When do subs get overridden?

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

I was reading through a pretty masterful bit of redirection in a recent obfu written by SarahM. (It really is quite nice; I recommend you check it out before reading the spoilers below) Something in it prompted me to wonder about the way perl deals with namespaces of packages, and I was a little confused. I knew what was happening, but I couldn't put my finger on why.

Here's the spoiler to the obfu: SarahM defined a sub q, and later used the builtin q to fool the reader into thinking she was using her own sub. So my question is: under what circumstances does the builtin q// get overridden (assuming one would want to do such a thing)? Is it possible to code the following so that a simple call of q("something") will execute main::q?

sub q { return "inside main sub q"; }
print "Ambiguity gets us the builtin: \n";
print q("not in sub q"), "\n";
print "Let us be more specific: \n";
print main::q("this does not get printed"), "\n";
__END__
Ambiguity gets us the builtin: 
"not in sub q"
Let us be more specific: 
inside main sub q
[download]

LAI

__END__

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Re: When do subs get overridden? (&) by tye (Sage) on Apr 03, 2003 at 21:28 UTC
My practice it to use mixed case in subroutine names or to use both & and () when calling my subroutines. See (tye)Re: A question of style for more details. `print &q("calling a subroutine"), $/;` [download] And, no, you can't override q//. You can override things like open. And you can tell this because `prototype("CORE::q")` returns `undef` while `prototype("CORE::open")` returns `"*;$@"`. See prototype. - tye	[reply] [d/l] [select]
Re: When do subs get overridden? by MrYoya (Monk) on Apr 03, 2003 at 21:34 UTC
Check out `perlsub`, under "Overriding Built-in Functions". Also, 12.11 in the Perl Cookbook. According to the Perl Cookbook, there are some functions that simply cannot be overridden -- 'q' is one of these.	[reply] [d/l]
Re: When do subs get overridden? by Mr. Muskrat (Canon) on Apr 03, 2003 at 21:12 UTC
You missed ~~one more way~~ two ways of calling main::q `print ::q("this isn't printed either"),$/; print &q("me either"),$/;` [download] Update: added second line of code and this: If I read the docs correctly, you should be able to override the built-ins by using prototypes. And I had better shut up before I give away all my obfu secrets ;-)	[reply] [d/l]
Re^2: When do subs get overridden? by LAI (Hermit) on Apr 03, 2003 at 21:29 UTC
I know there's different ways of calling `main::q`. What I'd like to do, though, is to override the builtin `q` so that I can call `main::q` with a plain ol' `q()`. Actually, no. I don't want to do that, I want to know how to do that, if possible. I actually have no plans to override builtins for any purpose, but I'm just curious as to if and how it's possible. LAI `__END__`	[reply] [d/l] [select]
Re: When do subs get overridden? by dakkar (Hermit) on Apr 05, 2003 at 11:24 UTC
You see, `q` is not a subroutine, it's a syntactic construct, like quotes or braces. There is nothing to "override". You can, however, change its semantic a bit with: a source filter (ok, this is obvious) redefinition of stringification for some datatype (`package MyQ;use overload '""' => sub {return "overridden"};`) -- dakkar - Mobilis in mobile	[reply] [d/l] [select]