Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions(3pm)

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.