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perldata doesn't tell what happens on list assignment, with less/more elements #16289

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p5pRT opened this issue Dec 7, 2017 · 8 comments

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@p5pRT p5pRT commented Dec 7, 2017

Migrated from rt.perl.org#132538 (status was 'resolved')

Searchable as RT132538$

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@p5pRT p5pRT commented Dec 7, 2017

From calestyo@scientia.net

This is a bug report for perl from calestyo@​scientia.net,
generated with the help of perlbug 1.40 running under perl 5.26.1.


perldata explains most things related to lists, arrays, etc. and
their assignments.

However, I feel it misses one crucial point, namely what happens
when one assigns a list/array with more respectively less elements
to a list, e.g.

@​more=(1, 2, 3, 4);
@​less=(1, 2);
$a="a"; $b="b"; $c="c";

and with that each of the following then the cases​:
($a, $b, $c) = @​more;
($a, $b, $c) = @​less;
($a, $b, $c) = (1, 2, 3, 4);
($a, $b, $c) = (1, 2);

Seems the result is, that if there is - as above - no array or hash
in the list like in​:
  ($a, $b, @​X) = ...
any additional elements on the right-hand side (be it from a list
or an array) are discarded... while if there are not enough
elements (again from a list or an array on the right-hand side),
their corresponding elements on the left-hand side list become
undef.
Especially the later, i.e. that after the assignment of a too "short"
list/array, $c doesn't retain the previous value of "c" isn't
necessarily obvious and should thus be documented :)

Cheers,
Chris.



Flags​:
  category=docs
  severity=wishlist


Site configuration information for perl 5.26.1​:

Configured by Debian Project at Tue Nov 28 17​:44​:14 UTC 2017.

Summary of my perl5 (revision 5 version 26 subversion 1) configuration​:
 
  Platform​:
  osname=linux
  osvers=4.9.0
  archname=x86_64-linux-gnu-thread-multi
  uname='linux localhost 4.9.0 #1 smp debian 4.9.0 x86_64 gnulinux '
  config_args='-Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-w2lU16/perl-5.26.1=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-z,relro -Dlddlflags=-shared -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.26 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.26 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.26 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.26.1 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.26.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl
-Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.26.1'
  hint=recommended
  useposix=true
  d_sigaction=define
  useithreads=define
  usemultiplicity=define
  use64bitint=define
  use64bitall=define
  uselongdouble=undef
  usemymalloc=n
  default_inc_excludes_dot=define
  bincompat5005=undef
  Compiler​:
  cc='x86_64-linux-gnu-gcc'
  ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
  optimize='-O2 -g'
  cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
  ccversion=''
  gccversion='7.2.0'
  gccosandvers=''
  intsize=4
  longsize=8
  ptrsize=8
  doublesize=8
  byteorder=12345678
  doublekind=3
  d_longlong=define
  longlongsize=8
  d_longdbl=define
  longdblsize=16
  longdblkind=3
  ivtype='long'
  ivsize=8
  nvtype='double'
  nvsize=8
  Off_t='off_t'
  lseeksize=8
  alignbytes=8
  prototype=define
  Linker and Libraries​:
  ld='x86_64-linux-gnu-gcc'
  ldflags =' -fstack-protector-strong -L/usr/local/lib'
  libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
  libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
  perllibs=-ldl -lm -lpthread -lc -lcrypt
  libc=libc-2.25.so
  so=so
  useshrplib=true
  libperl=libperl.so.5.26
  gnulibc_version='2.25'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs
  dlext=so
  d_dlsymun=undef
  ccdlflags='-Wl,-E'
  cccdlflags='-fPIC'
  lddlflags='-shared -L/usr/local/lib -fstack-protector-strong'

Locally applied patches​:
  DEBPKG​:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
  DEBPKG​:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
  DEBPKG​:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
  DEBPKG​:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @​INC directories.
  DEBPKG​:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
  DEBPKG​:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
  DEBPKG​:fixes/respect_umask - Respect umask during installation
  DEBPKG​:debian/writable_site_dirs - Set umask approproately for site install directories
  DEBPKG​:debian/extutils_set_libperl_path - EU​:MM​: set location of libperl.a under /usr/lib
  DEBPKG​:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
  DEBPKG​:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
  DEBPKG​:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
  DEBPKG​:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
  DEBPKG​:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
  DEBPKG​:debian/mod_paths - Tweak @​INC ordering for Debian
  DEBPKG​:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
  DEBPKG​:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules
  DEBPKG​:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
  DEBPKG​:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.26.1-3 in patchlevel.h
  DEBPKG​:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
  DEBPKG​:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN​::Distribution with correct name of html2text
  DEBPKG​:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
  DEBPKG​:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN​::FirstTime defaults with nonexisting site dirs if a parent is writable
  DEBPKG​:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize​::Storable​: respect 'nstore' option not respected
  DEBPKG​:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories
  DEBPKG​:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU​::MakeMaker honour MANnEXT settings in generated manpage headers
  DEBPKG​:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798
  DEBPKG​:fixes/autodie-scope - https://bugs.debian.org/798096 Fix a scoping issue with "no autodie" and the "system" sub
  DEBPKG​:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize
  DEBPKG​:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd
  DEBPKG​:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math​::Trig​: clarify definition of great_circle_midpoint
  DEBPKG​:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math​::Trig​: add missing SEE ALSO
  DEBPKG​:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math​::Trig​: document angle units
  DEBPKG​:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN​: Add link to main CPAN web site
  DEBPKG​:fixes/time_piece_doc - https://bugs.debian.org/817925 Time​::Piece​: Improve documentation for add_months and add_years
  DEBPKG​:fixes/extutils_makemaker_reproducible - https​://bugs.debian.org/835815 https://bugs.debian.org/834190 Make perllocal.pod files reproducible
  DEBPKG​:fixes/file_path_hurd_errno - File-Path​: Fix test failure in Hurd due to hard-coded ENOENT
  DEBPKG​:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems
  DEBPKG​:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters
  DEBPKG​:fixes/file_path_chmod_race - https://bugs.debian.org/863870 [rt.cpan.org #121951] Prevent directory chmod race attack.
  DEBPKG​:fixes/extutils_file_path_compat - Correct the order of tests of chmod(). (#294)
  DEBPKG​:fixes/getopt-long-2 - [rt.cpan.org #120300] Withdraw part of commit 5d9947fb445327c7299d8beb009d609bc70066c0, which tries to implement more GNU getopt_long campatibility. GNU
  DEBPKG​:fixes/getopt-long-3 - provide a default value for optional arguments
  DEBPKG​:fixes/getopt-long-4 - https://bugs.debian.org/864544 [rt.cpan.org #122068] Fix issue #122068.
  DEBPKG​:fixes/test-builder-reset - https://bugs.debian.org/865894 Reset inside subtest maintains parent
  DEBPKG​:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa
  DEBPKG​:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4
  DEBPKG​:fixes/json-pp-example - [rt.cpan.org #92793] https://bugs.debian.org/871837 fix RT-92793​: bug in SYNOPSIS
  DEBPKG​:debian/perldoc-pager - https://bugs.debian.org/870340 [rt.cpan.org #120229] Fix perldoc terminal escapes when sensible-pager is less
  DEBPKG​:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
  DEBPKG​:debian/configure-regen - https://bugs.debian.org/762638 Regenerate Configure et al. after probe unit changes
  DEBPKG​:fixes/rename-filexp.U-phase1 - regen-configure​: rename filexp.U to filexp_path.U, phase 1
  DEBPKG​:fixes/rename-filexp.U-phase2 - regen-configure​: rename filexp.U to filexp_path.U, phase 2
  DEBPKG​:fixes/packaging_test_skips - Skip various tests if PERL_BUILD_PACKAGING is set


@​INC for perl 5.26.1​:
  /etc/perl
  /usr/local/lib/x86_64-linux-gnu/perl/5.26.1
  /usr/local/share/perl/5.26.1
  /usr/lib/x86_64-linux-gnu/perl5/5.26
  /usr/share/perl5
  /usr/lib/x86_64-linux-gnu/perl/5.26
  /usr/share/perl/5.26
  /usr/local/lib/site_perl
  /usr/lib/x86_64-linux-gnu/perl-base


Environment for perl 5.26.1​:
  HOME=/home/calestyo
  LANG=en_DE.UTF-8
  LANGUAGE (unset)
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)
  PATH=/usr/local/bin​:/usr/bin​:/bin​:/usr/local/games​:/usr/games
  PERL_BADLANG (unset)
  SHELL=/bin/bash

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@p5pRT p5pRT commented Dec 7, 2017

From @jkeenan

On Thu, 07 Dec 2017 04​:00​:45 GMT, calestyo@​scientia.net wrote​:

This is a bug report for perl from calestyo@​scientia.net,
generated with the help of perlbug 1.40 running under perl 5.26.1.

-----------------------------------------------------------------
perldata explains most things related to lists, arrays, etc. and
their assignments.

However, I feel it misses one crucial point, namely what happens
when one assigns a list/array with more respectively less elements
to a list, e.g.

@​more=(1, 2, 3, 4);
@​less=(1, 2);
$a="a"; $b="b"; $c="c";

and with that each of the following then the cases​:
($a, $b, $c) = @​more;
($a, $b, $c) = @​less;
($a, $b, $c) = (1, 2, 3, 4);
($a, $b, $c) = (1, 2);

Seems the result is, that if there is - as above - no array or hash
in the list like in​:
($a, $b, @​X) = ...
any additional elements on the right-hand side (be it from a list
or an array) are discarded... while if there are not enough
elements (again from a list or an array on the right-hand side),
their corresponding elements on the left-hand side list become
undef.
Especially the later, i.e. that after the assignment of a too "short"
list/array, $c doesn't retain the previous value of "c" isn't
necessarily obvious and should thus be documented :)

Cheers,
Chris.

Please review the patch attached.

Thank you very much.

--
James E Keenan (jkeenan@​cpan.org)

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@p5pRT p5pRT commented Dec 7, 2017

From @jkeenan

132538-0001-Clarify-different-cases-of-assignment-to-list-of-sca.patch
From e01ff6d79771f0c623f6be7a477289c8edd26d35 Mon Sep 17 00:00:00 2001
From: James E Keenan <jkeenan@cpan.org>
Date: Thu, 7 Dec 2017 16:09:54 -0500
Subject: [PATCH] Clarify different cases of assignment to list of scalars.

Prepared in response to RT #132538.
---
 pod/perldata.pod | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/pod/perldata.pod b/pod/perldata.pod
index 9d714f2..b093e3a 100644
--- a/pod/perldata.pod
+++ b/pod/perldata.pod
@@ -778,6 +778,56 @@ As of Perl 5.22, you can also use C<(undef)x2> instead of C<undef, undef>.
 (You can also do C<($x) x 2>, which is less useful, because it assigns to
 the same variable twice, clobbering the first value assigned.)
 
+Note that assignment of one list of scalars to another list of scalars is
+B<not> the same as assigning that first list to an array.
+
+When you assign a list of scalars to an array, all previous values in that
+array are wiped out and number of elements in the array will now be equal to
+the number of elements in the right-hand list -- the list from which
+assignment was made.  The array will automatically resize itself to precisely
+accommodate each element in the right-hand list.
+
+    use warnings;
+    my (@xyz, $x, $y, $z);
+
+    @xyz = (1, 2, 3);
+    print "@xyz\n";                             # 1 2 3
+
+    @xyz = ('al', 'be', 'ga', 'de');
+    print "@xyz\n";                             # al be ga de
+
+    @xyz = (101, 102);
+    print "@xyz\n";                             # 101 102
+
+When, however, you assign a list of scalars to another list of scalars, the
+results differ according to whether the left-hand list -- the list being
+assigned to -- has the same, more or fewer elements than the right-hand list.
+
+    ($x, $y, $z) = (1, 2, 3);
+    print "$x $y $z\n";                         # 1 2 3
+
+    ($x, $y, $z) = ('al', 'be', 'ga', 'de');
+    print "$x $y $z\n";                         # al be ga
+
+    ($x, $y, $z) = (101, 102);
+    print "$x $y $z\n";                         # 101 102
+    # Use of uninitialized value $z in concatenation (.)
+    # or string at [program] line [line number].
+
+If the number of scalars in the left-hand list is less than that in the
+right-hand list, the "extra" scalars in the right-hand list will simply not be
+assigned.
+
+If the number of scalars in the left-hand list is greater than than in the
+left-hand list, the "missing" scalars will become undefined.
+
+    ($x, $y, $z) = (101, 102);
+    for my $el ($x, $y, $z) {
+        (defined $el) ? print "$el " : print "<undef>";
+    }
+    print "\n";
+                                                # 101 102 <undef>
+
 List assignment in scalar context returns the number of elements
 produced by the expression on the right side of the assignment:
 
-- 
2.7.4

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@p5pRT p5pRT commented Dec 7, 2017

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

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@p5pRT p5pRT commented Dec 8, 2017

From @cpansprout

On Thu, 07 Dec 2017 13​:18​:52 -0800, jkeenan wrote​:

Please review the patch attached.

+Note that assignment of one list of scalars to another list of scalars is
+B<not> the same as assigning that first list to an array.

I find this sentence confusing. I think the rest of your patch clarifies things sufficiently that you can just leave out this sentence.

+array are wiped out and number of elements in the array will now be equal to

*the* number

+If the number of scalars in the left-hand list is greater than than in the

s/than tha\Kn/t/

--

Father Chrysostomos

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@p5pRT p5pRT commented Dec 10, 2017

From @jkeenan

On Fri, 08 Dec 2017 00​:55​:01 GMT, sprout wrote​:

On Thu, 07 Dec 2017 13​:18​:52 -0800, jkeenan wrote​:

Please review the patch attached.

+Note that assignment of one list of scalars to another list of
scalars is
+B<not> the same as assigning that first list to an array.

I find this sentence confusing. I think the rest of your patch
clarifies things sufficiently that you can just leave out this
sentence.

+array are wiped out and number of elements in the array will now be
equal to

*the* number

+If the number of scalars in the left-hand list is greater than than
in the

s/than tha\Kn/t/

Pushed to blead with those corrections in commit 436908e. Marking ticket Resolved.

Thank you very much.

--
James E Keenan (jkeenan@​cpan.org)

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@p5pRT p5pRT commented Dec 10, 2017

@jkeenan - Status changed from 'open' to 'resolved'

@p5pRT p5pRT closed this Dec 10, 2017
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@p5pRT p5pRT commented Dec 14, 2017

From calestyo@scientia.net

Hi.

Thanks for the patch :)

My comments below​:

+Note that assignment of one list of scalars to another list of scalars is
+B<not> the same as assigning that first list to an array.

=> I also think this is confusing...

+When you assign a list of scalars to an array, all previous values in that
+array are wiped out and number of elements in the array will now be equal to
+the number of elements in the right-hand list -- the list from which
+assignment was made. The array will automatically resize itself to precisely
+accommodate each element in the right-hand list.

=> I would add here (just for convenience)​:
When you assign a list of scalars _(or an array)_ to an array

+ use warnings;
=> The documentation somewhere says this is always implicitly assumed.

+ my (@​xyz, $x, $y, $z);
+
+ @​xyz = (1, 2, 3);
+ print "@​xyz\n"; # 1 2 3
+
+ @​xyz = ('al', 'be', 'ga', 'de');
+ print "@​xyz\n"; # al be ga de
+
+ @​xyz = (101, 102);
+ print "@​xyz\n"; # 101 102

=> could also fit in 3 lines, but I'm fine with it :)
@​a = (1, 2, 3); # array of 3 elements holds​: 1 2 3
@​a = ("a", "b", "c", "d"); # array of 3 elements holds​: a b c d
@​a = (4, 5); # array of 2 elements holds​: 4 5

+
+When, however, you assign a list of scalars to another list of scalars

=> "... or an array"

, the
+results differ according to whether the left-hand list -- the list being
+assigned to -- has the same, more or fewer elements than the right-hand list.
+
+ ($x, $y, $z) = (1, 2, 3);
+ print "$x $y $z\n"; # 1 2 3
+
+ ($x, $y, $z) = ('al', 'be', 'ga', 'de');
+ print "$x $y $z\n"; # al be ga
+
+ ($x, $y, $z) = (101, 102);
+ print "$x $y $z\n"; # 101 102
+ # Use of uninitialized value $z in concatenation (.)
+ # or string at [program] line [line number].
+
+If the number of scalars in the left-hand list is less than that in the
+right-hand list, the "extra" scalars in the right-hand list will simply not be
+assigned.

=> one might add here, that 'de' is of course still evaluated, but just discarded
=> and I'd rather write either "not be assigned to anything" or "will be simply discarded".

+
+If the number of scalars in the left-hand list is greater than than in the
+left-hand list, the "missing" scalars will become undefined.

=> there are two times left-hand list... the 2nd should be right hand list
=> I'd add after the "undefined" something like "loosing it's previous value"

+
+ ($x, $y, $z) = (101, 102);
+ for my $el ($x, $y, $z) {
+ (defined $el) ? print "$el " : print "<undef>";
+ }
+ print "\n";
+ # 101 102 <undef>
+

Thanks :-)

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