Bareword Package Names

[martin]

This is a question about a perl5 feature I could find no documentation about so far. I would be glad to know whether it is intended this way and safe to use or even recommended.

A bareword ending in a double colon, such as Foo::Bar::, evaluates to a string without the double colon suffix, "Foo::Bar" in this example. Using such a bareword is not prohibited under strict 'subs', but it triggers a warning if perl hasn't seen a package of that name yet.

Example:

use strict;
use warnings;
package Foo::Bar;
package main;
my $p = Foo::Bar::;
print "$p\n";         # prints Foo::Bar
my $q = Baz::Qux::;   # warns:
# Bareword "Baz::Qux::" refers to nonexistent package
print "$q\n";         # prints Baz::Qux

I'll call this feature package-name-quoting, as appending two colons has the effect of quoting plus package name validation. I could use it where package name strings are needed, such as in arguments for isa or DOES. The warning would help me to catch typos.

So, after all, it is a useful feature. Is it documented? I would have expected perlmod to say something about it. Maybe I looked in the wrong place.

[tobyink]

I'm not sure how documented the behaviour you describe is, but the trailing double colons have another use. Consider the following:

use 5.010;

{
   package Foo;
   sub new { bless [] }
   sub Bar { 'Quux' }
}
{
   package Foo::Bar;
   sub new { bless [] }
}
{
   package Quux;
   sub new { bless [] }
}

my $object1 = Foo::Bar::->new;
say ref $object1;   # says "Foo::Bar"

my $object2 = Foo::Bar->new;
say ref $object2;   # says "Quux"

[martin]

What tobyink demonstrated is that a bareword will act as a procedure call if (and only if) a subroutine of that name is known. Perl's auto-quoting of barewords will only kick in in the absence of such a subroutine. Any kind of explicit quoting, of course, will also avoid procedure calls.

my $object3 = 'Foo::Bar'->new;
say ref $object3;   # says "Foo::Bar"

So we now have established by different examples that trailing double-colons can act like quoting.

There may be caveats though. In some places where perl looks for barewords, the package-quoted bareword does not seem to lose its bareword-likeness.

use strict;
use warnings;
package Foo::Bar;
package main;
print Foo::Bar::;    # warns:
# Name "Foo::Bar" used only once: possible typo
# print() on unopened filehandle Bar
print Foo::Bar::, "Baz\n"; # prints Foo::BarBaz

Perl has to distinguish whether print with a single argument is called as print HANDLE or print LIST, and apparently uses a rather simple heuristic for this.

I try to avoid ambiguity with print and always use the print HANDLE LIST syntax for printing to handles.

[jmlynesjr]

See Programming Perl 4th Edition page 389 - Symbol Tables.

"Symbol tables are stored in a hash whose name is the same as the package, but with two colons appended. The main symbol table's name is thus %main::."

"Likewise, the symbol table or the Red::Blue package is named %Red::Blue::."

James

Also see page 423 - Package-Quoted Classes.

"The final syntactic ambiguity with the indirect object style of method invocation is that it may not be parsed as a method call at all, because the current package may have a subroutine of the same name as the method...there is a way to resolve this ambiguity while still keeping the indirect object syntax: package-quote the class name by appending a double colon to it."

   $obj = method CLASS::;   # forced to be "CLASS"->method

[tobyink]

Interestingly...

use Test::More tests => 1;

# this test fails
is(
  do { package Foo::Bar::; __PACKAGE__ },
  do { package Foo::Bar;   __PACKAGE__ },
);

So there's at least one place the barewords Foo::Bar:: and Foo::Bar evaluate to different packages.

See also, Perl allows package names consisting entirely of colons.