How can I tell if a function has been forward declared but not yet defined?

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

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Re: How can I tell if a function has been forward declared but not yet defined? by dragonchild (Archbishop) on Mar 30, 2005 at 20:17 UTC
The scalar $hello doesn't exist. The subrouine &hello exists. `sub hello; $fn = \&hello; # ...` [download] Alternately: `sub hello; fn = hello; # ...` [download] But, either way, your check of $::{'hello'} will fail. Being right, does not endow the right to be rude; politeness costs nothing. Being unknowing, is not the same as being stupid. Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence. Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.	[reply] [d/l] [select]
Re^2: How can I tell if a function has been forward declared but not yet defined? by ihb (Deacon) on Mar 30, 2005 at 21:36 UTC
The subroutine `&hello` doesn't exist. In fact `$::{hello}` is `&hello`'s prototype (as a plain string). There's no CV or CV-ish value around yet--just a plain string being the prototype. When you do `hello`, `\&hello` or `$hello` you create the glob at comile-time and so `$::{hello}` returns a glob. This means that `sub hello; fh = $::{hello}; hello;` [download] works. See Re^3: Grabbing Variable Names for an elaboration. `ihb` See perltoc if you don't know which perldoc to read!*	[reply] [d/l] [select]
Re^2: How can I tell if a function has been forward declared but not yet defined? by mab (Acolyte) on Mar 30, 2005 at 20:31 UTC
%:: is the namespace for the main package and what I'm doing with $::{'hello'} is to get the glob of things pointed at by the name 'hello.' For example, do this and you'll see what I mean: `sub hello { }; # declared and defined fn = $::{'hello'}; print "declared\n" if fn{CODE}; print "defined\n" if defined &fn;` [download] So I know the technique works - I just need a way to differentiate functions that are only forward declared from ones that are actually defined.	[reply] [d/l]
Re^2: How can I tell if a function has been forward declared but not yet defined? by mab (Acolyte) on Mar 30, 2005 at 20:59 UTC
Hey, I see what you're saying now. Sorry for being slow. I am still curious why the namespace approach doesn't work but thanks for this suggestion.	[reply]
Re: How can I tell if a function has been forward declared but not yet defined? by ihb (Deacon) on Mar 30, 2005 at 21:31 UTC
If you would've waited two hours for me to answer instead of reposting the exact same question as a SoPW after just one hour, you would've seen my elaborate answer. `ihb` See perltoc if you don't know which perldoc to read!	[reply]
Re^2: How can I tell if a function has been forward declared but not yet defined? by mab (Acolyte) on Mar 30, 2005 at 21:35 UTC
my apologies for not waiting - when i realized that the thread i posted against was a few years old, i reposted it. in any case, thank you for the in depth answer - very excellent.	[reply]