This Week on perl5-porters - 22 December 2007-29 December 2007
"Remember, everyone is a beginner, but only for a small time." --
Abigail, on why you can't please everyone, everywhere, all the time.
Topics of Interest
The summaries are back! I had meant to get this out earlier this week,
but the season's festivities got the better of me. Enjoy! -- David
Consting goodness done to excess
Robin Barker, in a remarkable coincidence of increasing the
summariser's burden, went about adding "const" goodness to XS modules
in the core distribution.
MIME::Base64 and Digest::MD5 (not applied)
http://xrl.us/bdtgg
Compress::Raw::Zlib and Filter::Util::Call (not applied)
http://xrl.us/bdtgi
Cwd and ExtUtils::ParseXS (applied, with slight turbulence)
http://xrl.us/bdtgk
IO (not applied)
http://xrl.us/bdtgn
Storable (applied)
http://xrl.us/bdtgp
Digest::SHA (applied)
http://xrl.us/bdtgr
Not to mention the judicious application of a "NUM2PTR" macro
(applied)
http://xrl.us/bdtgt
"grep" and smart match should warn in void context
Michael G. Schwern wondered why a "grep" operation performed in void
context produced no warnings. The idea being that since a grep is a
filtering operation over a list, you either want to get back some, all
or none of the list, or possibly the number of elements that matched.
But to want nothing at all was either stupid, an error, or some sort
of obfuscation.
Nicholas Clark asked Michael to write some TODO tests to nail down the
desired behaviour, but Michael went one better and produced a first
cut at a patch to do what he wanted.
Yitzchak Scott-Thoennes wasn't against the idea, but wanted to make
sure there was a simple way to disable the warning message, just in
case. Rather than introduce a new "void" keyword, Michael pointed out
that "scalar grep foo(), list()" works just as well.
obfuscated map, anyone?
http://xrl.us/bdtgv
Big slowdown in 5.10 @_ parameter passing
Yves Orton forwarded a message from the DBIx-Class mailing list, where
they had discovered with horror that
my ($x, $y, $z) = @_;
has become unacceptably slow in 5.10 (whereas a semantically
equivalent code block using "shift" ran as swiftly as ever). Dave
Mitchell discovered that the optimiser had become confused, and was
running the list assignment through the code path that spends the
extra cycles that makes sure that
($x, $y) = ($y, $x)
works as expected. That is, extra care has to be taken when there are
variables common to both sides of the assignment, otherwise things get
clobbered.
wish they had taken 5.10 for a spin earlier
http://xrl.us/bdtgx
Paul Johnson identified the patch responsible. No solution as yet.
it was just a teeny bug fix
http://public.activestate.com/cgi-bin/perlbrowse?patch=28488
Should lexicals used only once warn?
Nicholas Clark asked innocently whether it would make sense to have
the interpreter warn about lexical variables that were defined and
then never used afterwards. A volley of reasons as to why this would
not be a good idea followed, and ten days later the thread was still
bubbling along nicely. Lots of nice idioms in there worth pondering; I
liked Hugo van der Sanden's resource lock trick.
don't call us, we'll call you
http://xrl.us/bdtgz
Extra warnings for beginners
Gabor Szabo kicked off a slightly shorter thread as to whether it
would help to have extra warnings that would help beginners not fall
prey to some types of simple errors. A concensus formed rather rapidly
that this would not be a Good Idea.
use brain;
http://xrl.us/bdtg3
"Configure -Dunknown_config_var" should exit with error
Jim Cromie wrote a simple patch to make "Configure" do sanity checking
on its command line. Andy Dougherty explained just how hard it was to
do such a thing correctly, as hints files complicate the picture.
H.Merijn Brand began to work on the scripts used to build the
Configure file itself, and pulled his hair out over the bizarro coding
style used. Jim Cromie was worried that if H.Merijn and Andy were
simultaneously unable to work on shepherding the configuration
infrastructure, The Perl project would be seriously stalled.
bus fault in undisclosed location
http://xrl.us/bdtg5
Optimising opcodes
Jim Cromie had been wondering whether all the null op-codes left
behind after the optimiser was done with an op-tree could cause cache
misses due to bloat, as the CPU ran ahead of the data pipeline.
Nicholas Clark wanted to find out whether adding another pass that
threw away the dead ops, rearranging the ops into a more sensible
order and storing them with a slab allocator would be a win.
Paul Johnson pointed out that one consequence would be that some error
messages would no longer be able to relate to line number, as
currently that information is stored in the very nullops that Nicholas
was proposing to throw away.
need some running code for concensus
http://xrl.us/bdtg7
Later on in the week, Jim returned with a patch to move op-sibling
pointers out of OPs, the theory being that it reduces the optree's
cache footprint by 20%. Unfortunately the MJD advice about getting the
wrong answer as fast as possible applies, since the patch (which Jim
admits is a work in progress) causes things to dump core.
it's a start
http://xrl.us/bdtg9
Storable for bytecode?
Since the decision to jettison the Perl compiler for 5.10, perl no
longer has a bytecode loader. Nicholas observed that "Storable" can
serialise stuff, so, how much effort would it take before it could
store an optree? And, at that point, would we have a byteloader? First
up, Storable doesn't do regular expressions or typeglobs or... a whole
lot of things it turns out, all of which are vital for an optree.
Nicholas wondered whether it was feasible, easy or impossible.
Hugo said that the first step (this no doubt applies to the above
thread of optimising opcodes as well) would be to overhaul the
optimiser to separate the manditory fixups (that if absent, would
cause the code to be unrunnable) from the true optimisations.
chromatic thought that a more promising avenue would be to remove the
pessimisation in Perl 5 that makes the interpreter to poke at
everything incessantly in case there's some sort of magic behaviour
lying in ambush to do something completely different.
the last frontier
http://xrl.us/bdthb
APC now includes perl-5.10
Philippe M. Chiasson updated the APC (All Perl Changes repository) to
include the 5.10 track. Since it has been over five years since the
5.8 track was created, there was considerable head-scratching on how
to do a couple of things that had been lost in the mists of time.
Philippe was hoping that Sarathy could chip in with what he recalled
of the process.
write it down for 5.12
http://xrl.us/bdthd
Array ~~ Any
Michael G. Schwern caught wind of a thread on Perlmonks discussing the
behaviour in something like
42 ~~ ['foo', 15]
warning, which some people find rather unsettling, about "foo" not
being numeric. Michael wondered if something could be done about the
matter.
JFDWIM
http://xrl.us/bdthf
Smart matching with objects
Ricardo Signes thought that smart matching and objects didn't mix very
well as things stand, and suggested that objects participate in smart
matching only if they overload "~~". This is a pretty reasonable
suggestion, as it gives the object the power to decide how it smart
matches, rather than the operator blindly assume that anything behind
the implementation curtain is fair game.
Tels was surprised to learn that it was even possible to overload "~~"
and began to wonder whether "Math::BigInt" objects and their relatives
needed to do so. (Answer: yes, probably).
Michael thought that the path of least resistance would be to make a
smart match against an object die, unless the class had overloaded
"~~". Ricardo was a little squeamish at first, but realised that it
solves future backward-compatibility concerns nicely: no need to
suffer the result of a poor choice in what tricksy dwimmery "~~" the
porters could invent on the spur of the moment to deal with a
not-smart-match-aware object.
Larry Wall chipped in to point out that the smart match, as
implemented in Perl 5 today, is now considered a misfeature in Perl 6,
and there smart match behaves differently. That is, the match is
determined solely by the nature of the RHS argument. Which means that
if Perl 5 wants to borrow from the future and implement Perl 6 ideas,
we need to track things more closely.
electing to match
http://xrl.us/bdthh
"~~" changing behaviour after using "=="?
In a similar vein, Gabor Szabo reported being tripped up by two
scalars being smart matched, then tested for numeric equality, and
then watching a subsequent smart match return a different result. This
was in spite of the fact that the scalars had not changed value.
No-one had a really good answer, but Mark-Jason Dominus thought it
reminded him of a heisenproblem with "vec" a few years back which in
his recollection wound up labelled as, at best, a misfeature.
http://xrl.us/bdthj
In the final thread of the week concerning smart matching, Jerry D.
Hedden patched the documentation to indicate that "~~" is not a
feature.
it just is (unapplied)
http://xrl.us/bdthm
Localising $@ in a "BEGIN" block
... prevents perl from noticing errors in the block. As related by
Yves Orton, following a thread on Perlmonks. No-one ventured a reason
as to why.
looks like a bug
http://xrl.us/bdtho
Always to use strict
Steffen Müller, whose post for some reason isn't showing up on Xray
(and thus we point you to the first follow-up in the thread),
suggested that it would be a really great idea to enable "use strict"
by default for 5.11 (and hence 5.12).
While I was expecting a flamefest, surprisingly, everyone was more or
less in agreement, although a push to enable "use warnings" as well
had Abigail wringing her hands in dismay.
there is no strict
http://xrl.us/bdthq
Steffen cooked up a first cut at a patch to implement the desired
behaviour, but Rafaël declined it, explaining that it felt a bit too
hackish and suggested a better approach.
the heat is on
http://xrl.us/bdths
Weird BEGIN block CV assignment behaviour
Peter Scott reported a question seen on c.l.p.m concerning typeglobs,
"BEGIN" blocks and comments, for which he had no satisfactory answer.
Nicholas and Dave Mitchell weren't able to do much better apart from
some sympathetic waving of hands.
http://xrl.us/bdthu
Regexps are people, too
Ricardo dreamt about the possibility letting an object knowing when it
was being used in a regexp context, which would allow it to be used
with ease in pattern matches and substitutions, and splits.
my life is made of patterns
http://xrl.us/bdthw
So Nicholas set about making regexps orange (no, don't ask). As part
of the fallout, Jerry D. Hedden noticed that it caused threads tests
to fail and proposed a patch to fix it.
http://xrl.us/bdthy
Jerry then fixed the "orange" regexps in threads.
http://xrl.us/bdth2
The shaking up of the code base in this way uncovered something else
we weren't testing for, so Nicholas added a test:
http://xrl.us/bdth4
And that brought us first class regexps (like "ARRAY" and "HASH"),
rather than "Regexp" objects. Nicholas tried to change it to "REGEXP"
but too many things broke. The underlying implementation structure is
nothing more (for the moment) than a PVMG (a thing of magic) with a
pointer to the regexp structure. The quest was on to see what could be
hoisted out of the latter structure (such as reference counting) into
the PVMG, since it had a slew of unused elements begging to be used.
Yves Orton liked the idea, and felt that it could go quite some way in
reducing the complexity of the regexp engine. Nicholas continued to
chip away at now-obsolete infrastructure code... until his monitor
gave up the ghost.
There was idle chatter about renaming the REGEXP type to REGEX, RULE
and PATTERN. ORANGE and PLUM also got a look in.
he makes it look so easy
http://xrl.us/bdth6
When "strict 'refs'" is not strict enough
Mark-Jason Dominus penned a thoughtful critique of why he thought
strict 'refs' was not good enough. it was well-written enough to be
summarised as: "stringifying a reference is probably an error". If you
really need to do so, you should be explicit about it. Having it done
implicitly is probably unwise.
Michael S. made a couple of pertinent remarks as to why things had to
be the way they were.
http://xrl.us/bdth8
Patches of Interest
Jan Dubois's 64-bit fix for "Time::Local" was applied.
http://xrl.us/bdtia
And his patch to bring blead up to 5.11 on Win32 was also applied.
http://xrl.us/bdtic
Vincent Pit found a two year old typo in op.c (using "&&" instead of
"&") and offered a patch. Nicholas applied it.
http://xrl.us/bdtie
Obsolete "Nullxx" macros
Jan Dubois spotted some now-obsolete Nullxx macros in "x2p" and took
out the axe. Applied by Marcus.
http://xrl.us/bdtig
Elsewhere in the tree, Jim Cromie found a couple of Nullstrs that had
been missed in the initial cull. Also applied by Marcus.
http://xrl.us/bdtii
One of these changes may have been the reason why Jerry D. Hedden had
to fix cygwin.c again.
http://xrl.us/bdtik
Loss of context with "return do { my $x; 1}" constructs
Some years back, Vincent Pit reported a bug (#38809) that reveals how
"return do {my $x; 1}" returns undef, yet "return do {1}" returns 1
(except in taint mode).
It's all slightly confusing until you look at the output from
"B::Concise", which shows how the result from the "do" block gets
discarded. Vincent Pit proposed a simple brute-force patch that does
better, pointing out that a more subtle approach would be better, and
requested comments (but received none) on the matter.
http://xrl.us/bdtin
Replace "FH" by "my $fh" in "open"
Gabor thought it would be a good idea to use lexical filehandles in
the documentation instead of globs. Michael G. Schwern thought so too.
http://xrl.us/bdtip
He also suggesting listing where $_ is used. This was applied.
http://xrl.us/bdtir
He followed up with another patch which received a lot more attention
and came back with a revised version. Not yet applied.
Http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-12/msg00823.html
Factor out duplicate code in "struct xpv*"
Marcus Holland-Moritz was puzzled, because with a short amount of
work, he removed 377 lines of code and reduced the duplication of some
hard-to-keep-in-sync code. He wondered if it was so blindingly obvious
that no-one had spotted it before, or whether it was that way because
of some impossibly subtle edge case.
On the downside, some struct members need to be wrapped in macros. On
the other hand, some structures become identical, for instance,
"XPVIV" and "XPVUV" are defined the same way.
Nicholas Clark was worried that the extra macroification might cause
cranky compilers, such as the one on AIX, to have a fit. H.Merijn
Brand took the patch for a spin on an AIX machine he had handy and
gave it a clean bill of health.
Marcus discovered that there are 63 macros in the codebase that expand
to more than 533 bytes, the grand-daddy of them all being
"REXEC_TRIE_READ_CHAR" weighing in at 3697. In the end the patch was
applied.
http://xrl.us/bdtit
Watching the smoke signals
Smoke [5.11.0] 32712 FAIL(m) MSWin32 WinXP/.Net SP2 (x86/2 cpu)
Steve Peters looked at one of Steve Hay's smoke failures, and thought
that it would be fixed by change #32713.
http://xrl.us/bdtiv
Smoke [5.11.0] 32718 FAIL(M) OSF1 V5.1 (EV6/4 cpu)
Jarkko Hietaniemi wondered if some recent change had broken the
ability to compile the source with a C++ compiler (other than g++).
and after all that effort
http://xrl.us/bdtix
New and old bugs from RT
"pod2html": Various markup errors with (nested) definition lists (#45211)
Steve Peters applied a patch that came from Debian.
http://xrl.us/bdtiz
"CGI::Util::escape" broken for iso-8859-1 data (#49055)
Slaven Rezic reported that this was broken in 5.10. Ævar Arnfjörð
Bjarmason traced the problem down to the change in "pack" formats.
http://xrl.us/bdti3
"SIGTERM" not terminating child process (#49073)
Jerry D. Hedden noted that a "SIGTERM" was not killing its child
properly, resulting in a failure in "Time::HiRes"'s test suite.
http://xrl.us/bdti5
$_[0] seems to get reused inappropriately (#49115)
schmorp filed a report that boils down to being yet another "Bizarre
copy of ARRAY in sassign at Carp/Heavy.pm"-type bug.
need to get to the bottom of this
http://xrl.us/bdti7
"\R" doesn't backtrack into "\r\n" (#49149)
Abigail discovered the above behaviour, but Yves was of the opinion
that the Unicode specification pretty much imposes it.
http://xrl.us/bdti9
$REGMARK not available in REPLACEMENT (#49190)
Abigail also reported that $REGMARK is not available on the right hand
side of an "s///" expression.
http://xrl.us/bdtjb
So naturally, Yves fixed it.
http://xrl.us/bdtjd
nested closures keep scalars alive (#49208)
schmorp reported a bug (on 5.8) regarding closures holding into
scalars for too long, thus preventing them from being destructed
before the end of the program.
hmm...
http://xrl.us/bdtjf
Dave Mitchell confirmed that the correct, desired behaviour is seen on
5.10
one reason to make the switch
http://xrl.us/bdtjh
Perl5 Bug Summary
307 new, 1465 open makes 1772 tickets (ouch). 28 created this week, 6
closed.
http://rt.perl.org/rt3/NoAuth/perl5/Overview.html
New Core Modules
constant 1.14
Sébastien Aperghis-Tramoni pushed out a new version of "constant"
and then a second version followed hot on its heels (1.14), with
the actual fix that 1.13 was supposed to contain.
this is the one
http://xrl.us/bdtjj
SelfLoader 1.13_01
Steffen Müller made himself co-maintainer of SelfLoader,
dual-lifed it, and pushed a development version out to CPAN.
pull yourself in
http://xrl.us/bdtjm
Prior to this, Andreas König reported that the previous version
was unhappy on 5.6.x
what does '<&' on open do, anyway?
http://xrl.us/bdtjo
This is the BBC
Andreas König runs a smoke rig that checks as many CPAN modules in the
shortest time possible against the most recent version of bleadperl,
to see what breaks. Hence, Blead Breaks CPAN, or BBC.
32013 broke Apache::DB (0.13)
http://xrl.us/bdtjq
32707 broke Data::Alias, Devel::Declare anD autobox
http://xrl.us/bdtjs
32734 broke Params::Validate and Clone
http://xrl.us/bdtju
(The final item was reported by Steve Peters).
In Brief
Michael discovered that blockless greps run as fast as blocky greps,
but both are about twice as slow as using a smart match.
The color of surprise
http://xrl.us/bdtjw
Tels had some Warnocked "Math::BigInt", Ubuntu and "Module::Install"
woes
http://xrl.us/bdtjy
Marcus readied himself for the push to "IPC::SysV" 2.0
if all goes according to plan
http://xrl.us/bdtj2
Nicholas responded to a May 2005 thread and replaced the C-level
assert mechanism in the "perl" source by the standard C library assert
mechanism.
not dead yet
http://xrl.us/bdtj4
He also noticed with some dismay that there was no simple, elegant
method for detecting the building of modules under the core. The
"PERL_CORE=1" signal was used in just about every way imaginable.
Build a better core trap
http://xrl.us/bdtj6
Continuing, we find that he considers "gcc -pedantic" to be a mixed
blessing, since it unfortunately disables a few important things like
inlining macros, which kill performance.
Bondage & discipline and speed: choose 1
http://xrl.us/bdtj8
Andy Lester reported on 5.10 coverage on the tech web sites
www.news.com
http://xrl.us/bdtka
Sam Vilain, who has been working on getting the entire perl source
history into a "git" repository reported that its tag for 5.004_05
returned the wrong set of files and in tracking down the reason,
discovered that "Module::CoreList" was probably wrong and issued a
patch to fix it (unapplied).
http://xrl.us/bdtkc
Jim Cromie, noticing that the Perforce repository was close to change
32768, or 2**15, tries to flog it off to someone willing to donate
TPF, only to discover that H.Merijn Brand had carelessly burnt it
several hours previously on a minor whitespace tweak.
changes 33333 and 34567 still up for grabs
http://xrl.us/bdtke
Ricardo proposed real exceptions for 5.12, picking up on some topics
that were discussed in the run up to 5.10. None of the few people who
answered were against the idea.
something to be pursued
http://xrl.us/bdtkg
Gabor encountered a surprising test failure when building with a
nonsensical path, as in "sh Configure -Dprefix==/path/to/perl". (Note
the "=="). As the failure was somewhere in ExtUtils, Schwern sighed
and said he'd have a look at it.
http://xrl.us/bdtki
Marcus discovered a 8 year old bug (or at least a performance drain),
and quietly committed a patch to fix it.
UTF-8 file to UTF-8 scalar now 8% faster!
http://xrl.us/bdtkk
About this summary
This summary was written by David Landgren as part of the Vienna.pm
Winter of Code project. Thanks Vienna!
Weekly summaries are published on http://use.perl.org/ and posted on a
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