'map' returns array instead of list #16044

p5pRT · 2017-06-25T12:06:57Z

Migrated from rt.perl.org#131652 (status was 'resolved')

Searchable as RT131652$

p5pRT · 2017-06-25T12:06:58Z

From @KES777

Created by @KES777

The [DOC](https://perldoc.perl.org/functions/map.html) says:

map ... evaluates ... and returns the list value

but it returns array:

$x = ( 1, 7, 5 ); #5 list: last element is returned
print $x;

@array = ( 1, 7, 5 );
$x = @array; #3 array: number of elements is returned
print $x;

$x = map{ $_ } ( 1, 7, 5 ); #EXPECTED: 5 (list), ACTUAL: 3 (array)
print $x;

OUTPUT is:
533

Perl Info


Flags:
    category=core
    severity=medium

Site configuration information for perl 5.24.1:

Configured by kes at Sun Apr 30 22:40:30 EEST 2017.

Summary of my perl5 (revision 5 version 24 subversion 1) configuration:
   
  Platform:
    osname=linux, osvers=4.4.0-53-generic, archname=x86_64-linux
    uname='linux kes-x751sa 4.4.0-53-generic #74-ubuntu smp fri dec 2 15:59:10 utc 2016 x86_64 x86_64 x86_64 gnulinux '
    config_args='-de -Dprefix=/home/kes/perl5/perlbrew/perls/perl-5.24.1 -Aeval:scriptdir=/home/kes/perl5/perlbrew/perls/perl-5.24.1/bin'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion='', gccversion='5.4.0 20160609', 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='cc', ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/5/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=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.23.so, so=so, useshrplib=false, libperl=libperl.a
    gnulibc_version='2.23'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'

Locally applied patches:
    Devel::PatchPerl 1.38


@INC for perl 5.24.1:
    /home/kes/perl5/perlbrew/perls/perl-5.24.1/lib/site_perl/5.24.1/x86_64-linux
    /home/kes/perl5/perlbrew/perls/perl-5.24.1/lib/site_perl/5.24.1
    /home/kes/perl5/perlbrew/perls/perl-5.24.1/lib/5.24.1/x86_64-linux
    /home/kes/perl5/perlbrew/perls/perl-5.24.1/lib/5.24.1


Environment for perl 5.24.1:
    HOME=/home/kes
    LANG=en_US.UTF-8
    LANGUAGE=en
    LC_ADDRESS=uk_UA.UTF-8
    LC_IDENTIFICATION=uk_UA.UTF-8
    LC_MEASUREMENT=uk_UA.UTF-8
    LC_MESSAGES=en_US.UTF-8
    LC_MONETARY=uk_UA.UTF-8
    LC_NAME=uk_UA.UTF-8
    LC_NUMERIC=uk_UA.UTF-8
    LC_PAPER=uk_UA.UTF-8
    LC_TELEPHONE=uk_UA.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/kes/perl5/perlbrew/bin:/home/kes/perl5/perlbrew/perls/perl-5.24.1/bin:/home/kes/bin:/home/kes/bin:/home/kes/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
    PERLBREW=command perlbrew
    PERLBREW_BASHRC_VERSION=0.78
    PERLBREW_HOME=/home/kes/.perlbrew
    PERLBREW_MANPATH=/home/kes/perl5/perlbrew/perls/perl-5.24.1/man
    PERLBREW_PATH=/home/kes/perl5/perlbrew/bin:/home/kes/perl5/perlbrew/perls/perl-5.24.1/bin
    PERLBREW_PERL=perl-5.24.1
    PERLBREW_ROOT=/home/kes/perl5/perlbrew
    PERLBREW_VERSION=0.78
    PERL_BADLANG (unset)
    SHELL=/bin/bash

p5pRT · 2017-06-25T12:23:34Z

From Eirik-Berg.Hanssen@allverden.no

On Sun, Jun 25, 2017 at 2:06 PM, KES <perlbug-followup@perl.org> wrote:

The [DOC](https://perldoc.perl.org/functions/map.html) says:

map ... evaluates ... and returns the list value

… and goes on to say "In scalar context, returns the total number of
elements so generated".

but it returns array:

$x = ( 1, 7, 5 ); #5 list: last element is returned
print $x;

Nope; this is the comma operator in scalar context, returning the RHS
operand.
(Bugbear mantra: There's no such thing as a list in scalar context – only
expressions are evaluated in different contexts.)

@array = ( 1, 7, 5 );

$x = @array; #3 array: number of elements is returned
print $x;

$x = map{ $_ } ( 1, 7, 5 ); #EXPECTED: 5 (list), ACTUAL: 3 (array)
print $x;

Since map in scalar context is documented to return the "total number of
elements so generated", 3 is to be expected.

Works as per documentation – no bug, just my particular bugbear, the idea
of lists in scalar context.

At most I might suggest making the documentation clearer that the return
value is context dependent, so that instead of the seemingly unconditional
"returns the list value composed of", we explicate the context: "in list
context returns the value composed of".

Eirik

p5pRT · 2017-06-25T12:23:35Z

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

p5pRT · 2017-06-25T13:08:41Z

From @cpansprout

On Sun, 25 Jun 2017 05:23:34 -0700, Eirik-Berg.Hanssen@allverden.no wrote:

On Sun, Jun 25, 2017 at 2:06 PM, KES <perlbug-followup@perl.org> wrote:

The [DOC](https://perldoc.perl.org/functions/map.html) says:

map ... evaluates ... and returns the list value

… and goes on to say "In scalar context, returns the total number of
elements so generated".

but it returns array:

$x = ( 1, 7, 5 ); #5 list: last element is returned
print $x;

Nope; this is the comma operator in scalar context, returning the RHS
operand.
(Bugbear mantra: There's no such thing as a list in scalar context – only
expressions are evaluated in different contexts.)

@array = ( 1, 7, 5 );

$x = @array; #3 array: number of elements is returned
print $x;

$x = map{ $_ } ( 1, 7, 5 ); #EXPECTED: 5 (list), ACTUAL: 3 (array)
print $x;

Since map in scalar context is documented to return the "total number of
elements so generated", 3 is to be expected.

Works as per documentation – no bug, just my particular bugbear, the idea
of lists in scalar context.

At most I might suggest making the documentation clearer that the return
value is context dependent, so that instead of the seemingly unconditional
"returns the list value composed of", we explicate the context: "in list
context returns the value composed of".

Or what about we eschew ‘value’ and say ‘a list composed of...’?

--

Father Chrysostomos

p5pRT · 2017-07-08T00:02:05Z

From @jkeenan

On Sun, 25 Jun 2017 13:08:41 GMT, sprout wrote:

On Sun, 25 Jun 2017 05:23:34 -0700, Eirik-Berg.Hanssen@allverden.no
wrote:

On Sun, Jun 25, 2017 at 2:06 PM, KES <perlbug-followup@perl.org>
wrote:

The [DOC](https://perldoc.perl.org/functions/map.html) says:

map ... evaluates ... and returns the list value

… and goes on to say "In scalar context, returns the total number of
elements so generated".

but it returns array:

$x = ( 1, 7, 5 ); #5 list: last element is returned
print $x;

Nope; this is the comma operator in scalar context, returning the RHS
operand.
(Bugbear mantra: There's no such thing as a list in scalar context –
only
expressions are evaluated in different contexts.)

@array = ( 1, 7, 5 );

$x = @array; #3 array: number of elements is
returned
print $x;

$x = map{ $_ } ( 1, 7, 5 ); #EXPECTED: 5 (list), ACTUAL: 3
(array)
print $x;

Since map in scalar context is documented to return the "total number
of
elements so generated", 3 is to be expected.

Works as per documentation – no bug, just my particular bugbear, the
idea
of lists in scalar context.

At most I might suggest making the documentation clearer that the
return
value is context dependent, so that instead of the seemingly
unconditional
"returns the list value composed of", we explicate the context: "in
list
context returns the value composed of".

Or what about we eschew ‘value’ and say ‘a list composed of...’?

Please review the patch attached.

Thank you very much.

--
James E Keenan (jkeenan@cpan.org)

p5pRT · 2017-07-08T00:02:05Z

From @jkeenan

131652-0001-Improve-documentation-of-map.patch

From f6d32eb695407668bf1f7b5973029574a84b31f2 Mon Sep 17 00:00:00 2001
From: James E Keenan <jkeenan@cpan.org>
Date: Fri, 7 Jul 2017 19:59:47 -0400
Subject: [PATCH] Improve documentation of 'map'.

Per discussion in RT # 131652.
---
 pod/perlfunc.pod | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index b1e4ac2..dadb3d0 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -4071,12 +4071,11 @@ X<map>
 =for Pod::Functions apply a change to a list to get back a new list with the changes
 
 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
-L<C<$_>|perlvar/$_> to each element) and returns the list value composed
-of the
-results of each such evaluation.  In scalar context, returns the
-total number of elements so generated.  Evaluates BLOCK or EXPR in
-list context, so each element of LIST may produce zero, one, or
-more elements in the returned value.
+L<C<$_>|perlvar/$_> to each element) and composes a list of the results of
+each such evaluation.  Each element of LIST may produce zero, one, or more
+elements in the generated list, so the number of elements in the generated
+list may differ from that in LIST.  In scalar context, returns the total
+number of elements so generated.  In list context, returns the generated list.
 
     my @chars = map(chr, @numbers);
 
-- 
2.7.4

p5pRT · 2017-07-08T01:28:06Z

From @cpansprout

On Fri, 07 Jul 2017 17:02:05 -0700, jkeenan wrote:

Please review the patch attached.

+one

--

Father Chrysostomos

p5pRT · 2017-07-11T14:50:23Z

From @jkeenan

On Sat, 08 Jul 2017 01:28:06 GMT, sprout wrote:

On Fri, 07 Jul 2017 17:02:05 -0700, jkeenan wrote:

Please review the patch attached.

+one

P5P: Any further comments on this documentation patch?

I am taking this ticket for the purpose of applying the patch within 7 days unless there is a valid objection.

Thank you very much.

--
James E Keenan (jkeenan@cpan.org)

p5pRT · 2017-07-17T17:18:35Z

From @davidnicol

this is a fine example of how "array" and "list" appear to be used
interchangeably in the documentation.

that said I think the patch is great.

--
everything has to be just so or the magic won't work

p5pRT · 2017-07-17T22:11:47Z

From @jkeenan

On Tue, 11 Jul 2017 14:50:23 GMT, jkeenan wrote:

On Sat, 08 Jul 2017 01:28:06 GMT, sprout wrote:

On Fri, 07 Jul 2017 17:02:05 -0700, jkeenan wrote:

Please review the patch attached.

+one

P5P: Any further comments on this documentation patch?

I am taking this ticket for the purpose of applying the patch within 7
days unless there is a valid objection.

Thank you very much.

Applied to blead in commit e1d3549; marking ticket Resolved.

Thank you very much.

--
James E Keenan (jkeenan@cpan.org)

p5pRT · 2017-07-17T22:11:47Z

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

p5pRT closed this as completed Jul 17, 2017

p5pRT added Severity Low distro-Linux documentation labels Oct 19, 2019

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'map' returns array instead of list #16044

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Comments

p5pRT commented Jun 25, 2017

p5pRT commented Jun 25, 2017

From @KES777

Created by @KES777

p5pRT commented Jun 25, 2017

From Eirik-Berg.Hanssen@allverden.no

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