[perl #75720] for(@a) optimisation causes problems

# New Ticket Created by  Father Chrysostomos 
# Please include the string:  [perl #75720]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=75720 >


$ perl -le' @a = (1,2,3); print shift @a for @a'
1
2

$ perl -le' @a = (1,2,3); print shift @a for (),@a'
1
2
3

There seems to be some optimisation for ‘for @array’ that has strange  
side-effects.

This can sometimes cause action-at-a-distance:
$node = ... expr returning an XML::DOM::Lite::Node
$node->removeChild($_) for @{ $node->childNodes };

And, apparently, I’m not the only person to have run into this. I  
stumbled across this In Silki::Web::Form 0.08 today:

sub _collapse_single_option_selects {
     my $self = shift;

     my @to_collapse;
     for my $select ( @{ $self->_dom()->getElementsByTagName 
('select') } ) {
         next if $select->id() =~ /^wpms-/;

         my @options = $select->options();

         next if @options != 1;

         push @to_collapse, [ $select, $options[0] ];
     }

     # Modifying the dom as we loop through it seems to cause weirdness
     # where some select elements get skipped.
     $self->_collapse_single_option_select( @{$_} ) for @to_collapse;
}

See that comment. A simple ‘for my $select( (), @{...} )’ would have  
worked around the problem.


Use of uninitialized value $category in concatenation (.) or string  
at /usr/local/bin/perlbug5.13.1 line 645.
Use of uninitialized value $severity in concatenation (.) or string  
at /usr/local/bin/perlbug5.13.1 line 645.
---
Flags:
     category=core
     severity=low
---
Site configuration information for perl 5.13.1:

Configured by sprout at Sun Jun  6 14:31:27 PDT 2010.

Summary of my perl5 (revision 5 version 13 subversion 1 patch  
v5.13.1-149-g6dd2be5) configuration:
   Snapshot of: 6dd2be570d715119e05672f6f0266d924022b65a
   Platform:
     osname=darwin, osvers=10.0.0, archname=darwin-2level
     uname='darwin pint.local 10.0.0 darwin kernel version 10.0.0: fri  
jul 31 22:47:34 pdt 2009; root:xnu-1456.1.25~1release_i386 i386 '
     config_args='-de -Dusedevel'
     hint=recommended, useposix=true, d_sigaction=define
     useithreads=undef, usemultiplicity=undef
     useperlio=define, d_sfio=undef, uselargefiles=define,  
usesocks=undef
     use64bitint=undef, use64bitall=undef, uselongdouble=undef
     usemymalloc=n, bincompat5005=undef
   Compiler:
     cc='cc', ccflags ='-fno-common -DPERL_DARWIN -no-cpp-precomp -fno- 
strict-aliasing -pipe -fstack-protector -I/usr/local/include',
     optimize='-O3',
     cppflags='-no-cpp-precomp -fno-common -DPERL_DARWIN -no-cpp- 
precomp -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/ 
include'
     ccversion='', gccversion='4.2.1 (Apple Inc. build 5646)',  
gccosandvers=''
     intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
     d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
     ivtype='long', ivsize=4, nvtype='double', nvsize=8,  
Off_t='off_t', lseeksize=8
     alignbytes=8, prototype=define
   Linker and Libraries:
     ld='env MACOSX_DEPLOYMENT_TARGET=10.3 cc', ldflags =' -fstack- 
protector -L/usr/local/lib'
     libpth=/usr/local/lib /usr/lib
     libs=-ldbm -ldl -lm -lutil -lc
     perllibs=-ldl -lm -lutil -lc
     libc=/usr/lib/libc.dylib, so=dylib, useshrplib=false,  
libperl=libperl.a
     gnulibc_version=''
   Dynamic Linking:
     dlsrc=dl_dlopen.xs, dlext=bundle, d_dlsymun=undef, ccdlflags=' '
     cccdlflags=' ', lddlflags=' -bundle -undefined dynamic_lookup -L/ 
usr/local/lib -fstack-protector'

Locally applied patches:


---
@INC for perl 5.13.1:
     /usr/local/lib/perl5/site_perl/5.13.1/darwin-2level
     /usr/local/lib/perl5/site_perl/5.13.1
     /usr/local/lib/perl5/5.13.1/darwin-2level
     /usr/local/lib/perl5/5.13.1
     /usr/local/lib/perl5/site_perl
     .

---
Environment for perl 5.13.1:
     DYLD_LIBRARY_PATH (unset)
     HOME=/Users/sprout
     LANG=en_US.UTF-8
     LANGUAGE (unset)
     LD_LIBRARY_PATH (unset)
     LOGDIR (unset)
     PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/ 
usr/local/bin
     PERL_BADLANG (unset)
     SHELL=/bin/bash

• [perl #75720] for(@a) optimisation causes problems by Father Chrysostomos
• Re: [perl #75720] for(@a) optimisation causes problems by Chas. Owens
• Re: [perl #75720] for(@a) optimisation causes problems by Dave Mitchell