Use a module only if it is available

This construct allows you to use a module only if it's installed. Example with Pod::Usage -- will print full doc if available, message otherwise.

# This is equivalent to 'use Pod::Usage;', except that it does not fai
+l if the
# module is not available:
eval { require Pod::Usage; import Pod::Usage; };

#....

# Usage just calls pod2usage, except in case the Pod::Usage module is 
+not
# available. Then it tries to give some hint on what's going on.
#
sub usage {
  if (defined *pod2usage{CODE}) {
    pod2usage (@_);
  }
  else {
    print STDERR "Can't print error message because the ",
                 "Pod::Usage Perl module is not available\n";
    $"=", ";
    print STDERR "\nAdditional info: @_\n";
    exit 3;
  }
[download]

Comment on Use a module only if it is available Download Code

Replies are listed 'Best First'.
Re: Use a module only if it is available by chromatic (Archbishop) on Dec 30, 2001 at 08:45 UTC
Instead of checking for the code slot in the current typeglob, you could check $@.	[reply]
Re: Use a module only if it is available by ehdonhon (Curate) on Dec 30, 2001 at 10:43 UTC
I can't take credit for this idea, but I've seen it done in places, and use it myself. I think I saw it in Term::ReadLine: `unless ( eval "use Packagename; 1" ) { print "Packagename not available!!\n"; }` [download] The idea is that if things go wrong with the use statement, then the 1 never gets evaluated and the eval will pass back a false value, but if things go ok, then eval will pass back a true value ( 1, the last statement evaluated).	[reply] [d/l]
Re: Re: Use a module only if it is available by rob_au (Abbot) on Jan 19, 2003 at 12:25 UTC
The `1` value within the `eval` statement is unnecessary given that all modules must return a true value upon loading - This is most commonly achieved through the addition of a single statement such as: `# Code here 1; __END__` [download] `perl -le 'print+unpack("N",pack("B32","00000000000000000000001000011101"))'`	[reply] [d/l] [select]