Recently while developing a script i thought i found a good use for closures. The script itself is a pretty simple log parsing script, and on the whole i didn't need to use a closure, but the chance to learn got hold of me and i decided to use one anyway :-) Simply put, i'm running through a loop and i want to initialize an array with certain settings every time through. The settings are actually user-dependant and read from a file. Since reading from a file many times is a slow-up on the rest of the code i only wanted to read it once.

Of course i wouldn't be posting unless there was a problem. It turns out that i can use simple scalars/arrays/hashes just fine, but a code ref doesn't get "parsed"¹ correctly. The problem i ran into is that the closure must be "parsed" before any code that calls it can run or else strict will die with the message:

Can't use string ("") as a subroutine ref while "strict refs" in use a
+t temp line 21.
[download]

<READ MORE> My solution was a BEGIN block, but i'm wondering if that's the best solution. Anything that requires magic to work makes me wonder about my design. So with that, here's a code example with documentation (using perl5.6.0). Please tell me if there's a better solution.

#!/usr/bin/perl -w
use strict;

# To test this uncomment these lines and (un)comment BEGIN {}
#my @strings = get_init_strings();
#print "strings: @strings\n";

BEGIN {
{ # begin closure

  my @strings;
  my $get_strings = sub {
    # Trivialized code here for brevity.
    # This actually reads in from a file with lots of error checking.
    return "this", "that", "the", "other";
  };

  # the actual subroutine, the only thing callable outside of the clos
+ure
  sub get_init_strings {
    if (!@strings) {                        # only call $get_strings o
+nce
      @strings = $get_strings->();
    }
    return @strings;
  }

} # close closure
} # close BEGIN block
[download]

Since i want to have the actual main code at the top of the file (for maintainability) and not the subroutines, this is what i resorted to. The only other way i could see to get it to "parse" this first is to have a subroutine return this code as a string, and then eval the string at the top of the file. That sounded too icky so i avoided it. Either way i have to document this well so later maintainers don't ponder for 3 days why i did this.

Also, i searched for related topics and came up with this but testing showed that mentioning $get_strings again didn't solve the problem, so i don't think it's related. And if there's something in merlyn's WebTechnique column about this i didn't catch it, and those were the only highly pertinent links i found on the matter (but i have been known to be blind :-)

Any help with style, suggestions, otherwise is greatly appreciated,
jynx

¹ "Parsed" is in quotes here because that's almost but not quite what i mean when i say that. IIRC there's a lot more that goes on than just "parsing"...

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