happened.
http://xrl.us/ktb
IMCC/Parrot leak
Clinton A. Pierce had reported a memory leak in Parrot, but tracked it
down to a situation where he was doing:
.arg 0
call _foo
And forgetting to take the 0 off the stack. However, even after he'd
fixed that, he had segfault issues, and posted a (largish) code fragment
that tweaked the bug.
It appears that Parrot wasn't throwing warnings when stacks get to big,
just failing silently. Leo added a check for too deeply nested stacks,
which at least avoids segfaulting on logic bugs.
Leo and Dan discussed other places where such limit checking should be
put in place. Dan also muttered something about turning stack chunks
into PMCs, allowing for the garbage collection of stack frames. Leo also
muttered about the proliferation of stack implementations in Parrot
(there are five) and thinks it should be possible to have one general
stack engine.
http://xrl.us/ktc
Making "+" a unary operator
Bernhard Schmalhofer found a problem with the Perl 6 implementation.
print +42, "\n";
printed '42', but omitted the carriage return. He fixed this by making
"+" into a unary operator as well as a binary operator and sent the
patch to the list, where it was applied. Good catch Bernhard.
http://xrl.us/ktd
ParrotIO File-Descriptors
Jürgen Bömmels is in the process of porting the IO subsystem from its
current "mem_sys_alloc/free" based implementation to the sunny, garbage
collected, uplands of a PMC based implementation. However, he's run into
a problem; some of the operations in op.ops use integer File
Descriptors, grabbing information from a table in the interpreter
structure. This gets in the way of garbage collection, since any integer
could be a file descriptor.
Jürgen proposed removing the integer file descriptors and mandating that
ParrotIO PMCs be the only way to access IO (including the standard
STDIN, STDOUT and STDERR). He proposed adding "get_std[in|out|err]" ops
to get at the standard streams.
Dan suggested that Jürgen Just Do It; the current IO system being more
than slightly hackish, it was essentially put in place until something
better came along.
http://xrl.us/kte
Small Perl task for the interested
Want to get involved in the Parrot development process? Don't know much
about Virtual Machine design and implementation? Do know Perl? Dan has a
small but interesting task for you.
At present, Parrot gets built without any compiler level optimizations
turned on because files like tsq.c can't have any optimizations turned
on (tsq.c is the thread safe queue module, which is "annoyingly
execution-order-dependent because it has to operate safely as interrupt
code potentially interrupting itself").
Dan would like a version of Configure.pl which can build a makefile (or
whatever build tool we end up using) with per-C-file compiler flags, and
it needs to be possible to override those flags, per file, by the
platform configuration module.
Interested? David Robins seems to be, and he asked whether the build
system had to be makefile based. Dan says not, but the really important
thing is that the resulting build script, or the config system that
generates the script be adequately understandable/maintainable.
http://xrl.us/ktf
Scoping, ".local" and IMCC
Bugfinder General, Clinton A Pierce is getting a headache trying to
understand ".local". When he executes the following code
.local int f
.sub _main
.local int x
.sub _foo1
f=1
x=2
call _foo2
end
.end
.sub _foo2
print "f is 1: "
print f
print "\n"
ret
.end
.end
the output looks like:
f is 1: 2
Which isn't quite what one would expect.
Leo explained what's going on; essentially it boils down to issues with
register allocation not being aware of ".local" scopes. He recommended
that Clint use either true globals or lexicals instead of ".local".
Clint isn't so sure that this is a good idea, pointing out that there
are occasions when having lexically scoped names at the IMCC level as
well as at the level of lexical pads would be very useful.
"In my mind, when I saw: 1. ".local", 2. automagical register spillage
in IMCC, and 3. nested compilation units I thought I'd found Assembler
Manna." -- Clint Pierce
http://xrl.us/ktg -- Clint is puzzled
http://xrl.us/kth -- Leo explains it all
Tentative valclone patch
Luke Palmer has been thinking about value and reference objects. He
wondered if there was any value in a "valclone" operator alongside "set"
and "clone" which would allow the target PMC to decide whether to use
"set" or "clone" semantics. He also offered a patch implementing the
operator if people thought it would be useful. Leo Tötsch wasn't sure
the new operator was necessary.
Klaas-jan Stol noted that he'd encountered problems with reference/value
confusion when he'd been working on his Lua compiler, but he wondered if
the problem couldn't be solved by having a general, language independent
"argument" PMC class. (I'm not sure I understood what he meant by this
so I'm hoping for an explanation with code fragments).
http://xrl.us/kti
Events, exceptions and threads. Oh my!
There is a story that UK prime minister Harold MacMillan was asked by a
student what it was that concerned him most as Prime Minister. Mac
replied "Events dear boy, events."
Leo Tötsch laid out his thoughts and ensuing questions about Exceptions,
events and threads, and how they played together. There has been a small
amount of discussion in response to this, but I think everyone's
currently thinking hard about the issue...
http://xrl.us/ktj
CPS and the call stack
Luke Palmer wondered if there would be a standard way of inspecting the
call stack (for debugging/"caller"/etc). (I think I'm going to switch to
using the phrase 'call chain' rather than call stack, as the presence of
continuations makes the call 'stack' look pretty unstacklike...).
Leo and Dan both thought that this would be a high level language issue
rather than a Parrot issue, though Dan did note that there might be
useful things that Parrot could do to make such introspection
easier/possible.
http://xrl.us/ktk
Continuation manipulation
Leo Tötsch has been thinking about occasions when one might need to
monkey with the internals of an existing continuation (he was thinking
about the 'warnings' state...) and proposed several solutions. Dan
favoured his new opcode, "updatecc" and thought it would be good to be
able to broaden the scope of what one could update in a
continuation/context. This scared Leo somewhat, but Dan came up with
some examples of where it might prove to be useful.
http://xrl.us/ktl
Meanwhile in perl6-language
Almost nothing happened, there were all of 15 messages.
Perl 6 Daydreams
Miko O'Sullivan engaged in some summer daydreaming by asking what
everyone was looking forward to most from Perl 6. Miko himself is
looking forward to more Pure Perl modules; if Perl 6 delivers on its
performance promises then there are going to be more and more things
where implementing directly in Perl will be fast enough, and Perl is so
much easier to implement in than C...
Jonathan Scott Duff incurred Cozeny when he said that he's hoping that
by this time next year we'll have an 85% complete Perl 6 that will be
usable in production (by brave people). Simon Cozens noted that we
already have such a beast and it's called Perl 5. For some reason this
led to a new marketing slogan being proposed: Perl 6, the reconstituted
cheeseburger of programming languages. Somehow I don't think that one's
going to fly. (I just read this bit out to my wife and she says that she
really doesn't like the thought of a flying reconstituted cheeseburger,
so I think we'd best leave it at that.)
http://xrl.us/ktm
Acknowledgements, Announcements and Apologies
Tcha! I announce the retirement of Leon Brocard from his post as Perl 6
Summary Running Joke and put the right to choose the next joke up for
auction at YAPC. And what do you know, the winner of the auction
nominates Leon Brocard as the new running joke. So, settle in for
another year of desperate rationalizations for mentioning Leon in these
summaries. Who knows, maybe Leon's Parrot related workrate will go up to
such an extent that it'll be easy, but somehow I doubt it.
Thanks to, in chronological order, Adam Turoff and Lisa Wolfisch; Walt
Mankowski, Mark Jason and Laurie Dominus; Dave Adler; Dan and Karen
Sugalski; and Uri and Linda Guttman for being such fine hosts in
Washington, Philadelphia, New York, Hartford and Boston respectively.
Next time we do this, we will not be attempting to visit quite so many
cities on the Eastern Seaboard in such a short time. At one point all we
were seeing was Perl nerds and freeways in rapid succession.
As ever, if you've appreciated this summary, please consider one or more
of the following options:
* Send money to the Perl Foundation at
http://donate.perl-foundation.org/ and help support the ongoing
development of Perl.
* Get involved in the Perl 6 process. The mailing lists are open to
all. http://dev.perl.org/perl6/ and http://www.parrotcode.org/
are good starting points with links to the appropriate mailing
lists.
* Send feedback, flames, money, photographic and writing commissions,
or a cute little iPod with a huge capacity to satisfy my technolust
[EMAIL PROTECTED]
--
Piers
