undef ($a, $b, $c) doesn't work

[p5pRT]

Migrated from rt.perl.org#8039 (status was 'rejected')

Searchable as RT8039$

[p5pRT]

From @nwc10

Created by @nwc10

I was going to stick this on wishlist. Then I RTFM​:

  =item undef

  Undefines the value of EXPR, which must be an lvalue. Use only on

[at which point it implicitly excludes a list]. But why can't I do this​:

perl -we 'undef ($a, $b, $c)'

The error I get is​:

Too many arguments for undef operator at -e line 1, at end of line
Execution of -e aborted due to compilation errors.

A list is an lvalue​:

perl -lwe '($a, $b, $c) = @​INC; print $a; print $b; print $c'
/usr/local/lib/perl5/5.7.2/armv4l-linux
/usr/local/lib/perl5/5.7.2
/usr/local/lib/perl5/site_perl/5.7.2/armv4l-linux

so why can't I undef it?
[and I realise that changing undef to a list operator is not on, but if I
explicitly specific a list, surely that can work. Is there any case where
a list would currently be anything other than that syntax error]

If this is a bug, should we fix the docs to say it is known deficiency?
If this isn't a bug but a feature, should we fix the docs to make it explicit.

Nicholas Clark

Perl Info

Flags:
    category=core
    severity=low

Site configuration information for perl v5.7.2:

Configured by nick at Sat Nov 17 21:08:50 GMT 2001.

Summary of my perl5 (revision 5.0 version 7 subversion 2 patch 13060) configuration:
  Platform:
    osname=linux, osvers=2.4.5-rmk6, archname=armv4l-linux
    uname='linux bagpuss.unfortu.net 2.4.5-rmk6 #4 tue jun 19 23:26:57 bst 2001 armv4l unknown '
    config_args='-Doptimize=-O1 -Dusedevel -Uinstallusrbinperl [email protected] [email protected] -Dinc_version_list=  -Dinc_version_list_init=0 -Uuse64bitint -de'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=define
  Compiler:
    cc='cc', ccflags ='-fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O1',
    cppflags='-fno-strict-aliasing -I/usr/local/include'
    ccversion='', gccversion='2.95.4 20010319 (Debian prerelease)', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=8
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -lgdbm -ldbm -ldb -ldl -lm -lc -lcrypt -lutil
    perllibs=-lnsl -ldl -lm -lc -lcrypt -lutil
    libc=/lib/libc-2.2.4.so, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    DEVEL13048


@INC for perl v5.7.2:
    /usr/local/lib/perl5/5.7.2/armv4l-linux
    /usr/local/lib/perl5/5.7.2
    /usr/local/lib/perl5/site_perl/5.7.2/armv4l-linux
    /usr/local/lib/perl5/site_perl/5.7.2
    /usr/local/lib/perl5/site_perl
    .


Environment for perl v5.7.2:
    HOME=/home/nick
    LANG (unset)
    LANGUAGE (unset)
    LC_CTYPE=en_GB.ISO-8859-1
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/nick/bin:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:/sbin:/usr/sbin:/usr/local/sbin
    PERL_BADLANG (unset)
    SHELL=/bin/bash

[p5pRT]

From [Unknown Contact. See original ticket]

On Dec 11, Nicholas Clark said​:

A list is an lvalue​:

perl -lwe '($a, $b, $c) = @​INC; print $a; print $b; print $c'
/usr/local/lib/perl5/5.7.2/armv4l-linux
/usr/local/lib/perl5/5.7.2
/usr/local/lib/perl5/site_perl/5.7.2/armv4l-linux

I would say the elements of a list are potentially lvalues, but a list
itself is immutable.

--
Jeff "japhy" Pinyan japhy@​pobox.com http​://www.pobox.com/~japhy/
RPI Acacia brother #734 http​://www.perlmonks.org/ http​://www.cpan.org/
** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
<stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.

[p5pRT]

From @rgs

undef() is a unary operator. That means that C<undef $a, $b> will
compile, and undefine only $a. The proposed change would break that, and
make the backward compatibility police not happy.

[p5pRT]

The RT System itself - Status changed from 'stalled' to 'open'

[p5pRT]

@rgs - Status changed from 'open' to 'rejected'