Hi fellow perl monks.

I have had a nasty surprise when I discovered that Iterator prefetches values when instantiated. This causes non-pure functions to behave very, very strange. E.g.:

use Iterator;

my $i = 0;

my $iterator = new Iterator(sub { return ++$i });

print "Before calling anything: $i\n";
print "First call yields: ", $iterator->value, "\n";
print "After first call: $i\n";
[download]

This SHOULD print:

Before calling anything: 0
First call yields: 1
After first call: 1
[download]

But it does in fact print this:

Before calling anything: 1
First call yields: 1
After first call: 2
[download]

It seems that the author of Iterator wants it to contain a buffer of one value, but I have a very hard time understanding why. Could someone please enlighten me?

use Iterator;

my $i = 0;
sub get_a {
    return new Iterator(sub { print "a called\n"; return $i++ });
}

sub get_b {
    my $a = get_a;
    return new Iterator(sub { print "b called\n"; return $a->value });
}

print "Now I will get_b ($i):\n";
$b = get_b;
print "Got b ($i)\n";
print "First value is ($i):\n";
print "b: ", $b->value, " ($i)\n";
[download]

Now I will get_b (0):
Got b (0)
First value is (0):
b called
a called
b: 0 (1)
[download]

Now I will get_b (0):
a called
b called
a called
Got b (2)
First value is (2):
b called
a called
b: 0 (3)
[download]

Please let me know if there is any reasonable explaination for this behaviour.

Regards,

Michael Zedeler (MADZ).