Conditionally faking a module

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

Hello monks,

I've built an application which is designed to run both normally on Linux, or packaged up on Win32 with Cava Packer. Cava Packer comes with Cava::Pack, which I need to fake out, because as far as I can tell, there is no equivalent module for Linux.

I've worked around this with the following code:

BEGIN {
    package Cava::Pack;
    sub Resource { $Cava::Pack::dir.'/'.$_[0]; }
    sub SetResourcePath { $Cava::Pack::dir = $_[0]; }
    sub GetInfoProductVersion { "0.9.9.0"; }
}
[download]

This works fine in Linux, but in Windows I need to use the real Cava::Pack, and if I add a use Cava::Pack; line, I get function redefinition warnings (not to mention compilation fails on Linux because there is no Cava/Pack.pm in my @INC.

What I need to do is conditionally use the real Cava::Pack if it is available, or run my above code if it is not available. How do I go about that?

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Re: Conditionally faking a module by Anonymous Monk on Jan 13, 2010 at 18:38 UTC
see use, require, eval `BEGIN { if( eval { require Cava::Pack; 1 } ) { import Cava::Pack; } else { no strict 'refs'; Cava::Pack::Resource = sub { $Cava::Pack::dir . '/' . $_[0]; }; Cava::Pack::SetResourcePath = sub { $Cava::Pack::dir = $_[0]; }; *Cava::Pack::GetInfoProductVersion = sub { "0.9.9.0"; }; } ## end unless ( eval { require Cava::Pack...}) } ## end BEGIN` [download]	[reply] [d/l]
Re^2: Conditionally faking a module by wanna_code_perl (Friar) on Jan 13, 2010 at 19:03 UTC
This was the ticket. Thanks!	[reply]
Re: Conditionally faking a module by bobf (Monsignor) on Jan 13, 2010 at 18:40 UTC
If you want to run some code if a module isn't installed, just test the `use` statement for failure: `eval "use Cava::Pack" or do { # your code goes here };` [download] Super Search will find several threads on this topic, including Auto install of a perl package if 'use' statement fails.	[reply] [d/l] [select]
Re^2: Conditionally faking a module by wanna_code_perl (Friar) on Jan 13, 2010 at 19:01 UTC
Unfortunately, your example doesn't work: `eval "use Cava::Pack" or do { die "Using faked out Cava::Pack\n"; }` [download] I see the "Using faked out Cava::Pack" message on both Linux and Win32 (where Cava::Pack is indeed available).	[reply] [d/l]
Re^3: Conditionally faking a module by bobf (Monsignor) on Jan 13, 2010 at 19:14 UTC
Apparently it may not always work, as I learned when I researched this further (see RE: Checking to see if a particular Module is installed). merlyn's response was to use the following: `eval 'require GD'; if ($@) { # problems with GD, fall back to non-GD } else { # we have GD, go for it! }` [download] although there appears to be some caveats to that approach. If you are open to using other modules to do the check, you could look into ExtUtils::Installed and Module::Load::Conditional. See also a recent thread on this topic: Check if module is installed.	[reply] [d/l]
Re^3: Conditionally faking a module by Anonymous Monk on Jan 13, 2010 at 21:27 UTC
That is the reason for a true value, 1 `eval "use Cava::Pack; 1" or die "Oh noes! $@"; eval "use HOW::Cava::Pack; 1" or die "HOW!?! $@"; __END__ HOW!?! Can't locate HOW/Cava/Pack.pm in @INC (@INC contains: C:/perl/5 +.10.1/lib/MSWin32-x86-multi-thread C:/perl/5.10.1/lib C:/perl/site/5. +10.1/lib/MSWin32-x86-multi-thread C:/perl/site/5.10.1/lib .) at (eval + 2) line 1. BEGIN failed--compilation aborted at (eval 2) line 1.` [download] Hmm, I don't recall installing Cava::Pack :)	[reply] [d/l]
Re^3: Conditionally faking a module by ikegami (Patriarch) on Jan 15, 2010 at 18:01 UTC
The code being executed by `eval` (`""`) returns nothing (`()`), which is indistinguishable from the value `eval` returns on failure. You need `eval "use Cava::Pack; 1"` [download]	[reply] [d/l] [select]
Re: Conditionally faking a module by NiJo (Friar) on Jan 13, 2010 at 18:59 UTC
`# add Cava\Pack.pm at the end of include path, # real Cava::Pack is found earlier push @INC, './dummy' use Cava::Pack` [download]	[reply] [d/l]
Re^2: Conditionally faking a module by Anonymous Monk on Jan 15, 2010 at 09:16 UTC
use happens at BEGIN time, so to make your strategy work you would need `BEGIN { push @INC, './dummy'; }` [download] Its shorter to use lib `use lib './dummy';` [download]	[reply] [d/l] [select]
Re^3: Conditionally faking a module by NiJo (Friar) on Jan 15, 2010 at 17:29 UTC
You are right with the BEGIN block, but 'use lib' adds to the high priority end of @INC, so the real package is never found.	[reply]