Several longs threads will be summarized for your enjoyment and education in this week's P5P summary. Learn about how new features are added in perl! How to enhance a core module like Carp! How to change the implementation of a command-line switch! How to change the semantics of require()! How to introduce a new warning! And, last but not least, how to modify the perl parser itself! Ain't it exciting ?
Brent Dax proposed a patch to the Carp module, to allow
use Carp 'die';
use Carp 'warn';
to install references to cluck() and confess() in the $SIG{__DIE__} and
$SIG{__WARN__} handlers, respectively.
Michael G Schwern commented that overriding warn() and die() in the
package that uses Carp may be more useful, since signal handlers are
global.
Hugo van der Sanden made a similar remark : changing $SIG{__DIE|WARN__}
this way feels like pragmatic behaviour, expected to be reversible by a
simple no Carp '...'. Hence he rejected the proposed patch to avoid
confusion.
Matt Sergeant asked why the behavior of require is different when it's
followed by a string or by a bareword, as in
require IO::Socket;
require 'IO/Socket.pm';
Tim Bunce explained that the bareword argument is intended to provide a package name, while the string argument is intended to provide a file name. Jan Dubois helpfully added that this enables to distinguish between the two forms
require 'Foo'; # loads a file Foo
require Foo; # loads a file Foo.pm
Michael G Schwern then pointed out his UNIVERSAL::require module, that
provides an equivalent syntax Foo->require().
Jos Boumans then proposed to provide a new function, via a carefully named module, to load modules and files at runtime with an appropriate amount of DWIMmery.
Eduardo Pérez Ureta remarked that the -i command-line switch, used in
one-liners to edit files in place, first performs an unlink of the
original file (after having opened it for reading), then reads it while it
writes to a new file, just opened for writing under the same name.
Potentially, this can cause some data to be lost, when some failure occurs.
It was pointed out that doing the rename by hand, or using a backup file
(as with -i.bak) makes the operation more reliable ; however that
doesn't necessarily mean that -i by itself shouldn't be made more
reliable if it's reasonably possible. Eduardo proposed that a temporary
file should be used for output instead. This change also would make
possible to use -i without a backup extension on Windows systems, where
it's not possible to unlink an opened file.
During the discussion, I recalled a little-known feature of -i,
described in perlrun : its ability to create backup files in any
directory, with the syntax -i/tmp/*.bak.
In any case, true atomicity for every possible failure mode can't be achieved. Mark Jason Dominus submitted a bug report for the universe.
Yves Orton asked for a way to determine if a sub was called as a subroutine or as a method.
Apparently there is no way to get this information from perl : digging
into the internals is necessary. Richard Clamp provided an update to his
Devel::Caller module that adds a called_as_method() function, to answer
this exact question.
? : Stephen McCamant provided a one-line patch to the perl parser that
modifies the way the ? ternary operator binds its second argument.
In other words : currently the following are syntax errors
$a = $b ? $c, $d : $e;
$a = $b ? $c and $d : $e;
because the precedence of the comma-operator and of textual logical operators are very low. However it's possible to parse them unambiguously, and that's what Stephen's patch does, without introducing any backward incompatibility (if I'm not mistaken).
Rafael Garcia-Suarez proposed a new compile-time warning for Perl, to detect potential misleading uses of split(), namely when the first argument is written as a string (but interpreted, as always, as a pattern.) The canonical example is
my @array = split '|', $string;
that doesn't probably do what was meant.
It appears that running regression tests with this new warning reveals
that eleven core modules use this syntax (sometimes unreasonably, as in
split '\.', $string). Brent Dax suggested that the warning should be
triggered only on strings with metacharacters, the other cases not leading
to confusion. Hugo liked this last idea, though being still dubious that
the benefits outweigh the irritation.
Ronald Otto Valentin Fischer reported bug #18479, about use strict
'subs' not detecting barewords in an eval("")'d BEGIN block.
H.Merijn Brand reported that the test t/io/openpid.t began to hang on AIX 4.3.3. No workaround is known, so no smoke reports have been produced for this system.
Bugs #18489, #18579 and #18571 are about small snippets of code that make perl dump core. (In fact, the first two are equivalent bugs.) They all involve modifying an array while a foreach loop iterates over it (or kind of). Dave Mitchell commented : in the longer term, Perl should be fixed so that it doesn't coredump in such situations (but this is hard to do efficiently).
Bug #18573 demonstrates a strange memory allocation problem : eval
q("\c") produces an Out of memory error.
Joost van Baal and Slaven Rezic tried to work out a solution to a Sys::Syslog portability problem on Solaris (bug #18180).
Dave Mitchell proposed a new version of his patch to allow eval("") to see
the full enclosing lexical scope. This patch follows a discussion about
lexicals in evals that occured two weeks ago.
Rafael Garcia-Suarez fixed bug #17920, a case of parser coredump, and added a new test file t/comp/parser.t aimed at testing that syntax errors are correctly reported. He moved also some tests there from the infamous t/run/fresh_perl.t (aimed at testing things that used to dump core, a sloppy classification for tests indeed.)
This summary brought to you by Rafael Garcia-Suarez, both on http://use.perl.org/ and via a mailing list, which subscription address is perl5-summary-subscribe@perl.org . Comments and corrections are, as always, welcome.
That was a long time ago. I got an O'Reilly C book and taught myself C in order to provide the patch. You should all consider yourself fortunate that Sarathy rewrote it