Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions(3pm)
                                           User Contributed Perl Documentation


NAME
       Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions
       - Don't modify "$_" in list functions.

AFFILIATION
       This Policy is part of the core Perl::Critic distribution.

DESCRIPTION
       "map", "grep" and other list operators are intended to transform arrays
       into other arrays by applying code to the array elements one by one.
       For speed, the elements are referenced via a $_ alias rather than
       copying them.  As a consequence, if the code block of the "map" or
       "grep" modify $_ in any way, then it is actually modifying the source
       array.  This IS technically allowed, but those side effects can be
       quite surprising, especially when the array being passed is @_ or
       perhaps values(%ENV)!  Instead authors should restrict in-place array
       modification to "for(@array) { ... }" constructs instead, or use
       List::SomeUtils:apply() or List::MoreUtils::apply().

CONFIGURATION
       By default, this policy applies to the following list functions:

           map grep
           List::Util qw(first)
           List::MoreUtils qw(any all none notall true false firstidx
                              first_index lastidx last_index insert_after
                              insert_after_string)
           List::SomeUtils qw(any all none notall true false firstidx
                              first_index lastidx last_index insert_after
                              insert_after_string)

       This list can be overridden the .perlcriticrc file like this:

           [ControlStructures::ProhibitMutatingListFunctions]
           list_funcs = map grep List::Util::first

       Or, one can just append to the list like so:

           [ControlStructures::ProhibitMutatingListFunctions]
           add_list_funcs = Foo::Bar::listmunge

LIMITATIONS
       This policy deliberately does not apply to "for (@array) { ... }" or
       List::MoreUtils::apply() List::SomeUtils::apply().

       Currently, the policy only detects explicit external module usage like
       this:

           my @out = List::MoreUtils::any {s/^foo//} @in;
           my @out = List::SomeUtils::any {s/^foo//} @in;

       and not like this:

           use List::MoreUtils qw(any);
           my @out = any {s/^foo//} @in;

           use List::SomeUtils qw(any);
           my @out = any {s/^foo//} @in;

       This policy looks only for modifications of $_.  Other naughtiness
       could include modifying $a and $b in "sort" and the like.  That's
       beyond the scope of this policy.

SEE ALSO
       There is discussion of this policy at
       <http://perlmonks.org/index.pl?node_id=743445>.

AUTHOR
       Chris Dolan <cdolan@cpan.org>

       Michael Wolf <MichaelRWolf@att.net>

COPYRIGHT
       Copyright (c) 2006-2021 Chris Dolan.

       This program is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

perl v5.38.0                      2023-07-26
   Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions(3pm)