This Week on perl5-porters - 30 December 2007-5 January 2008

This Week on perl5-porters - 30 December 2007-5 January 2008

   "That's now twice in this thread that I've been accused of favouring
   languages that I actually dislike [...] . I think the Java model of
   exception signatures is awful." Zefram (I hope you know this will go
   down on your permanent record).

Topics of Interest

Exception roles, take 1

   Ricardo Signes wrapped up the year with a first cut at doing exception
   roles in Perl (the idea being that one would manage errors with some
   formal mechanism, anything, than matching $! with regexps).

   Zefram quoted lisp back at him for more background on handling error
   conditions.

     http://xrl.us/bd45y

   The discussion continued in the new year, with people discussing how
   to avoid creating elaborate Exception hierarchies that would wind up
   looking like Java.

     ewww
     http://xrl.us/bd452

Lexicals used only once should warn (redux I)

   This thread from last week, err month, um year continued in full swing
   this week. There are two main points to come out of it. Firstly, given
   the following statement:

     my $opaque = xyzzy();

   if $opaque appears nowhere else in the current scope, it is not
   possible to determine at compile time whether or not its purpose is to
   hold a reference to an acquired resource (and thus cannot be
   considered "unused").

   Secondly, and in light of the above, is it worth expending so much
   effort to hunt down and carp about truly unused lexicals? From a
   purist's point of view, the answer is yes, but from a pragmatist's
   point of view, the better solution lies in a "lint"-like analysis.

     http://xrl.us/bd454

Lexicals used only once should warn (redux II)

   Another part of the thread from last week reminded Fergal Daly how
   much he would like, in a loop like

     for my $x (@list) { ... }

   to obtain a trace that not only shows the call stack, but also shows
   the value of $x. Even knowing that the loop was in the Nth iteration
   would be better than nothing. chromatic suspected that if code were
   written to handle that, it would come in handy for dealing with
   tail-call optimisations.

   That reminded Yves Orton about a "$^SUB" variable which would provide
   a hinting mechanism to the compiler, thereby side-stepping the issue
   of introducing unnecessary slowdowns in the general case, and putting
   the onus on the programmer to get things right.

     http://xrl.us/bd456

     RAII: not an Italian TV station
     http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization

"sub 2007{ ... goto &2008 }"

   David Nicol broke out of the monster warnings thread and wondered
   about tail recursion the light of RAII. The trouble is that an opaque
   scalar in @_ holding a resource lock would get wiped out during the
   tail recursion call. He was of the opinion that tail recursion could
   not be done automatically by the compiler, but that it could be
   possible to set it up yourself, if one was able to set up a new @_
   explicitly, just before transfer.

     finding the right syntax
     http://xrl.us/bd458

   In the new year, Aristotle Pagaltzis cautioned against diddling with
   @_ since it is built for speed and thus behaves oddly in a number of
   edge cases. Better to invent some sort of syntax that looks like a
   regular function call, and deals with @_ itself away from prying eyes.

   Jim Cromie pondered whether some sort of "pp_goto"/"pp_entersub"
   mashup would do the trick.

     http://xrl.us/bd46a

Optimising opcodes

   Rafaël Garcia-Suarez, Nicholas Clark and Paul Johnson continued to
   ponder ways to obtain line number information for warning messages if
   all the nullops were thrown away.

     pointer games
     http://xrl.us/bd46c

Negated smart match

   Ron Blaschke wanted to know why there was no "!~~" negated smart match
   operator. There's one in Perl 6. In Perl 5.10, it gets parsed as the
   slightly useless "!~ ~". In the meantime, one can get by with "!($thud
   ~~ @qux)".

     one way to do it
     http://xrl.us/bd46e

strictly by default

   The debate continued over how and when strict would be enabled by
   default in future versions of Perl. The key was to specify some sort
   of "use 5.12" pragma or a feature, but there was disagreement over
   whether they should behave the same, or differently.

     http://xrl.us/bd46g

SV leak

   Nicholas Clark looked at some code in "XS_PerlIO_get_layers" and
   realised that it was probably leaking "SV"s. This made him wonder if
   other XS code committed the same sin.

   Marcus Holland-Moritz seemed to think so, and committed change #32816
   it. And then committed #32817 when he realised that the routine's
   "dTARGET" was now unused.

   Vincent Pit had a look around and came up with a number of other
   places where the same kind of misbehaviour was occurring. He whipped
   up a patch, that Marcus applied.

     http://xrl.us/bd46i

Handling of SET magic in "mX?PUSH" macros

   After the above adventure, Marcus noted that there was no need to
   handle the setting of magic in the "mPUSH" family of macros, since
   they are creating new mortals that cannot have any magic on them
   (yet). Which makes for less make-work code.

     http://xrl.us/bd46k

MRO and "av_clear"

   Torsten Schönfeld was having problems with perl 5.10 and XS code
   diddling @ISA to change package hierarchies. As it happens, Torsten
   was using the "av_clear" API call to clear out @ISA.

   The problem is that @ISA has a certain amount of magic associated with
   it. Rick Delaney had a look at what Torsten was trying to do, and once
   he understood what was needed, was able to cook up a patch and toss in
   a couple of regression tests to wrap things up.

     introducing magic_clearisa()
     http://xrl.us/bd46n

When will perl 5.10 be stable?

   Alberto Simões asked why stable.tar.gz refers to 5.8.8 and not 5.10.0.
   A long discussion ensued. Most people were happy to accept that it is
   probably premature to label 5.10.0 to be stable, but it's going to
   happen sooner or later. So when?

   Michael G. Schwern suggested adopting Debian's
   stable/testing/unstable/experimental labels. The main problem was that
   people had difficulty trying to match Perl releases into the above
   four categories. Dave Mitchell came up with an alternative eminently
   pragmatic approach.

     5.8 is the new 5.6
     http://xrl.us/bd46p

"SvOOK()" now doesn't (ab)use "SvIVX"

   Nicholas Clark, looking more closely at how macros expand, put forward
   an alternative technique to deal with strings that get clipped from
   the beginning. Instead of recopying the string, perl has always kept
   the string as is, and moved a pointer forwards to point to the new
   beginning.

   Until now, the macro expanded to some bit-twiddling and possibly a
   function call. By rearranging things, Nicholas was able to get rid of
   the function call, but wondered if there was a way to trip things up
   because of that.

   After having played with it a bit more, Nicholas determined that it
   was simpler to store an offset.

     ook!
     http://xrl.us/bd46r

Solving the "~~" changing behaviour after using "=="

   Following on from the thread from last week where Gabor Szabo reported
   that smart match could return a differing results from the same
   inputs, Nicholas changed the behaviour in bleadperl so that "42x" (a
   numeric value with trailing garbage) never gets the IOK or NOK flags
   set. Thus solving the problem neatly.

   It turns out that doing so didn't break the test suite, but the
   question to ask is whether there were no tests for it. In which case,
   careful analysis will be required to see whether it is safe to
   backport to the 5.10 line.

     http://xrl.us/bd46t

Patches of Interest

"mg_magical()" sometimes turns "SvRMAGICAL" on when it shouldn't

   Vincent Pit detected problems in the chain of magic whereby different
   ordering of magic would produce different results. Steve Peters wanted
   to see some tests.

     http://xrl.us/bd46v

refactor "PL_opargs" generation in opcode.pl and fix "helem"

   Marcus Holland-Moritz was in the mood for adding a new op flag, and
   suffered considerable pain when he gazed upon opcode.pl, as well as
   some 8-year old code contributed by Ilya Zakharevich which he thought
   was a "can't happen" scenario.

   In tightening things up, he discovered a dormant bug that meant that
   "helem" had an incorrect specification so he corrected it.

     all applied
     http://xrl.us/bd46x

Loading a "loadable object" with a non-standard file extension

   One of things that was pushed for in 5.10 was to embed
   platform-specific decisions into DynaLoader.pm when it was generated
   during the build of Perl instead of deferring things until run-time.
   The move proved to be a shade too aggressive and broke established
   behaviour in 5.6 and 5.10.

   Jan Dubois restored the old behaviour with a patch that was applied by
   Rafaël.

     http://xrl.us/bd46z

Clean up "File::Temp" test file

   Jerry D. Hedden fixed up a leaking temporary file in
   lib/File/Temp/t/fork.t. Applied by Rafaël.

     http://xrl.us/bd463

Clean up lib/B

   He also ensured that the "realclean" target removed lib/B. Not
   applied.

     http://xrl.us/bd465

"~~" is not a feature

   Jerry also redelivered a Warnocked patch which, happily, was applied
   the second time around.

     if at first you do not succeed
     http://xrl.us/bd467

"File::Temp::_gettemp" should ignore dir -w test on Cygwin

   Jari Aalto could not install CPAN modules on Cygwin because of a
   pointless check to see whether the directory was writable (which it
   always is). Applied.

     http://xrl.us/bd469

This is the BBC

   "Params::Validate" and "Clone"

   Andreas König, Rafaël Garcia-Suarez, Nicholas Clark and Steve Peters
   had a closer look at this failure and tried to figure what could be
   done to blead to reduce the breakage. To a certain extent, however,
   some changes have been advertised for a long time, patches have been
   sent to authors of problematic modules, but few distributions have
   seen new releases.

     not much we can do
     http://xrl.us/bd47m

New and old bugs from RT

"say" behaves as just "print" on tied filehandle (#49264)

   Ambrus Zsbán noticed that "say" on a tied filehandle lacks the "\n"
   tacked on the end, and traced the problem as far as pp_hot.c but
   didn't know how to fix it.

   Schwern weighed in with a first cut at a patch. Graham Barr saw that
   it leaked. Rafaël and Nicholas started debating internals, discussing
   hitherto unknown macros (at least to the summariser). Something was
   applied, in any event

     say can you see
     http://xrl.us/bd47o

"IO::Handle" method "say" should ignore $\ (#49266)

   Ambrus, on a roll, found another edge case where "say" misbehaved.
   This was either ignored, or solved by the same patch that fixed bug
   #49264.

     say it ain't so
     http://xrl.us/bd47q

"B::Deparse" fails to deparse a reference to an anonymous hash (#49298)

   David Leadbeater noticed that "B::Deparse" was incapable of dealing
   with coderef that returns a reference to an anonymous array or hash.
   Rafaël muttered something about someone having to teach something
   about something, and then did just exactly that.

     special ops
     http://xrl.us/bd47s

"[[:print:]]" *versus* "\p{Print}" (#49302)

   According to the documentation, any "[[:...:]]" and "\p{Is....}" pair
   should match the same thing. Robin Barker showed that this was not
   always the case.

     http://xrl.us/bd47u

   segfault in 5.10 (and earlier) (#49322)

   A bug report from Will Coleda showed that

     @r=eval {@c=(@n=(1,2) && ($n[1],$n[0]))};
     @r=eval {@c=(@n=(1,2) && ($n[1],$n[0]))};

   will crash on any number of different versions of Perl.

     so that's pretty sick code
     http://xrl.us/bd47w

   Steve Peters noted that in 5.10... it still dumps core, but with a new
   error message!

     progress at last
     http://xrl.us/bd47y

Segfault with with "tie" and "STDOUT" (#49366)

   Steve Peters noted that if you are not careful when creating "tie"d
   objects that print, and the thing tied is "STDOUT", perl goes into a
   loop of infinite recursion and dumps core (after exhausting its C
   stack). Ways to have interpreter deal with the situation more
   gracefully foundered on the problem of determining the maximum stack
   size in C.

     and portably, while you're at it
     http://xrl.us/bd472

Perl5 Bug Summary

   Ticket Counts: 310 new + 1470 open = 1780 (8 created, 4 closed this
   week)

     http://rt.perl.org/rt3/NoAuth/perl5/Overview.html

New Core Modules

   Math-Complex 1.38
         documentation and test tweaks, courtesy Jarkko Hietaniemi (applied)
         http://xrl.us/bd474

   Sys-Syslog 0.24
         tests that skip, courtesy Sébastien Aperghis-Tramoni (applied)
         http://xrl.us/bd476

   constant 1.15
         tests that behave on 5.8.[0-3], also Sébastien, also applied
         http://xrl.us/bd478

   ExtUtils-MakeMaker 6.43_01
         lots of bug fixes, courtesy Michael G. Schwern
         http://xrl.us/bd48a

In Brief

   Nicholas Clark thought that having "gcc -pedantic" on by default would
   be useful more for debugging builds than production builds.

     http://xrl.us/bd48c

   Jan Dubois traced a perl 5.10 failure on Linux 2.4 to a bug report and
   its corresponding patch. Even so, he couldn't see why it caused the
   failure, but an environment variable tweak to the system provided a
   reasonable work-around.

     more fun with glibc
     http://xrl.us/bd48e

   Nicholas responded to Larry's remarks about the fact that the Perl 5
   smart match was not quite the same as the Perl 6 smart match. The
   problem is that there is insufficient cross-pollination between the
   two development camps. Certainly, there have been no patches from Perl
   6 developers to adjust the Perl 5 implementations of Perl 6 ideas to
   keep them in line with the functionality du jour.

     obscured by crowds
     http://xrl.us/bd48g

   Vincent Pit thought that "DEBUG_S" should meet the thin end of a
   chainsaw.

     remnants of 5.005 threads
     http://xrl.us/bd48i

   After having manually expanded macros once too often, Nicholas finally
   got fed up enough to write a short Perl program to automate the task.

     good laziness
     http://xrl.us/bd48k

   Rafaël killed the v-string portability warning in 5.10, declaring that
   it would no longer be present in 5.10.1.

     no-one shed a tear
     500 Can't connect to metamark.net:80 (connect: Operation not permitted)

   Robin Barker's consting goodness to "Compress::Raw::Zlib" and
   "Filter::Util::Call" were applied.

     http://xrl.us/bd48n

   Sébastien Aperghis-Tramoni delivered some small documentation tweaks,
   applied.

     http://xrl.us/bd48p

   Marcus Holland-Moritz eliminated some magic numbers in "NewOp()"
   calls. Code that invents new ops is likely to break.

     but code that invents new ops is unlikely
     http://xrl.us/bd48r

   Michael G. Schwern tweaked t/test.pl to make it resistant to changes
   to $\, $" and $,.

     no more havoc
     http://xrl.us/bd48t

   The "strict by default for 5.12" discussion got bogged down in details
   of whether it should be a feature and how should it really be enabled
   but sometimes we don't unless we do although maybe we might if we
   should.

     or words to that effect
     http://xrl.us/bd48v

About this summary

     get last week's here
     http://xrl.us/bd48x

   This summary was written by David Landgren. It does not exactly cover
   the entire week, as I want to move from Monday through Sunday to
   Sunday through Saturday. So some threads will be dealt next week.

   Weekly summaries are published on http://use.perl.org/ and posted on a
   mailing list, (subscription: [email protected]). The
   archive is at http://dev.perl.org/perl5/list-summaries/. Corrections
   and comments are welcome.

   If you found this summary useful, please consider contributing to the
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