Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions - Don't modify $_ in list functions.
$_
This Policy is part of the core Perl::Critic distribution.
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().
map
grep
@_
values(%ENV)
for(@array) { ... }
List::SomeUtils:apply()
List::MoreUtils::apply()
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
This policy deliberately does not apply to for (@array) { ... } or List::MoreUtils::apply() List::SomeUtils::apply().
for (@array) { ... }
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.
$a
$b
sort
There is discussion of this policy at http://perlmonks.org/index.pl?node_id=743445.
Chris Dolan <cdolan@cpan.org>
Michael Wolf <MichaelRWolf@att.net>
Copyright (c) 2006-2021 Chris Dolan.
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cpanm
cpanm Perl::Critic
CPAN shell
perl -MCPAN -e shell install Perl::Critic
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