Removal of the perl4 ' package seperator

2018-10-12

The removal of the old and deprecated perl4 single quote character ' as valid package seperator in cperl went through various steps. It’s also explained in perldata.

There are still two package separators in perl5:

A double colon (::) and a single quote ('). Normal identifiers can start or end with a double colon, and can contain several parts delimited by double colons. Single quotes within perl5 have similar rules, but with the exception that they are not legal at the end of an identifier: That is, $'foo and $foo'bar are legal, but $foo'bar' is not.

In cperl only the double colon (::) is a legal package separator, the perl4 package seperator ' was made illegal with 5.26c, and since 5.28c is now legal part of the identifer. Full support for the single quote ' as part of an identifer came with 5.28c, and matches now the behavior of perl6.

sub isn't { } # parsed as isn't since 5.28c, illegal with 5.26c,
              # before parsed as isn::t
isn't()       # parsed as isn't since 5.28c, illegal with 5.26c,
              # before parsed as isn::t

Differences and caveats, when being expanded in strings:

Note that hashes are not expanded, e.g. "%file's" stays "%file's", only scalars and arrays are expanded.

my $file = "test";
my @file = ("a", "b");
if ($^O !~ /c$/ or $] < 5.026) { # pre 5.26c
  is("$file's test", " test");   # parsed as file::s
  is("@file's test", " test");
}
elsif ($^O =~ /c$/ and $] >= 5.026 and $] < 5.028) { # 5.26c
  is("$file's test", "test's test"); # parsed as $file . "'s test"
  is("@file's test", "a b's test");
}
elsif ($^O =~ /c$/ and $] >= 5.028) { # 5.28c
  is("$file's test", " test");   # parsed as $file's. " test"
  is("@file's test", " test");
}

perl6:

no strict;
my $file = "test";
my @file = "a", "b";
is("$file's test", " test");        # parsed as $file's. " test"
is("@file's test", "@file's test"); # @ is not expanded anymore

There’s no plan to follow the perl6 syntax decision to not expand the @ sigil in strings anymore. This is a fundamental perl5 feature and widely (ab)used for various cases, which cannot be changed. We also don’t find that breaking change useful.

Additionally, if the identifier is preceded by a sigil - that is, if the identifier is part of a variable name - it may optionally be enclosed in braces.

So to properly expand such a scalar string without the quote you need to use "${file}'s".

  is("${file}'s test", "test's test");

It worked for a short time in 5.26c without the {} quoting, but only in this particular version.

There are still some modules on CPAN which rely on the wrong outdated perl4 behaviour. Mostly test related modules which insist on using isn't as a method name. There’s nothing wrong in using isn't as name, just beware that it is not expanded to isn::t anymore. I.e. it’s in the main module namespace, and not a method called "t" in the "isn" sub namespace. It’s also illegal with 5.26c and cannot be parsed.

For XS modules there’s the PERL_NO_QUOTE_PKGSEPERATOR definition. It is defined in cperl since 5.26c. Affected are only Sub::Name and Sub::Util. Sub::Name is broken for a long time and should not be used, and Sub::Util is distributed in cperl and perl5 core.

Removal of the perl4 ' package seperator

Comments on /r/cperl.

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