Re: exit this way

[sundialsvc4]

Another strategy that I have sometimes seen used goes something like this:

#!/usr/bin/perl

exit main();

sub main {
   ...
}

Even though the Perl language does not have the concept of a main() subroutine, these authors created one, and ensured that only it would be executed.   An interesting idea ...

[Your Mother]

This seems like a mistake or a bad practice in Perl. exit codes are a complete mismatch for subroutine return expectations. I suppose something like this might work but still… I say, nay. Also neigh.

#!/usr/bin/env perl
exit ! main();
sub main { $success = [ undef, "true" ]->[rand 2] }

[GrandFather]

Boiler plate code for the build scripts in our build system looks like:

package main;

my $build = BuildGeneral->new(buildtype => 'release');

exit(!$build || $build->runFailed());

which often causes a double take at first glance, but accurately reflects the sense of the exit value.

[1nickt]

Hi, Your Mother. Can you please explain this snippet? It does print out the randomly assigned value of $success if I change it to:

exit ! main();
sub main { $success = [ undef, "true" ]->[rand 2]; print "$success\n" 
+}

But it appears to produce the same output when I omit the !:

exit main();
sub main { $success = [ undef, "true" ]->[rand 2]; print "$success\n" 
+}

Which appears to be the same code as The Vague One posted ...

What is the function of the bang?

[Your Mother]

:P The last expression of a sub/block/package is its return value; when there is no explicit return. So that little snippet just rotates returning "true" and "false" randomly; idiom: return 0 or 1 element of anon array with 2 elements, one true, one false. By adding a print you made the sub always return "true."

exit values are approximately opposed to return/success values in most Perl. The bang (!) performs this inversion. exit == 0 means success, any other value (-1, 1 .. 255) means some flavor of fail (on *nix anyway). Why I suggest it is cognitively a bad idea for a Perl script. To test the original, try these instead (with 1140673.pl being the first scriptlet I posted)-

moo@cow~>1140673.pl && echo "I CAN HAZ SUCCEED?"
moo@cow~>1140673.pl || echo "FAIL!"

[RonW]

In many other languages, main is "special". The unnamed programmers were attempting to make a facsimile of that "specialness".

Perhaps:

#!perl
use strict;
use warnings;
main();
exit 99; # or other reasonable default exit value

might be better.

[Your Mother]

Yes, and the attempt at the idiom is a reasonable idea, I have in fact used it for service/daemon style scripts in the past, but exit 0; # undef works too is the only "correct" exit code. Everything else is an error state. Since returning false is suggestive of an error in Perl I am reticent to put that lipstick on this pig... or this pig stick on those lips... And that's the way the news goes!