in reply to Re: static-like persistence of my variable due to trailing conditional
in thread static-like persistence of my variable due to trailing conditional

The problem is definitely that the whole line "my $foo = "blah" if $var;" isn't getting executed (including the my $foo declaration) ... you can see the expected behavior if you change it to : "my $foo; $foo = "blah" if $var;"

But a global $foo isn't getting set -- otherwise "-w/use strict" would complain.

I honestly have no idea what in the semantic rules of Perl describes the behavior you are seeing. Why this compiles, and does what it does without complianing is beyond me. I would expect it to complain the same way this does... 

#!/usr/bin/perl -w
use strict;
sub mycode {
  my $var = $_[0];
  my $foo = "blah" if $var; 
  print "foo is $foo\n" if $foo;
  $bar ||= "BAR";
}
mycode("test");
mycode();
mycode();
mycode();

[download]

Replies are listed 'Best First'.
Re: Re: Re: static-like persistence of my variable due to trailing conditional
by jsprat (Curate) on Jun 29, 2002 at 22:29 UTC
    It looks like you are absolutely right. The output of perl -MO=Deparse,-p a.pl follows (after removing Data::Dumper):

    sub mycode {
    (my $var = $_[0]);
    (defined($var) ? print("mycode() called with $var\n") :
print("mycode() called with no operands.\n"));
    ($var and (my $foo = 'blah'));
#^^^^^^^^^^^^^^^ Look here
    ($foo and print("foo is $foo\n"));
    ($foo ||= 'FOO');
}
mycode('test');
mycode();
mycode();
mycode();
a.pl syntax OK

    [download]

    my $foo = 'blah' is optimized away.

    In order to make it behave as expected, my $foo; needs to be its own statement, with no conditional.

    Update: Removed bit about strict, see the follow up

[reply]
[d/l]
[select]
      my $foo = 'blah' is optimized away. It looks like $foo is still in the symbol table for mycode, so strict doesn't complain on any subsequent calls.
      Hmm. I don't agree and/or don't completely understand what you're saying. (1) What do you mean by "optimized away"? (2) if $foo is in a symbol table, can you provide example code to print foo outside of the sub mycode{} block?

      On a side note, does anyone know of links/books/perldoc that I can look at which document this "feature"?

      ---
      "A Jedi uses the Force for knowledge and defense, never for attack."
[reply]
[d/l]
[select]
        1. ($var and (my $foo = 'blah'));The second part of this statement will never be executed if $var is undefined. The 'and' is short-circuited, so the second part of the statement (the re-initialization of $foo) is optimized away never evaluated.
        2. You are right. Poor use of the term "symbol table" on my part, since $foo is a lexical variable. I just got typing too fast ;-) However, I never meant to say that $foo was available outside of the block. What I meant was that it was initialized on the first call to mysub and retains the original value within that block since it is never re-initialized, just like using a sub to create a static variable as in perlfaq7. You can also get the same result (a static variable) by using my $i = 0 if 0;
        On a side note, does anyone know of links/books/perldoc that I can look at which document this "feature"?
        I think this feature should be spelled 'b-u-g'

        And about strict not complaining, I was just plain wrong.

        Update: Changed "optimized away" - Aristotle++.

[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom