Provided by: libperl-critic-pulp-perl_96-1_all bug

NAME

       Perl::Critic::Policy::Compatibility::ConstantLeadingUnderscore - new enough "constant"
       module for leading underscores

DESCRIPTION

       This policy is part of the "Perl::Critic::Pulp" add-on.  It asks that if you have a
       constant with a leading underscore,

           use constant _FOO ...  # leading underscore on name

       then you explicitly declare "use 5.6" or "use constant 1.02", or higher, since
       "constant.pm" before that did not allow leading underscores.

           use constant _FOO => 123;        # bad

           use 5.006;
           use constant _FOO => 123;        # ok

           use constant 1.02;
           use constant _FOO => 123;        # ok

           use constant 1.02 _FOO => 123;   # ok

       The idea is to avoid trouble in code which might run on Perl 5.005, or might in principle
       still run there.  On that basis this policy is under the "compatibility" theme (see
       "POLICY THEMES" in Perl::Critic).

       Asking for the new enough module "use constant 1.02" is suggested, since it's the module
       feature which is required and the code might then still run on Perl 5.005 or earlier if
       the user has a suitable "constant.pm" from CPAN.

   Details
       A version declaration must be before the first leading underscore, so it's checked before
       the underscore is attempted (and gives an error).

           use constant _FOO => 123;        # bad
           use 5.006;

       A "require" for the Perl version is not enough since the "use constant" is at "BEGIN"
       time, before plain code.

           require 5.006;                   # doesn't run early enough
           use constant _FOO => 123;        # bad

       But a "require" within a "BEGIN" block is ok (an older style, still found occasionally).

           BEGIN { require 5.006 }
           use constant _FOO => 123;        # ok

           BEGIN {
             require 5.006;
             and_other_setups ...;
           }
           use constant _FOO => 123;        # ok

       Currently ConstantLeadingUnderscore pays no attention to any conditionals within the
       "BEGIN", it assumes any "require" there always runs.  It might be tricked by obscure tests
       but hopefully anything like that is rare.

       A quoted version number like

           use constant '1.02';    # no good

       is no good, only a bare number is recognised by the "use" statement.  A string like that
       in fact goes through to "constant" as a name to define (which it will reject).

       Leading underscores in the multi-constant hash are not flagged, since if you've got multi-
       constants then you've got underscores.  See Compatibility::ConstantPragmaHash for checking
       multi-constants.

           use constant { _FOO => 1 };      # not checked

       Leading double-underscore is disallowed by all versions of "constant.pm".  That's not
       reported by this policy since the code won't run at all.

           use constant __FOO = 123;  # not allowed by any constant.pm

   Drawbacks
       Explicitly adding required version numbers in the code can be irritating, especially if
       other things you're using only run on 5.6 up anyway.  But declaring what code needs is
       accurate, it allows maybe for backports of modules, and explicit versions can be grepped
       out to create or check Makefile.PL or Build.PL prereqs.

       As always if you don't care about this and in particular if you only ever use Perl 5.6
       anyway then you can disable "ConstantLeadingUnderscore" from your .perlcriticrc in the
       usual way (see "CONFIGURATION" in Perl::Critic),

           [-Compatibility::ConstantLeadingUnderscore]

OTHER WAYS TO DO IT

       It's easy to write your own constant subr and it can have any name at all (anything
       acceptable to Perl), bypassing the sanity checks or restrictions in "constant.pm".  Only
       the "()" prototype is a bit obscure.

           sub _FOO () { return 123 }

       The key benefit of subs like this, whether from "constant.pm" or explicitly, is that the
       value is inlined and can be constant-folded in an arithmetic expression etc (see "Constant
       Functions" in perlsub).

           print 2*_FOO;   # folded to 246 at compile-time

       The purpose of a leading underscore is normally a hint that the sub is meant to be private
       to the module and/or its friends.  If you don't need the constant folding then a "my"
       scalar is even more private, being invisible to anything outside the relevant scope,

           my $FOO = 123;         # more private
           # ...
           do_something ($FOO);   # nothing to constant-fold anyway

       The scalar from a constant sub is flagged read-only, which might prevent accidental when
       passed around.  The "Readonly" module can have a similar effect on scalars.  If you've got
       "Readonly::XS" then it's just a flag too (no performance penalty on using the value).

           use Readonly;
           Readonly::Scalar my $FOO => 123;

SEE ALSO

       Perl::Critic::Pulp, Perl::Critic, Perl::Critic::Policy::Compatibility::ConstantPragmaHash,
       Perl::Critic::Policy::ValuesAndExpressions::ProhibitConstantPragma,
       Perl::Critic::Policy::Modules::RequirePerlVersion

       "Constant Functions" in perlsub

HOME PAGE

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

COPYRIGHT

       Copyright 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017 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 v5.26.2                  Perl::Critic::Policy::Compatibility::ConstantLeadingUnderscore(3pm)