perl - two stage conditional compilation

Question

I have pretty big perl script executed quite frequently (from cron).
Most executions require pretty short & simple tests.

How to split single file script into two parts with "part two" compiled based on "part 1" decision?

---

Considered solution:

1. using BEGIN{ …; exit if …; } block for trivial test.
2. two file solution with file_1 using require to compile&execute file_2.
   I would prefer single file solution to ease maintenance if the cost is reasonable.

Answer 1

First, you should measure how long the compilation really takes, to see if this "optimization" is even necessary. If it does happen to be, then since you said you'd prefer a one-file solution, one possible solution is using the __DATA__ section for code like so:

use warnings;
use strict;

# measure compliation and execution time
use Time::HiRes qw/ gettimeofday tv_interval /;
my $start;
BEGIN { $start = [gettimeofday] }
INIT  { printf "%.06f\n", tv_interval($start) }
END   { printf "%.06f\n", tv_interval($start) }

my $condition = 1; # dummy for testing
# conditionally compile and run the code in the DATA section
if ($condition) {
    eval do { local $/; <DATA>.'; 1' } or die $@;
}

__DATA__
# ... lots of code here ...

Answer 2

I see two ways of achieving what you want. The simple one would be to divide the script in two parts. The first part will do the simple tests. Then, if you need to do more complicated tests you may "add" the second part. The way to do this is using eval like this:

<first-script.pl>
...
eval `cat second-script.pl`;
if( $@ ) {
  print STDERR $@, "\n";
  die "Errors in the second script.\n";
}

Or using File::Slurp in a more robust way:

eval read_file("second-script.pl", binmode => ':utf8');

Or following @amon suggestion and do:

do "second-script.pl";

Only beware that do is different from eval in this way:

It also differs in that code evaluated with do FILE cannot see lexicals in the enclosing scope; eval STRING does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop.

The eval will execute in the context of the first script, so any variables or initializations will be available to that code.

Related to this, there is this question: Best way to add dynamic code to a perl application, which I asked some time ago (and answered myself with the help of the comments provided and some research.) I took some time to document everything I could think of for anyone (and myself) to refer to.

The second way I see would be to turn your testing script into a daemon and have the crontab bit call this daemon as necessary. The daemon remains alive so any data structures that you may need will remain in memory. On the down side, this will take resources in a continuos way as the daemon process will always be running.