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.de Sp \" Vertical space (when we can't use .PP)
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.de Vb \" Begin verbatim text
.ft CW
.nf
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.IX Title "Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic 3pm"
.TH Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic 3pm 2023-07-26 "perl v5.38.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.if n .ad l
.nh
.SH NAME
Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic \- Don't pass $_ to built\-in functions that assume it, or to most filetest operators.
.SH AFFILIATION
.IX Header "AFFILIATION"
This Policy is part of the Perl::Critic distribution.
.SH DESCRIPTION
.IX Header "DESCRIPTION"
There are a number of places where \f(CW$_\fR, or "the topic" variable,
is unnecessary.
.SS "Topic unnecessary for certain Perl built-in functions"
.IX Subsection "Topic unnecessary for certain Perl built-in functions"
Many Perl built-in functions will operate on \f(CW$_\fR if no argument
is passed.  For example, the \f(CW\*(C`length\*(C'\fR function will operate on
\&\f(CW$_\fR by default.  This snippet:
.PP
.Vb 2
\&    for ( @list ) {
\&        if ( length( $_ ) == 4 ) { ...
.Ve
.PP
is more idiomatically written as:
.PP
.Vb 2
\&    for ( @list ) {
\&        if ( length == 4 ) { ...
.Ve
.PP
In the case of the \f(CW\*(C`split\*(C'\fR function, the second argument is the
one that defaults to \f(CW$_\fR.  This snippet:
.PP
.Vb 2
\&    for ( @list ) {
\&        my @args = split /\et/, $_;
.Ve
.PP
is better written as:
.PP
.Vb 2
\&    for ( @list ) {
\&        my @args = split /\et/;
.Ve
.PP
There is one built-in that this policy does \fBnot\fR check for:
\&\f(CW\*(C`reverse\*(C'\fR called with \f(CW$_\fR.
.PP
The \f(CW\*(C`reverse\*(C'\fR function only operates on \f(CW$_\fR if called in scalar
context.  Therefore:
.PP
.Vb 2
\&    for ( @list ) {
\&        my $backwards = reverse $_;
.Ve
.PP
is better written as:
.PP
.Vb 2
\&    for ( @list ) {
\&        my $backwards = reverse;
.Ve
.PP
However, the distinction for scalar vs. list context on \f(CW\*(C`reverse\*(C'\fR
is not yet working.  See KNOWN BUGS below.
.SS "Topic unnecessary for most filetest operators"
.IX Subsection "Topic unnecessary for most filetest operators"
Another place that \f(CW$_\fR is unnecessary is with a filetest operator.
.PP
.Vb 3
\&    # These are identical.
\&    my $size = \-s $_;
\&    my $size = \-s;
\&
\&    # These are identical.
\&    if ( \-r $_ ) { ...
\&    if ( \-r ) { ...
.Ve
.PP
The exception is after the \f(CW\*(C`\-t\*(C'\fR filetest operator, which instead of
defaulting to \f(CW$_\fR defaults to \f(CW\*(C`STDIN\*(C'\fR.
.PP
.Vb 3
\&    # These are NOT identical.
\&    if ( \-t $_ ) { ...
\&    if ( \-t ) { ...  # Checks STDIN, not $_
.Ve
.SH "KNOWN BUGS"
.IX Header "KNOWN BUGS"
This policy flags a false positive on \f(CW\*(C`reverse\*(C'\fR called in list
context, since \f(CW\*(C`reverse\*(C'\fR in list context does \fInot\fR assume \f(CW$_\fR.
.PP
.Vb 2
\&    my $s = reverse( $_ ); # $_ is useless.
\&    my @a = reverse( $_ ); # $_ is not useless here.
.Ve
.SH CONFIGURATION
.IX Header "CONFIGURATION"
This Policy is not configurable except for the standard options.
.SH AUTHOR
.IX Header "AUTHOR"
Andy Lester <andy@petdance.com>
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright (c) 2013\-2022 Andy Lester <andy@petdance.com>
.PP
This library is free software; you can redistribute it and/or modify it
under the terms of the Artistic License 2.0.