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

I'm wondering if there's a better way to avoid the warning of "Ambiguous use of X resolved to Y".
Here's what I did: 
   # issues "Ambiguous use of X resolved to Y" warning that 
   # I don't know how to turn off, these don't work:
   # $SIG{__WARN__} = sub{}
   # $^W = 0
   # no strict 'refs'
   my $old_scan_podpath = *{Pod::Html::scan_podpath}{CODE};

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I couldn't get rid of the warning generated from that line of code, so I went with this: 
   # issues "Subroutine X redefined at Y" warning, but
   # I can turn that one off with $^W
   local( $^W ) = 0;

   # Needed because I'm using a symbolic ref now.
   no strict 'refs';
   my $old_scan_podpath = *{'Pod::Html::scan_podpath'}{CODE};

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That seems to work.

After that's set up, I can do this: 
   local(*Pod::Html::scan_podpath) = sub {
      # do stuff here
      # call the old implementation
      &$old_scan_podpath;  # uses @_
   };

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And yes, I'm aware of a few wrap modules out there, but for some reason I didn't want to add more dependencies to this script than I already have. Besides, it seemed like a good opportunity for me to learn how to do this.

So, am I avoiding that warning the right way? Or is there a better way?

Thanks!
-Dave

Replies are listed 'Best First'.
Re: Ambiguous use of X resolved to Y
by chromatic (Archbishop) on Feb 05, 2004 at 23:28 UTC

    I believe you're running into magic autodisambiguation of hash keys versus non-words. In short, the first example is also a symbolic reference. It's just not in quotes, which makes it hard for Perl to tell whether you want it to be a quoted string (since it contains non-identifier characters) or you want to use the result of a function call as the hash key.

    Wow, that's an unclear paragraph.

    I seem to recall that just taking a reference to the function doesn't quite do the right thing when local is involved, but I'd try my $old_sub = \&Pod::Html::scan_podpath; first. Hook::LexWrap is definitely a better general choice, though.

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Re: Ambiguous use of X resolved to Y
by diotalevi (Canon) on Feb 05, 2004 at 23:30 UTC 
    my $old = \&Pod::HTML::scan_podpath;
{
    local $^W;
    *Pod::HTML::scan_podpath = sub {
        &{$old}( @_ );
    };
}

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[d/l]
      I seem to recall trying an incantation very similiar to that, but it ended up causing deep recursion. It seemed that the *{name}{CODE} syntax was required to make it work right.

      I suspect that what happens, is that a reference to the actual function in the module is lost when the glob gets reassigned. The reference held in $old is to a glob, and calling a function on it essentially references *{$old}{CODE}, which isn't really what you wanted (because it's the new one). That causes recursion.

      If I'm understanding that wrong, someone please correct me.

      -Dave
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        That could happen if the function didn't already exist before you took a reference from it. Be sure to have loaded the code for that function before you run that snippet. Consider also checking for recursion:
        my $old = \&Pod::HTML::scan_podpath;
{
    local $^W;
    *Pod::HTML::scan_podpath = sub {
        if ( $old == \&Pod::HTML::scan_podpath ) {
            croak "...";
        }
        &{$old}( @_ );
    };
}

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