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Id: 132537
Status: open
Priority: 0/
Queue: perl5

Owner: Nobody
Requestors: calestyo [at] scientia.net
Cc:
AdminCc:

Operating System: (no value)
PatchStatus: (no value)
Severity: low
Type: unknown
Perl Version: (no value)
Fixed In: (no value)



Date: Thu, 7 Dec 2017 03:02:30 +0100 (CET)
To: perlbug [...] perl.org
Subject: possible documentation error in perldata
From: calestyo [...] scientia.net
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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. ----------------------------------------------------------------- Hi. https://perldoc.perl.org/perldata.html says: Show quoted text
>LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, >each element of the list is evaluated in list context, and the resulting list >value is interpolated into LIST just as if each individual element were a member of LIST.
This seems to be wrong, in the sense, that the list itself doesn't mandate its elements to be evaluated in list context. It's rather the context in which the list is evaluated which determines the context which can be scalar or list contect. E.g. $ perl -de1 Loading DB routines from perl5db.pl version 1.51 Editor support available. Enter h or 'h h' for help, or 'man perldebug' for more help. main::(-e:1): 1 DB<1> @a=("a", "b", "c", "d"); DB<2> @b=(1, 2, @a); DB<3> print "@b"; 1 2 a b c d DB<4> $b=(1, 2, @a); DB<5> print "$b"; 4 DB<6> @a=("a", "b", "c", "d", "e", "f"); DB<7> $b=(1, 2, @a); DB<8> print "$b"; 6 DB<9> When doing $b=(1, 2, @a); the list is evaluated in scalar context and thus $b gets the last element of the list assigned. If the list itself, i.e. it's elements, would be evaluated in list context, as the above text claims, the last element should become "d", however it seems to be evaluated in scalar context, which means @a evaluates to the length of the array a, which is then also the last element of the list, being assigned to $b. Cheers, Chris. ----------------------------------------------------------------- --- Flags: category=docs severity=medium --- 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
Subject: Re: [perl #132537] possible documentation error in perldata
From: Dave Mitchell <davem [...] iabyn.com>
To: perl5-porters [...] perl.org
Date: Fri, 8 Dec 2017 09:12:18 +0000
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On Wed, Dec 06, 2017 at 06:02:52PM -0800, via RT wrote: Show quoted text
> https://perldoc.perl.org/perldata.html says:
> >LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, > >each element of the list is evaluated in list context, and the resulting list > >value is interpolated into LIST just as if each individual element were a member of LIST.
> > This seems to be wrong, in the sense, that the list > itself doesn't mandate its elements to be evaluated > in list context.
It does by definition: Show quoted text
> When doing $b=(1, 2, @a); the list is evaluated in scalar context
No, in scalar context, there isn't a list; instead there is merely a sequence of comma operators (which return their RH argument), with parentheses added for the purpose of precedence. -- Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
From: Christian Millour <cm.perl [...] abtela.com>
Subject: Re: [perl #132537] possible documentation error in perldata
Date: Fri, 8 Dec 2017 10:47:49 +0100
To: perl5-porters [...] perl.org
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Le 08/12/2017 à 10:12, Dave Mitchell a écrit : Show quoted text
> On Wed, Dec 06, 2017 at 06:02:52PM -0800, via RT wrote:
>> https://perldoc.perl.org/perldata.html says:
>>> LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, >>> each element of the list is evaluated in list context, and the resulting list >>> value is interpolated into LIST just as if each individual element were a member of LIST.
>> >> This seems to be wrong, in the sense, that the list >> itself doesn't mandate its elements to be evaluated >> in list context.
> > It does by definition: >
>> When doing $b=(1, 2, @a); the list is evaluated in scalar context
> > No, in scalar context, there isn't a list; instead there is merely > a sequence of comma operators (which return their RH argument), with > parentheses added for the purpose of precedence. >
FWIW I find the sentence above clearer than the current wording in perldata 677 =head2 List value constructors 678 X<list> 679 680 List values are denoted by separating individual values by commas 681 (and enclosing the list in parentheses where precedence requires it): 682 683 (LIST) 684 685 In a context not requiring a list value, the value of what appears 686 to be a list literal is simply the value of the final element, as 687 with the C comma operator. For example, 688 689 @foo = ('cc', '-E', $bar); 690 691 assigns the entire list value to array @foo, but 692 693 $foo = ('cc', '-E', $bar); 694 695 assigns the value of variable $bar to the scalar variable $foo. YMMV. From experience this is a significant hurdle for beginners. @array = (...): # OK %hash = (...); # OK $scalar = (...); # Possibly not what was intended, # consider $scalar = [...] instead ?
Subject: Re: [perl #132537] possible documentation error in perldata
To: perlbug-followup [...] perl.org
Date: Sun, 10 Dec 2017 05:11:24 +0100
From: Christoph Anton Mitterer <calestyo [...] scientia.net>
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On Fri, 2017-12-08 at 01:12 -0800, Dave Mitchell via RT wrote: Show quoted text
> It does by definition: > No, in scalar context, there isn't a list; instead there is merely > a sequence of comma operators (which return their RH argument), with > parentheses added for the purpose of precedence.
That's at least not how the text in perldata or perlop does it make look like. It's rather written as it's still a list (and it even says, that all elements are actually evaluated), just that it doesn't "return" the whole list, but rather only it's last element. - (x, y, z) seems to be clearly what is defined to be a list, every beginner will consider it as such, regardless of its context - other text places say that in both contexts (scalar/list) all elements are evaluated, just that in scalar context everything but the last is discarded So conceptually it still seems to be a list, and then the text that I was mentioning is still ambiguous... People would think (by operator precedence): 1. evaluation of the x, y, z (i.e. the list) and the text says always in list context 2. for list context = return all evaluated elements as a list for scalar context = return only the last element (which would however, by the documentation, be evaluated in list context, which it is not. Cheers, Chris.
CC: Dave Mitchell via RT <perlbug-followup [...] perl.org>
To: Christoph Anton Mitterer <calestyo [...] scientia.net>
Date: Thu, 14 Dec 2017 12:04:22 -0500
Subject: Re: [perl #132537] possible documentation error in perldata
From: Dan Book <grinnz [...] gmail.com>
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On Sat, Dec 9, 2017 at 11:11 PM, Christoph Anton Mitterer <calestyo@scientia.net> wrote:
Show quoted text
On Fri, 2017-12-08 at 01:12 -0800, Dave Mitchell via RT wrote:
> It does by definition:
> No, in scalar context, there isn't a list; instead there is merely
> a sequence of comma operators (which return their RH argument), with
> parentheses added for the purpose of precedence.

That's at least not how the text in perldata or perlop does it make
look like.
It's rather written as it's still a list (and it even says, that all
elements are actually evaluated), just that it doesn't "return" the
whole list, but rather only it's last element.


- (x, y, z) seems to be clearly what is defined to be a list, every
  beginner will consider it as such, regardless of its context
- other text places say that in both contexts (scalar/list) all
  elements are evaluated, just that in scalar context everything but
  the last is discarded

So conceptually it still seems to be a list, and then the text that I
was mentioning is still ambiguous...
People would think (by operator precedence):
1. evaluation of the x, y, z (i.e. the list)
   and the text says always in list context
2. for list context   = return all evaluated elements as a list
   for scalar context = return only the last element (which would
   however, by the documentation, be evaluated in list context, which
   it is not.

The definition of 'list' in perl is more precise than that. In scalar context x, y, z (with or without parentheses) is not a list, it is instead a sequence of expressions like in C. They are evaluated in order and then the last one is returned.

-Dan 


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