Use a module only if it is in use

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

I want to use a module M from another module X, only if the calling program P currently uses it.

I have the following test case that works, but X asumes that P uses M:

P.pl:

#!perl

use strict;
use warnings;

use M;
use X;

hello("world");
bye();
[download]

M.pm:

package M;

require Exporter;
@ISA = (Exporter);
@EXPORT = qw(hello);

use warnings;
use strict;

sub hello {
  print "Hello @_!\n";
}
[download]

X.pm:

package X;

require Exporter;
@ISA = (Exporter);
@EXPORT = qw(bye);

use warnings;
use strict;

sub bye {
  M::hello("again"); # if exists M:hello();
  print "That's all, folks!\n";
}
[download]

What should be checked in X to recognize that M is loaded and available? If it's not, do nothing!

BTW, in real life, M could be any CPAN's module.

Comment on Use a module only if it is in use Select or Download Code

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Re: Use a module only if it is in use by almut (Canon) on Sep 02, 2009 at 14:45 UTC
`M::hello("again"); # if exists M:hello();` Not exactly sure what you want to achieve... but maybe you just want `M::hello("again") if defined &M::hello;` [download]	[reply] [d/l] [select]
Re^2: Use a module only if it is in use by vitoco (Hermit) on Sep 02, 2009 at 14:57 UTC
The `&` was what I forgot in my tests!!! `M::hello("again") if exists &M::hello;` [download] also works. Thank you!	[reply] [d/l] [select]
Re^3: Use a module only if it is in use by ikegami (Patriarch) on Sep 02, 2009 at 15:13 UTC
`exists` returns true if the sub is declared, whether it's been defined or not. `defined` returns true if the sub is defined. `sub foo; # exists but not defined sub bar {} # exists and defined` [download]	[reply] [d/l] [select]
Re: Use module only if it is in use by ikegami (Patriarch) on Sep 02, 2009 at 14:27 UTC
Why? Check `%INC` to see if a module was loaded. Update: Fixed markup	[reply] [d/l]
Re^2: Use module only if it is in use by vitoco (Hermit) on Sep 02, 2009 at 14:50 UTC
It was that simple!!! `M::hello("again") if $INC{"M.pm"};` [download] Is it safe to asume that double colons in module names get always translated into slashes ("::" -> "/") instead of backslashes or whatever is used by the OS for subdirs?	[reply] [d/l]
Re^3: Use module only if it is in use by JavaFan (Canon) on Sep 02, 2009 at 15:56 UTC
Is it safe to asume that double colons in module names get always translated into slashes ("::" -> "/") instead of backslashes or whatever is used by the OS for subdirs? No, it's not safe to assume that. And be lucky. Your code wouldn't work if Perl changed the '::' to a '/', because that it would try to divide `M` by `hello("again")`, which most likely results in an error. Perl maps '::' to the appropriate directory separator when it's used as a bareword argument to `use` and `require`. But that isn't always.	[reply] [d/l] [select]
Re^4: Use module only if it is in use by bart (Canon) on Sep 02, 2009 at 18:32 UTC
Re^5: Use module only if it is in use by ikegami (Patriarch) on Sep 02, 2009 at 19:29 UTC
Re^4: Use module only if it is in use by ikegami (Patriarch) on Sep 02, 2009 at 16:08 UTC
Re^4: Use module only if it is in use by vitoco (Hermit) on Sep 02, 2009 at 16:08 UTC
Re^3: Use module only if it is in use by ikegami (Patriarch) on Sep 02, 2009 at 15:20 UTC
Yes, but again, why? I can't think of a scenario where this would possibly be useful. It feels like you're testing for a side-effect of the problem, and not for the problem itself.	[reply]
Re^4: Use module only if it is in use by vitoco (Hermit) on Sep 02, 2009 at 15:47 UTC
Re^5: Use module only if it is in use by ikegami (Patriarch) on Sep 02, 2009 at 16:13 UTC
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