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

Just a quick question -- what's the best (eg. fastest, least amount of code) way to test to see if a subroutine is defined in the current namespace?

I looked around here a bit, but couldn't find anyone else's comments on this. So I've been using eval, but I figure there must a perl shortcut for this sort of thing..

Here's what I've got: 
my $sub_to_test = "test_sub";   

# try to fire an EVAL_ERROR
eval "$sub_to_test()";                          

# if no EVAL_ERROR
if (!$@) {                                      
        # do the thing
        eval "$sub_to_test ('testing, 1,2,3')"; 
}

sub test_sub {
        return unless @_;
        print "TEST_SUB: $_[0]\n";
}

[download]

can I do any better?
--twerq

Replies are listed 'Best First'.
Re: test for subroutine existence
by Abigail-II (Bishop) on Aug 12, 2002 at 13:59 UTC
    To see whether something is defined, you use the defined function: 
    #!/usr/bin/perl

use warnings 'all';
use strict;

sub my_sub {1}

print "my_sub ", defined &my_sub ? "exists\n" : "doesn't exist\n";
print "no_sub ", defined &no_sub ? "exists\n" : "doesn't exist\n";

__END__
my_sub exists
no_sub doesn't exist

    [download]
    Abigail
[reply]
[d/l]
[select]
      Sorry, I probably wasn't clear enough in my original post (but that's why I added the code)...
      I need to be able to test for a subroutine's existence, a subroutine who'se name is only available to me in a scalar -- so I can't very well use defined &my_sub (or can I?)

      fuzzyping's right, eval does work pretty well, except that I end up actually executing the subroutine itself just to see if it's there -- I'd prefer not to actually run it at test time...

      --twerq
[reply]
[d/l]
        No eval needed. 
        #!/usr/bin/perl

use warnings 'all';
use strict;

sub my_sub {1}

foreach my $sub (qw /my_sub no_sub/) {
    print "$sub ", defined &$sub ? "exists\n" : "doesn't exist\n";
}

__END__
my_sub exists
no_sub doesn't exist

        [download]
        Abigail
[reply]
[d/l]
        Okay, anyway -- now it seems pretty obvious:
        print "my_sub ", test_for_sub("my_sub") ? "exists\n" : "doesn't exist\
+n";
print "no_sub ", test_for_sub("no_sub") ? "exists\n" : "doesn't exist\
+n";

sub test_for_sub {
        my ($expr) = @_;
        return eval "defined &$expr";
}

sub my_sub { 1; }

        [download]


        --twerq
[reply]
[d/l]
Re: test for subroutine existence
by fuzzyping (Chaplain) on Aug 12, 2002 at 14:02 UTC
    Seems like eval() works fine for you, but, if you simply want to check for a sub's existence without having to perform a call on it, just use defined() (or the implied defined):

    You may also use defined(&func) to check whether subroutine &func has ever been defined. The return value is unaffected by any forward declarations of &foo. Note that a subroutine which is not defined may still be callable: its package may have an AUTOLOAD method that makes it spring into existence the first time that it is called -- see perlsub.

    -fp
[reply]
Re: test for subroutine existence
by ehdonhon (Curate) on Aug 12, 2002 at 17:00 UTC

    As long as you are using a simple module with no inheritance, you could rely on can to do this:

      if ( __PACKAGE__->can( 'sub_to_test' ) ) {
    ...
  }

    [download]
[reply]
[d/l]
Re: Test for Subroutine Definition
by tadman (Prior) on Aug 15, 2002 at 07:42 UTC
    Just my two cents: 
    sub foo { }

foreach (qw[ foo bar ])
{
        print "$_? ",(defined(*$_{CODE})? "defined":"undef"), "\n";
}

    [download]
    Using this kind of test, you can check for other stuff, like GLOB, filehandles and such, which aren't easily prefixed with something like '$', '&', '%' or '@'.
[reply]
[d/l]

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