Why doesn't Perl file glob() work outside of a loop in scalar context?

Answer 1

(Scratching away at my rusty memory of Perl...) I think that multiple lexical instances of <*> are treated as independent invokations of glob, whereas in the while loop you are invoking the same "instance" (whatever that means).

Imagine, for instance, if you did this:

while (<*>) { ... }
...
while (<*>) { ... }

You certainly wouldn't expect those two invocations to interfere with each other.

Answer 2

Also from perlop:

A (file)glob evaluates its (embedded) argument only when it is starting a new list.

Calling glob creates a list, which is either returned whole (in list context) or retrieved one element at a time (in scalar context). But each call to glob creates a separate list.

Answer 3

The following code also seems to create 2 separate instances of the iterator...

for ( 1..3 )
{
   $filename = <*>;
   print "$filename\n" if defined $filename;
   $filename = <*>;
   print "$filename\n" if defined $filename;
}

I guess I see the logic there, but it is kind of counter intuitive and contradictory to the documentation. The docs don't mention anything about having to be in a loop for the iteration to work.

Answer 4

Here's a way to capture the magic of the <> glob operator's state into an object that you can manipulate in a normal sort of way: anonymous subs (and/or closures)!

sub all_files {
    return sub { scalar <*> };
}

my $iter = all_files();
print $iter->(), "\n";
print $iter->(), "\n";
print $iter->(), "\n";

or perhaps:

sub dir_iterator {
    my $dir = shift;
    return sub { scalar glob("$dir/*") };
}
my $iter = dir_iterator("/etc");
print $iter->(), "\n";
print $iter->(), "\n";
print $iter->(), "\n";

Then again my inclination is to file this under "curiosity". Ignore this particular oddity of glob() / <> and use opendir/readdir, IO::All/readdir, or File::Glob instead :)