perl string interpolation when using the package delimiter

Question

Am fighting some legacy perl looking like the following:

sub UNIVERSAL::has_sub_class {
    my ($package,$class) = @_;
    my $all = all_packages();
    print "$package - $class", "\n";
    print "$package::$class", "\n";
    return exists $all->{"$package::$class"};
}

On two different systems, two different PERL installations / versions, this code behaves differently, i.e. the "$package::$class" construct is properly resolved to the correct package name on one system, but not on the other.

The following different print outputs can be seen when running has_sub_class on the two different systems:

# print output on system 1 (perl v5.8.6):
webmars::parameter=HASH(0xee93d0) - webmars::parameter::date
webmars::parameter::date

# print output on system 2 (perl v5.18.1):
webmars::parameter=HASH(0x251c500) - webmars::parameter::date
webmars::parameter=HASH(0x251c500)::webmars::parameter::date

Has there been any string interpolation changes in between perl v5.8.6 and perl v5.18.1 that you know might cause this behaviour ? Or should I look somewhere else ? I've really tried google-ing around and reading through the perl change notes, but could not find anything of interest.

With my limited knowledge of perl, I've tried getting the smallest piece of code that could reproduce the problem I am having. I have come up with the following which I hope is relevant:

# system 1 (perl v5.8.6):
$ perl -e 'my %x=(),$x=bless(\%x),$y='bar';print "$x::$y\n";'
bar

# system 2 (perl v5.18.1):
$ perl -e 'my %x=(),$x=bless(\%x),$y='bar';print "$x::$y\n";'
main=HASH(0xec0ce0)::bar

The outputs are different ! Any ideas ?

perl -Mstrict -we 'my %x=(),$x=bless(\%x),$y='bar';print "$x::$y\n";' — mpapec
– mpapec, Commented May 12, 2015 at 15:20
Could quite easily be a bug fixed in the intervening 6 years between the releases :). But well done for such a concise MCVE! — Sobrique
– Sobrique, Commented May 12, 2015 at 15:37

mob · Accepted Answer · 2015-05-12 18:13:41Z

4

Shorter demonstration:

($x::, $x) = (1,2);  print "$x::$x"

$ perl5.16.3 -e '($x::, $x) = (1,2);  print "$x::$x"'
12

$ perl5.18.1 -e '($x::, $x) = (1,2);  print "$x::$x"'
2::2

Getting warmer.

$ perl5.16.3 -MO=Concise =e 'print "$x::$x"'
8  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <@> print vK ->8
3        <0> pushmark s ->4
-        <1> ex-stringify sK/1 ->7
-           <0> ex-pushmark s ->4
6           <2> concat[t3] sK/2 ->7
-              <1> ex-rv2sv sK/1 ->5
4                 <#> gvsv[*x::] s ->5          <-     $x::
-              <1> ex-rv2sv sK/1 ->6
5                 <#> gvsv[*x] s ->6            <-     $x
-e syntax OK

$ perl5.18.1 -MO=Concise -e 'print "$x::$x"'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
9     <@> print vK ->a
3        <0> pushmark s ->4
-        <1> ex-stringify sK/1 ->9
-           <0> ex-pushmark s ->4
8           <2> concat[t4] sKS/2 ->9
6              <2> concat[t2] sK/2 ->7
-                 <1> ex-rv2sv sK/1 ->5
4                    <#> gvsv[*x] s ->5         <-     $x
5                 <$> const[PV "::"] s ->6      <-     "::"
-              <1> ex-rv2sv sK/1 ->8
7                 <#> gvsv[*x] s ->8            <-     $x
-e syntax OK

TL;DR. v5.16 parses "$x::$x" as $x:: . $x. v5.18 as $x . "::" . $x. I don't see any obvious reference to this change in the delta docs, but I'll keep looking.

edited May 12, 2015 at 18:13

answered May 12, 2015 at 16:35

mob

119k18 gold badges159 silver badges291 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Cricri Over a year ago

Indeed, it looks like there was a bug in the code which was being silenced due tot "$x::$x" parsing as $x:: . $x, i.e. "$package::$class" would look for $package:: instead of $package and end up replacing it with `` in the string since it's undefined.

mob Over a year ago

Rewrite that as ${package}::$class to get it work in any version of perl.

Sobrique · Accepted Answer · 2015-05-12 16:16:18Z

3

So, my very quick test confirms the issue - using

perl -Mstrict -we 'my %x=(),$x=bless(\%x),$y="bar";print "$x::$y\n";'

(With your version, I get a bareword warning for 'bar').

The error I get in 5.8.8 is "use of uninitialized value in concatenation".

The difference seems to be that when I run with perl -MO=Deparse I get:

my ( %x ) = ();
my $x = bless ( \%x );
my $y = 'bar';
print "$x::$y\n";

If I run on 5.20.2 though, I get:

my ( %x ) = ();
my $x = bless ( \%x );
my $y = 'bar';
print "${x}::$y\n";

So yes, there has been a change in how the same code is parsed. But I'm not entirely sure how that helps you, apart from perhaps enlightening you as to what's going on?

answered May 12, 2015 at 16:16

Sobrique

53.6k8 gold badges63 silver badges107 bronze badges

1 Comment

Cricri Over a year ago

It does help me a lot to understand where the problem is coming from, if it's the perl interpreter, or issues in the code. And in this case, it's a combination of both.

Collectives™ on Stack Overflow

perl string interpolation when using the package delimiter

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related