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 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.

perl v5.20.2                    Perl::Critic::Policy::BuiltinFunctions::ProhibitUselessTopic(3pm)