Perl::Critic::Policy::Compatibility::ConstantPragmaHash - new enough "constant" module for multiple constants


This policy is part of the Perl::Critic::Pulp add-on. It requires that when you use the hash style multiple constants of use constant that you explicitly declare either Perl 5.8 or constant 1.03 or higher.

    use constant { AA => 1, BB => 2 };       # bad

    use 5.008;
    use constant { CC => 1, DD => 2 };       # ok

    use constant 1.03;
    use constant { EE => 1, FF => 2 };       # ok

    use constant 1.03 { GG => 1, HH => 2 };  # ok

The idea is to keep you from using the multi-constant feature in code which might run on Perl 5.6, or might in principle still run there. On that basis this policy is under the "compatibility" theme (see "POLICY THEMES" in Perl::Critic).

If you declare constant 1.03 then the code can still run on Perl 5.6 and perhaps earlier if the user gets a suitably newer constant module from CPAN. Or of course for past compatibility just don't use the hash style at all!


A version declaration must be before the first multi-constant, so it's checked before the multi-constant is attempted (and gives an obscure error).

    use constant { X => 1, Y => 2 };       # bad
    use 5.008;

A require for the perl version is not adequate since the use constant is at BEGIN time, before plain code.

    require 5.008;
    use constant { X => 1, Y => 2 };       # bad

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

    BEGIN { require 5.008 }
    use constant { X => 1, Y => 2 };       # ok

    BEGIN {
      require 5.008;
      and_other_setups ...;
    use constant { X => 1, Y => 2 };       # ok

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

A quoted version number like

    use constant '1.03';    # no good

is no good, only a bare number is recognised by use and acted on by ConstantPragmaHash. A string like that goes through to constant as if a name to define (which you'll see it objects to as soon as you try run it).


Explicitly adding version numbers to your code can be irritating if other modules you're using only run on 5.8 anyway. But declaring what your own code wants is accurate, it allows maybe for backports of those other things, 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 5.8 anyway then you can disable ConstantPragmaHash from your .perlcriticrc in the usual way (see "CONFIGURATION" in Perl::Critic),



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

"Constant Functions" in perlsub



Copyright 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017 Kevin Ryde

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