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Perl-Critic-1.156
River stage three • 132 direct dependents • 384 total dependents
/ Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic

NAME

Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic - Don't pass $_ to built-in functions that assume it, or to most filetest operators.

AFFILIATION

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

DESCRIPTION

There are a number of places where $_, or "the topic" variable, is unnecessary.

Topic unnecessary for certain Perl built-in functions

Many Perl built-in functions will operate on $_ if no argument is passed. For example, the length function will operate on $_ by default. This snippet:


            

              
              
for ( @list ) {
    if ( length( $_ ) == 4 ) { ...

is more idiomatically written as:


            

              
              
for ( @list ) {
    if ( length == 4 ) { ...

In the case of the split function, the second argument is the one that defaults to $_. This snippet:


            

              
              
for ( @list ) {
    my @args = split /\t/, $_;

is better written as:


            

              
              
for ( @list ) {
    my @args = split /\t/;

There is one built-in that this policy does not check for: reverse called with $_.

The reverse function only operates on $_ if called in scalar context. Therefore:


            

              
              
for ( @list ) {
    my $backwards = reverse $_;

is better written as:


            

              
              
for ( @list ) {
    my $backwards = reverse;

However, the distinction for scalar vs. list context on reverse is not yet working. See KNOWN BUGS below.

Topic unnecessary for most filetest operators

Another place that $_ is unnecessary is with a filetest operator.


            

              
              
# These are identical.
my $size = -s $_;
my $size = -s;
# These are identical.
if ( -r $_ ) { ...
if ( -r ) { ...

The exception is after the -t filetest operator, which instead of defaulting to $_ defaults to STDIN.


            

              
              
# These are NOT identical.
if ( -t $_ ) { ...
if ( -t ) { ...  # Checks STDIN, not $_

KNOWN BUGS

This policy flags a false positive on reverse called in list context, since reverse in list context does not assume $_.


            

              
              
my $s = reverse( $_ ); # $_ is useless.
my @a = reverse( $_ ); # $_ is not useless here.

CONFIGURATION

This Policy is not configurable except for the standard options.

AUTHOR

Andy Lester <andy@petdance.com>

COPYRIGHT

Copyright (c) 2013-2022 Andy Lester <andy@petdance.com>

This library is free software; you can redistribute it and/or modify it under the terms of the Artistic License 2.0.