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Perl::Critic::Policy::ValuesAndExpressions::UnexpandedSpecialLiteral - specials like __PACKAGE__ used literally



This policy is part of the Perl::Critic::Pulp add-on. It picks up some cases where the special literals __FILE__, __LINE__ and __PACKAGE__ (see Special Literals in perldata) are used with => or as a hash subscript and so don’t expand to the respective filename, line number or package name.

    my $seen = { __FILE__ => 1 };          # bad
    return (At:.__LINE__ => 123);        # bad
    $obj->{__PACKAGE__}->{myextra} = 123;  # bad

In each case you get a string "__FILE__", "__LINE__" or "__PACKAGE__", as if

    my $seen = { __FILE__ => 1 };
    return (At:__LINE__ => 123);
    $obj->{__PACKAGE__}->{myextra} = 123;

where almost certainly it was meant to expand to the filename etc. On that basis this policy is under the bugs theme (see POLICY THEMES in Perl::Critic).

Expression forms like

    MyExtra::.__PACKAGE__ => 123    # bad

are still bad because the word immediately to the left of a => is quoted even when that word is part of an expression.

If you really do want a string "__FILE__" etc then the suggestion is to write the quotes, even if you’re not in the habit of using quotes in hash constructors etc. It’ll pass this policy and make it clear to everyone that you really did want the literal string.

The __PACKAGE__ literal is new in Perl 5.004 but this policy is applied to all code. Even if you’re targeting an earlier Perl extra quotes will make it clear to users of later Perl that a literal string "__PACKAGE__" is indeed intended.

    Fat Comma After Newline

A => fat comma only quotes when it’s on the same line as the preceding bareword, so in the following __PACKAGE__ is not quoted and is therefore not reported by this policy,

    my %hash = (__PACKAGE__   # ok, expands

Of course whether or not writing this is a good idea is another matter. It might be a bit subtle to depend on the newline. Probably a plain , comma would make the intention clearer than =>.

    Class Data

A bad $obj->{__PACKAGE__} can arise when you’re trying to hang extra data on an object using your package name to hopefully not clash with the object’s native fields. Unexpanded __PACKAGE__ like that is a mistake you’ll probably only make once; after that the irritation of writing extra parens or similar will keep it fresh in your mind!

As usual there’s more than one way to do it when associating extra data to an object. As a crib here are some ways,
Subhash $obj->{(__PACKAGE__)}->{myfield} The extra parens ensure expansion, and you get a sub-hash (or sub-array or whatever) to yourself. It’s easy to delete the single entry from $obj if/when you later want to cleanup.
Subscript $obj->{__PACKAGE__,myfield} This makes entries in $obj, with the $; separator emulating multidimensional arrays/hashes (see $; in perlvar).
Concated key $obj->{__PACKAGE__.--myfield} Again entries in $obj, but key formed by concatenation and an explicit unlikely separator. The advantage over , is that the key is a constant (after constant folding), instead of a join on every access because $; could change.
Separate Tie::HashRef::Weak Use the object as a hash key and the value whatever data you want to associate. Keeps completely out of the object’s hair and also works with objects which use a restricted hash (see Hash::Util) to prevent extra keys.
Inside-Out Hash::Util::FieldHash Similar to HashRef with object as key and any value you want as the data outside the object, hence the jargon inside out. The docs are very hard to follow (as of its version 1.04), especially if you’re not into OOP, but it’s actually fairly simple.
Scalar::Footnote Key/value pairs attached to an object using its magic list. Doesn’t touch the object’s contents but separate footnote users must be careful not to let their keys clash.


Perl::Critic::Pulp, Perl::Critic, Special Literals in perldata



Copyright 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015 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 <>.

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