The Perl 6 Summary for the week ending 20030824
Another week, another Perl 6 summary. I'm running late writing this and
I don't care because I spent the bank holiday weekend at a folk festival
and didn't get back 'til Monday evening.
Predictably enough, we'll start with the shenanigans in perl6-internals.
Timely Destruction: Luke Palmer's Scheme
The timely destruction meme just won't go away. This week Luke Palmer
suggested a possible improvement to Parrot's current scheme (Parrot
keeps a counter of 'eager' objects needing timely destruction, and a
language's compiler emits conditional "sweep" opcodes at appropriate
points. In Perl's case, for instance, the conditional sweeps would be
triggered on scope exit, other languages may have different
requirements).
Luke's scheme involves terminating the DOD (Dead Object Detection) run
if every outstanding eager object is found alive at any point in the
run. For added performance, there would also be a 'high priority' flag
on all objects. The idea is that the high priority flag could be used to
ensure that, during a sweep, the paths through the object group that are
likely to lead to an eager object get checked first, hopefully ensuring
that the "sweep" gets terminated as quickly as possible in the case
where no eager objects need to be finalized.
The proposal was discussed, but I don't think anyone has gone so far as
to implement anything based on the scheme.
This discussion led Klaas-Jan Stol to ask what the heck timely
destruction was, anyway. This sparked a longish discussion, covering
what timely destruction is all about, why you'd want it, and why having
it could be problematic.
http://xrl.us/pu7
http://xrl.us/qxf
The String API
Benjamin Goldberg is concerned with 'a number of shortcomings in the
[string] API' and outlined the problems he sees, along with some
proposed improvements. Luke Palmer generally liked the ideas, but Leo
Tötsch had concerns about some of Benjamin's proposals. The discussion
that ensued got fairly technical. (One of these days I'm actually going
to go and get my head properly around the details of the various
encodings and charactersets. And then I'll probably go insane.)
Dan also thought that Benjamin was confusing language level strings with
low-level strings, and that this was a Bad Thing. He outlined the
difference between language level (PMC) and low-level (Parrot S
register) strings, and explained what each was appropriate for.
http://xrl.us/qxg
Making the Perl 6 Compiler use Continuation Passing Style
Allison Randal sent in a patch to switch languages/perl6 -- Parrot's
(incomplete) Perl 6 Compiler -- over to using Continuation Passing
Style, which is a useful step in the right direction. Leo applied it.
http://xrl.us/qxh
Fun with "search_lex"
Jos Visser (who in an earlier post confided that he's working on writing
a parrot compiler back end for his personal 'computer language
personality disorder: Comal', and that he does this for 'therapeutical
value') wants to generate some parrot code that is capable of searching
the lexical pads for the existence of a lexical. His current code tries
to look up a lexical and catches the exception when the lexical doesn't
exist, but this is apparently very slow. So, he proposes an op,
"search_lex" which, instead of throwing an exception, returns a null PMC
if the lexical is not found.
Leo Tötsch wasn't convinced by the need for the op, but he is looking
into why the exception handling route is so slow (indeed, he seems to
have found the reason, now he's trying to fix it).
http://xrl.us/qxi
Syntax Highlighting
Inspired by Luke Palmer's cunning, core.ops parsing vim syntax
highlighter, Andy Bussey wrote something similar for the KDE Kate
editor. Now, if someone could modify pasm.el, the Emacs highlighting
mode to use the autogeneration trick too...
http://xrl.us/qxj
RFC: Constant PMCs and Classes
Leo Tötsch kicked off a discussion of the handling of constants within
Parrot and proposed adding a bunch of functions for creating constant
strings (symbols?) within the core. He also proposed a system for
creating constant classes. In the ensuing discussion I think the
proposals for strings were pretty much accepted, but it's not yet known
if the proposal for constant classes is quite the Right Thing.
http://xrl.us/qxk
Should Tom Locke target parrot?
Tom Locke is about to start on a project to create a new language and
wondered if Parrot might be a good target VM for him. In short, we think
so. But we would, wouldn't we?
http://xrl.us/qxl
More fun with set/assign
The semantics of "set" in Parrot are, somewhat complex. Leo Tötsch
showed a fragment of code which got a value from out of a hash, set the
fetched value to a new value and then printed out the value from the
hash. Depending on whether the original hash entry existed or not, the
fragment of code printed different things. He wondered if things
shouldn't be fixed to make it print the same thing each time. Consensus
has not yet been reached.
http://xrl.us/qxm
Registers vs. Stacks
Tom Locke reopened the register or stack machine can of worms. He even
offered to recan them afterwards (by writing up the discussion for
adding to the FAQ). In particular, he wanted to know what we thought
Miguel de Icaza meant when he claimed that Parrot was "based on
religion". Klaas-Jan Stol pointed us at Dan's blog where he'd addressed
this issue some time ago.
http://xrl.us/qxn
http://xrl.us/qxo
Weak References
Benjamin Goldberg hoped that Parrot would have some way of creating Weak
References, as he felt that they were probably a vital feature. He
outlined a scheme for implementing them. Various people disagreed about
their level of vitality.
Dan pointed out that weak refs could be implemented with no special core
support beyond a general notification mechanism (which we're getting
anyway) and a weak reference PMC. Dan seemed to think that this had
already been discussed, but I confess that I can't remember the
discussion and, as Benjamin pointed out, it looks like Google can't
remember it either.
http://xrl.us/qxp
Print warning location
Leo kicked off a discussion of embedding debugging information (line
numbers, both for parrot code and the original high level code) in
parrot bytecode. Right now, only the slow core tracks lines within the
pasm file, and he wondered if using the "-w" switch should automatically
force using the slow core.
Nobody actually answered that question, choosing to discuss the various
means of storing meta data about line numbers and the like in the
bytecode file. Consensus hasn't been attained yet, but I think we're
getting there.
http://xrl.us/qxq
WARNING: Incompatible changes to IMCC
As he warned some time ago, Leo has finally removed the 'feature'
whereby IMCC would parse code it found outside of subs. All IMCC code
must now be contained in ".sub"/".pcc_sub"/".emit" blocks. If you're
writing code that doesn't behave properly, please fix it.
http://xrl.us/qxr
Program exit status
Leo noted that Parrot programs get commandline info passed in in P0, but
there is currently no way for a parrot program to communicate an exit
status to the shell. Leo offered up 3 different possibilities for fixing
this and asked for comments. Dan came up with a 4th option involving a
status PMC. Discussion is ongoing.
http://xrl.us/qxs
Meanwhile in perl6-language
There were all of 6 messages. Gordon Henriksen moved the set/assign
discussion over from perl6-internals, asking for clarification of what
various bits of Perl 6 should do. Essentially it boils down to 'when
should Perl 6 autovivify?'. The discussion is ongoing.
http://xrl.us/qxt
Next Week in the Perl 6 Summary
* The perl6-language list will erupt in a massive discussion of the
meaning of "my Dog $spot"
* Leo Tötsch will post precisely no patches to Parrot
* Dan will finish up the objects implementation
* Leon Brocard will say something on one of the lists
* Damian will release Perl6::Rules
* Your summarizer will have written some real Perl code and released
the code he talked about in Paris
Acknowledgements, Announcements and Apologies
Does anyone want to buy a four bedroomed house in Newark upon Trent,
dating back to the late 16th century? The house is on the market, Gill's
term starts on September 15th and so far the Estate Agents haven't sent
any prospective buyers. Well, at least it gives me time to write the
summary.
Check out http://xrl.us/mt4 for more of my writing (and thanks to
those who have already popped by).
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, requests for consultancy, photographic
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