Perl::Critic::Policy::ValuesAndExpressions::ProhibitNullStatements - disallow empty statements (stray semicolons)

Perl-Critic-Pulp documentation Contained in the Perl-Critic-Pulp distribution.

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NAME

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Perl::Critic::Policy::ValuesAndExpressions::ProhibitNullStatements - disallow empty statements (stray semicolons)

DESCRIPTION

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This policy is part of the Perl::Critic::Pulp|Perl::Critic::Pulp addon. It prohibits empty statements, ie. bare ; semicolons. This can be a typo doubling up a semi like

    use Foo;;    # bad

Or a stray left at the end of a control structure like

    if ($foo) {
      print "foo\n";
      return;
    };           # bad

An empty statement is completely harmless, so this policy is only under the "cosmetic" theme (see POLICY THEMES in Perl::Critic). It's surprisingly easy to leave a semi behind when chopping code around, especially when changing a statement to a loop or a conditional.

Allowed forms

A C style for (;;) { ...} loop is ok. Those semicolons are expression separators and empties are quite usual.

    for (;;) {   # ok
      print "infinite loop\n";
    }

A semicolon at the start of a map or grep block is allowed. It's commonly used to ensure Perl parses it as a block, not an anonymous hash. (Perl decides at the point it parses the {. A ; there forces a block when it might otherwise guess wrongly. See map in perlfunc for more on this.)

    map {; $_, 123} @some_list;      # ok

    grep {# this is a block
          ;                          # ok
          length $_ and $something } @some_list;

The map form is much more common than the grep, but both suffer the same ambiguity. grep doesn't normally inspire people to quite such wildly convoluted forms as map does.

CONFIGURATION

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allow_perl4_semihash (boolean, default false)

If true then Perl 4 style documentation comments like the following are allowed.

    ;# Usage: 
    ;#      require 'mypkg.pl';
    ;#      ...

The ; must be at the start of the line. This is fairly outdated, so it's disabled by default. If you're crunching through some old code you can enable it by adding to your .perlcriticrc file

    [ValuesAndExpressions::ProhibitNullStatements]
    allow_perl4_semihash=1

SEE ALSO

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Perl::Critic::Pulp, Perl::Critic

HOME PAGE

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http://user42.tuxfamily.org/perl-critic-pulp/index.html

COPYRIGHT

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Copyright 2008, 2009, 2010, 2011 Kevin Ryde

Perl-Critic-Pulp is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

Perl-Critic-Pulp is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Perl-Critic-Pulp. If not, see <http://www.gnu.org/licenses/>.

Perl-Critic-Pulp documentation Contained in the Perl-Critic-Pulp distribution. 
# Copyright 2008, 2009, 2010, 2011 Kevin Ryde

# This file is part of Perl-Critic-Pulp.

# Perl-Critic-Pulp is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by the
# Free Software Foundation; either version 3, or (at your option) any later
# version.
#
# Perl-Critic-Pulp is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
# for more details.
#
# You should have received a copy of the GNU General Public License along
# with Perl-Critic-Pulp.  If not, see <http://www.gnu.org/licenses/>.


package Perl::Critic::Policy::ValuesAndExpressions::ProhibitNullStatements;
use 5.006;
use strict;
use warnings;
use base 'Perl::Critic::Policy';
use Perl::Critic::Utils;

our $VERSION = 61;


use constant supported_parameters =>
  ({ name           => 'allow_perl4_semihash',
     description    => 'Whether to allow Perl 4 style ";#" comments.',
     behavior       => 'boolean',
     default_string => '0',
   });
use constant default_severity => $Perl::Critic::Utils::SEVERITY_MEDIUM;
use constant default_themes   => qw(pulp cosmetic);
use constant applies_to       => 'PPI::Statement::Null';

sub violates {
  my ($self, $elem, $document) = @_;

  # if allow_perl4_semihash then ";# comment ..." ok
  if ($self->{'_allow_perl4_semihash'} && is_perl4_semihash($elem)) {
    return; # ok
  }

  # "for (;;)" is ok, like
  #
  #   PPI::Structure::ForLoop  	( ... )
  #     PPI::Statement::Null
  #       PPI::Token::Structure  	';'
  #     PPI::Statement::Null
  #       PPI::Token::Structure  	';'
  #
  # or the incompatible change in ppi 1.205
  #
  #   PPI::Token::Word         'for'
  #    PPI::Structure::For     ( ... )
  #      PPI::Statement::Null
  #       PPI::Token::Structure        ';'
  #      PPI::Statement::Null
  #       PPI::Token::Structure        ';'

  my $parent = $elem->parent;
  if ($parent->isa('PPI::Structure::For')
      || $parent->isa('PPI::Structure::ForLoop')) {
    return; # ok
  }

  # "map {; ...}" or "grep {; ...}" ok
  if (is_block_disambiguator ($elem)) {
    return; # ok
  }

  # any other PPI::Statement::Null is a bare ";" and is not ok, like
  #
  #   PPI::Statement::Null
  #     PPI::Token::Structure  	';'
  #
  return $self->violation ('Null statement (stray semicolon)',
                           '',
                           $elem);
}

# is_block_disambiguator($elem) takes a PPI::Statement::Null $elem and
# returns true if it's at the start of a "map {; ...}" or "grep {; ...}"
#
# PPI structure like the following, with the Whitespace optional of course,
# and allow comments in there too in case someone wants to write "# force
# block" or something
#
#   PPI::Token::Word    'map'
#   PPI::Token::Whitespace      ' '
#   PPI::Structure::Block       { ... }
#     PPI::Token::Whitespace    ' '
#     PPI::Statement::Null
#       PPI::Token::Structure   ';'
#
sub is_block_disambiguator {
  my ($elem) = @_;

  my $block = $elem->parent;
  $block ->isa('PPI::Structure::Block')
    or return 0;  # not in a block

  # not "sprevious" here, don't want to skip other null statements, just
  # whitespace and comments
  my $prev = $elem->previous_sibling;
  while ($prev && ($prev->isa ('PPI::Token::Whitespace')
                   || $prev->isa ('PPI::Token::Comment'))) {
    $prev = $prev->previous_sibling;
  }
  if ($prev) {
    return 0;  # not at the start of the block
  }

  my $word = $block->sprevious_sibling
    or return 0;   # nothing preceding the block
  $word->isa('PPI::Token::Word')
    or return 0;
  my $content = $word->content;
  return ($content eq 'map' || $content eq 'grep');
}

# is_perl4_semihash($elem) takes a PPI::Statement::Null $elem and returns
# true if it's a Perl 4 style start-of-line ";# comment ..."
#
# When at the very start of a document,
#
#   PPI::Document
#     PPI::Statement::Null
#       PPI::Token::Structure       ';'
#     PPI::Token::Comment   '# foo'
#
# When in the middle,
#
#   PPI::Token::Whitespace        '\n'
#   PPI::Statement::Null
#     PPI::Token::Structure       ';'
#   PPI::Token::Comment   '# hello'
#
sub is_perl4_semihash {
  my ($elem) = @_;

  # must be at the start of the line
  # though not sure about this, the pl2pm program allows whitespace before
  ($elem->location->[1] == 1)
    or return 0;

  # must be immediately followed by a comment
  my $next = $elem->next_sibling;
  return ($next && $next->isa('PPI::Token::Comment'));
}


1;
__END__