NAME

       Perl::Critic::Policy::ValuesAndExpressions::ProhibitDuplicateHashKeys - disallow duplicate
       literal hash keys

DESCRIPTION

       This policy is part of the "Perl::Critic::Pulp" add-on.  It reports duplicate literal hash
       keys in a hash assignment or anonymous hashref.

           my %hash = (red   => 1,
                       green => 2,
                       red   => 3,    # bad
                      );

           my $hashref = { red   => 1,
                           red   => 3,   # bad
                         };

       Writing duplicate literal keys is probably a mistake or too much cut and paste, and if the
       values are different will make it unclear to human readers what was meant.  On that basis
       this policy is under the "bugs" theme and medium severity (see "POLICY THEMES" in
       Perl::Critic).

       Perl is happy to run code like the above.  The value of the last "red" is stored.  As
       runtime behaviour, this is good since you can give defaults which further values from a
       caller or similar can replace.  For example,

           sub new {
             my $class = shift;
             return bless { foo => 'default',
                            bar => 'default',
                            @_ }, $class;
           }

           MyClass->new (foo => 'caller value'); # overriding 'default'

   Expressions
       Expressions within a hash list cannot be checked in general.  Some concatenations of
       literals are recognised though they're probably unusual.

           my %hash = (ab      => 1,
                       'a'.'b' => 2);  # bad

           my %hash = (__PACKAGE__.'a' => 1,
                       __PACKAGE__.'a' => 2);  # bad

       Function calls etc within a list might return an odd or even number of values.  Fat commas
       "=>" are taken as indicating a key when in doubt.

           my %hash = (blah()    => 1,  # guided by =>
                       a         => 2,
                       a         => 3); # bad

           my %hash = (blah(),
                       a         => 2,  # guided by =>
                       a         => 3); # bad

       A hash substitution is always an even number of arguments,

           my %hash = (a         => 1,
                       %blah,           # even number
                       a         => 5); # bad, duplicate

       "qw()" words are recognised too

           my %hash = (qw(foo value1
                          foo value2));  # bad

   Disabling
       If you don't care about this you can always disable "ProhibitDuplicateHashKeys" from your
       .perlcriticrc file in the usual way (see "CONFIGURATION" in Perl::Critic),

           [-ValuesAndExpressions::ProhibitDuplicateHashKeys]

SEE ALSO

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

       Perl::Critic::Policy::CodeLayout::RequireTrailingCommas,
       Perl::Critic::Policy::CodeLayout::RequireTrailingCommaAtNewline

HOME PAGE

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

COPYRIGHT

       Copyright 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2019, 2020, 2021 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 v5.32.1           Perl::Critic::Policy::ValuesAndExpressions::ProhibitDuplicateHashKeys(3pm)