Problem with ``use if COND, Some::Module'' on Linux

stevieb has asked for the wisdom of the Perl Monks concerning the following question:

Hey all,

On Windows (2012 server, running perl 5.22.1), I get expected results using use if ...;:

use if $^O !~ /MSWin/, "Proc::Daemon";
use if $^O =~ /MSWin/, "Win32::Daemon";

print "$INC{'Win32/Daemon.pm'}\n";
exit;

__END__
C:/berrybrew/5.22.1_64/perl/vendor/lib/Win32/Daemon.pm
[download]

But on Ubuntu, running v5.22.1:

use if $^O !~ /MSWin/, 'Proc::Daemon';
use if $^O =~ /MSWin/, 'Win32::Daemon';

print "$INC{'Proc/Daemon.pm'}\n";
exit;

__END__
Too few arguments to 'use if' (some code returning an empty list in li
+st context?) at /home/ubuntu/perl5/perlbrew/perls/perl-5.22.1/lib/5.2
+2.1/if.pm line 7.
BEGIN failed--compilation aborted at /home/ubuntu/perl5/perlbrew/perls
+/perl-5.22.1/bin/bbtester line 6.
[download]

I can't really find anything about this being a known problem. Proc::Daemon is properly installed (as I can use it in a module without the conditional use):

perl -MProc::Daemon -E 'say $Proc::Daemon::VERSION'
0.23
[download]

Can anyone spot anything I'm doing wrong, or is this something that just doesn't work reliably?

Update: I just realized that this was my 1,000th post here on PerlMonks :)

Comment on Problem with ``use if COND, Some::Module'' on Linux Select or Download Code

Replies are listed 'Best First'.
Re: Problem with ``use if COND, Some::Module'' on Linux by 1nickt (Canon) on Apr 08, 2016 at 17:30 UTC
" ... (some code returning an empty list in list context?) ... " Nothing to do with Proc::Daemon, but with the pattern match that returns an empty list: `use strict; use warnings; use if scalar ( $^O !~ /Win32/ ), 'Proc::Daemon'; use if scalar ( $^O =~ /Win32/ ), 'Win32::Daemon'; print $INC{'Proc/Daemon.pm'}; exit; __END__` [download] Output: `perl 1159936.pl /Users/nick/perl5/perlbrew/perls/perl-5.22.0/lib/site_perl/5.22.0/Proc +/Daemon.pm` [download] Hope this helps! Updated: force scalar context so we can keep pattern matches The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^2: Problem with ``use if COND, Some::Module'' on Linux by stevieb (Canon) on Apr 08, 2016 at 17:39 UTC
Yep, that works if I do `ne 'MSWin32'`, but I'm very curious to know why my way works on Windows but not on nix. The whole reason for the `!~, =~` is so I don't have to check for both `MSWin32` and `MSWin64`.	[reply] [d/l] [select]
Re^3: Problem with ``use if COND, Some::Module'' on Linux by VinsWorldcom (Prior) on Apr 08, 2016 at 19:41 UTC
Not for nothing, but is 'MSWin64' an actual OS string? I'm on Windows 7 x64 with Strawberry Perl 5.22 64-bit and $^O still returns "MSWin32" for me. `VinsWorldcom@C:\Users\VinsWorldcom> ver Microsoft Windows [Version 6.1.7601] VinsWorldcom@C:\Users\VinsWorldcom> perl -v This is perl 5, version 22, subversion 1 (v5.22.1) built for MSWin32-x +64-multi-thread [...] VinsWorldcom@C:\Users\VinsWorldcom> perl -e "print $^O" MSWin32` [download]	[reply] [d/l]
Re^3: Problem with ``use if COND, Some::Module'' on Linux by stevieb (Canon) on Apr 08, 2016 at 17:45 UTC
The following works... `my $is_win = $^O =~ /MSWin/ ? 1 : 0; use if $is_win, 'Win32::Daemon'; use if ! $is_win, 'Proc::Daemon';` [download]	[reply] [d/l]
Re^4: Problem with ``use if COND, Some::Module'' on Linux by Laurent_R (Canon) on Apr 08, 2016 at 17:51 UTC
Re^5: Problem with ``use if COND, Some::Module'' on Linux by Anonymous Monk on Apr 08, 2016 at 19:30 UTC
Re^3: Problem with ``use if COND, Some::Module'' on Linux by 1nickt (Canon) on Apr 08, 2016 at 17:48 UTC
Updated to force scalar context on the pattern match output. The way forward always starts with a minimal test.	[reply]
Re: Problem with ``use if COND, Some::Module'' on Linux by RonW (Parson) on Apr 08, 2016 at 17:46 UTC
The "hint" in the error message provides a big clue. The arguments to `use if` are evaluated in list context. Therefore the match expressions are being evaluated in list context. From the Perl documentation: "When there are no parentheses in the pattern, the return value is the list (1) for success. With or without parentheses, an empty list is returned upon failure." So, on a non-MSwindows platform, `$^O =~ /MSWin/` is returning an empty list. Of course, on MSwindows, `$^O !~ /MSWin/` is failing, but because it has an implied `not`, it is equivalent to `not ($^O =~ /MSWin/)` so is returning a non-empty false value.	[reply] [d/l] [select]
Re^2: Problem with ``use if COND, Some::Module'' on Linux by stevieb (Canon) on Apr 08, 2016 at 17:50 UTC
aahhhh, yep, that does make sense. Rubber D^H^H^H^H^H^H^H^HPerlmonks Debugging saves the day once again :) Cheers!	[reply]
Re^3: Problem with ``use if COND, Some::Module'' on Linux by RonW (Parson) on Apr 08, 2016 at 18:14 UTC
Quack	[reply]
Re: Problem with ``use if COND, Some::Module'' on Linux by Anonymous Monk on Apr 08, 2016 at 19:55 UTC
See Devel::CheckOS `use Devel::CheckOS qw/ os_is /; use if os_is(qw/ Win32 /), 'Win32::Daemon'; use if !!os_is(qw/ Win32 /), 'Proc::Daemon';` [download]	[reply] [d/l]
Re: Problem with ``use if COND, Some::Module'' on Linux by ww (Archbishop) on Apr 08, 2016 at 17:48 UTC
Kludge, alternate, but seems that Ubuntu may ("MAY!") prefer this syntax, which works on both MSWin7 (64) and Ubuntu 14.04 and produces the expected output: `# 1159936 revising stevieb's OP use if ( $^O !~ /MSWin/ ), 'Proc::Daemon'; { print "$INC{'Proc/Daemon.pm'}\n"; } use if ($^O =~ /MSWin/), 'Win32::Daemon'; { print "$INC{'Win32/Daemon.pm'}\n"; } exit;` [download] If you didn't program your executable by toggling in binary, it wasn't really programming!	[reply] [d/l]
Re^2: Problem with ``use if COND, Some::Module'' on Linux by 1nickt (Canon) on Apr 08, 2016 at 18:02 UTC
That doesn't fix the problem on my Ubuntu (or Darwin): `$ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 14.04.3 LTS Release: 14.04 Codename: trusty` [download] Code: `use if ( $^O !~ /MSWin/ ), 'Proc::Daemon'; { print "$INC{'Proc/Daemon.pm'}\n"; } use if ($^O =~ /MSWin/), 'Win32::Daemon'; { print "$INC{'Win32/Daemon.pm'}\n"; } exit;` [download] Output: `Too few arguments to 'use if' (some code returning an empty list in li +st context?) at /home/vagrant/perl5/perlbrew/perls/perl-5.16.3/lib/5. +16.3/if.pm line 7.` [download] The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^2: Problem with ``use if COND, Some::Module'' on Linux by stevieb (Canon) on Apr 08, 2016 at 18:04 UTC
Update: I was mistaken. I didn't test both conditions... the latter one did break. Thanks ww, ~~I honestly could have sworn that I tested that, but I couldn't have, as wrapping the condition within parens does in fact work properly.~~	[reply]
Re^3: Problem with ``use if COND, Some::Module'' on Linux by 1nickt (Canon) on Apr 08, 2016 at 18:09 UTC
Really? On Ubuntu 14.04? Not for me ... The way forward always starts with a minimal test.	[reply]


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