p5pRT
commented
20 years ago From @jlokier
Created by @jlokier
perl -le 'for (1..10) { my $x; $x .= "A"; $$x .= "B"; print length($x)."\, ".length($$x)."\, $x\, $$x"; }'
Prints:
1\, 1\, A\, B 1\, 2\, A\, BB 1\, 3\, A\, BBB
What a stonking great pair of surprises!
Firstly\, that $x can be both a string _and_ a reference at the same time. That's a surprise to me\, but probably not to long-time Perl 5 133t ha%0rz.
Secondly\, that $$x is magically not destroyed and recreated\, even though $x is! That\, surely\, is just plain wrong?
To illustrate why it's dubious\, try this:
perl -le 'for (1..3) { my $x; $$x .= "B"; print length($x)."\, ".length($$x)."\, $x\, $$x"; }'
The output is:
17\, 1\, SCALAR(0x804c988)\, B 17\, 1\, SCALAR(0x804c988)\, B 17\, 1\, SCALAR(0x804c988)\, B
Note how _now_ we are still auto-vivifying $$x and appending to it\, but each time round the loop $$x is destroyed and recreated anew. As expected.
There is some serious mystery in the Perl lexical variable implementation\, and I'm sure it's not compatible with what we expect lexical variables to do.
It's nice that taking a reference to a lexical (\$x) keeps the lexical alive. But lexicals that don't go away when there aren't any program-visible references to them are extremely impolite. Object users _expect_ objects to be destroyed when a lexical scope is left - that's one of the lovely things about Perl's lexical model - file handles being a fine example of those objects. (I've just tried\, and as expected\, objects accessed through $$x aren't destroyed when the scope of $x is left).
More illustrative is this code:
perl -le 'for (1..3) { my $x="A"; $$x .= "B"; print "$x\, $$x"; $x="A"; $$x .= "D"; print "$x\, $$x"; }'
A\, B C\, D A\, BB C\, DD A\, BBB C\, DDD
Oh. That looks a lot like the string is used as a hash key of some kind. It looks quite deliberate.
Is there something I've not noticed in the documentation? Is it even useful? Hmm\, lexicals that behave like C local statics...
Seriously\, I understand Perl 5 has a _lot_ of oddities in the lexical variables department\, as I seem to be discovering at a remarkeably high rate. (And I've been using Perl 5 for years; it's strange how they're all showing up now). I never noticed these in the manual. Are they know\, expected\, normal behaviour\, or are they really bugs?
In particular\, "man perlsub" explains a lot about lexical variables and mentions nothing about lexicals that retain their value (or aspects of it) when a scope is exited and reentered\, in fact it says this:
The parameter list to my() may be assigned to if desired\, which allows you to initialize your variables. (If no initializer is given for a particular variable\, it is created with the undefined value.) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
One could be forgiven for believing that it means C\
Btw\, C\<use strict 'refs'> does prevent these kind of assignments. It's not the end of the world.
-- Jamie
Migrated from rt.perl.org#27591 (status was 'resolved')
Searchable as RT27591$