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.IX Title "Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions 3pm"
.TH Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions 3pm 2023-07-26 "perl v5.38.0" "User Contributed Perl Documentation"
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.SH NAME
Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions \- Don't modify "$_" in list functions.
.SH AFFILIATION
.IX Header "AFFILIATION"
This Policy is part of the core Perl::Critic
distribution.
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`map\*(C'\fR, \f(CW\*(C`grep\*(C'\fR and other list operators are intended to transform
arrays into other arrays by applying code to the array elements one by
one.  For speed, the elements are referenced via a \f(CW$_\fR alias rather
than copying them.  As a consequence, if the code block of the \f(CW\*(C`map\*(C'\fR
or \f(CW\*(C`grep\*(C'\fR modify \f(CW$_\fR in any way, then it is actually modifying the
source array.  This IS technically allowed, but those side effects can
be quite surprising, especially when the array being passed is \f(CW@_\fR
or perhaps \f(CWvalues(%ENV)\fR!  Instead authors should restrict in-place
array modification to \f(CW\*(C`for(@array) { ... }\*(C'\fR constructs instead, or
use \f(CWList::SomeUtils:apply()\fR or \f(CWList::MoreUtils::apply()\fR.
.SH CONFIGURATION
.IX Header "CONFIGURATION"
By default, this policy applies to the following list functions:
.PP
.Vb 8
\&    map grep
\&    List::Util qw(first)
\&    List::MoreUtils qw(any all none notall true false firstidx
\&                       first_index lastidx last_index insert_after
\&                       insert_after_string)
\&    List::SomeUtils qw(any all none notall true false firstidx
\&                       first_index lastidx last_index insert_after
\&                       insert_after_string)
.Ve
.PP
This list can be overridden the \fI.perlcriticrc\fR file like this:
.PP
.Vb 2
\&    [ControlStructures::ProhibitMutatingListFunctions]
\&    list_funcs = map grep List::Util::first
.Ve
.PP
Or, one can just append to the list like so:
.PP
.Vb 2
\&    [ControlStructures::ProhibitMutatingListFunctions]
\&    add_list_funcs = Foo::Bar::listmunge
.Ve
.SH LIMITATIONS
.IX Header "LIMITATIONS"
This policy deliberately does not apply to \f(CW\*(C`for (@array) { ... }\*(C'\fR or
\&\f(CWList::MoreUtils::apply()\fR \f(CWList::SomeUtils::apply()\fR.
.PP
Currently, the policy only detects explicit external module usage like
this:
.PP
.Vb 2
\&    my @out = List::MoreUtils::any {s/^foo//} @in;
\&    my @out = List::SomeUtils::any {s/^foo//} @in;
.Ve
.PP
and not like this:
.PP
.Vb 2
\&    use List::MoreUtils qw(any);
\&    my @out = any {s/^foo//} @in;
\&
\&    use List::SomeUtils qw(any);
\&    my @out = any {s/^foo//} @in;
.Ve
.PP
This policy looks only for modifications of \f(CW$_\fR.  Other naughtiness
could include modifying \f(CW$a\fR and \f(CW$b\fR in \f(CW\*(C`sort\*(C'\fR and the like.
That's beyond the scope of this policy.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
There is discussion of this policy at
<http://perlmonks.org/index.pl?node_id=743445>.
.SH AUTHOR
.IX Header "AUTHOR"
Chris Dolan <cdolan@cpan.org>
.PP
Michael Wolf <MichaelRWolf@att.net>
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright (c) 2006\-2021 Chris Dolan.
.PP
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.