Workings of UNIVERSAL::can

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

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Re: Workings of UNIVERSAL::can by Anonymous Monk on Jul 05, 2011 at 00:18 UTC
Suddenly UNIVERSAL::can no longer find a number of taglib functions that were previously visible. More than likely this nothing to do with UNIVERSAL. Try http://search.cpan.org/~dongxu/Audio-TagLib-1.50_01/	[reply]
Re^2: Workings of UNIVERSAL::can by geoffleach (Scribe) on Jul 05, 2011 at 16:47 UTC
Not likely to be the problem as that's the source base I'm working on. Although it is true that that distribution does not have my problem. Just to be clear, I'm not alleging that UNIVERSAL::can is buggy. If I understand what's it algorithm is -- how it finds out what files it searches to determine if a script 'can', then I can figure out what I'm doing wrong. I hope that this clears things up.	[reply]
Re^3: Workings of UNIVERSAL::can by Anonymous Monk on Jul 05, 2011 at 17:11 UTC
I hope that this clears things up..... It really doesn't :) Anyway, It just walks @ISA asking if it can, See UNIVERSAL, perlobj, perlboot	[reply]
Re^4: Workings of UNIVERSAL::can by Anonymous Monk on Jul 05, 2011 at 17:22 UTC
Re^4: Workings of UNIVERSAL::can by geoffleach (Scribe) on Jul 05, 2011 at 22:46 UTC
Re^5: Workings of UNIVERSAL::can by Anonymous Monk on Jul 06, 2011 at 00:12 UTC
Re: Workings of UNIVERSAL::can by Somni (Friar) on Jul 06, 2011 at 22:48 UTC
The algorithm is a simple symbol table search in method resolution order. If the method is not in the symbol table (perhaps it's loaded with AUTOLOAD, or otherwise loaded after the ->can call) it won't be found. If it's loaded via AUTOLOAD, the module itself will need to overload `can` so it can answer effectively for those functions; this is somewhat tricky to get right, as you need to allow the search to continue further up the tree as necessary. You mentioned later that ->can is not seeing your C++ functions. Unless these are exposed to the Perl level, they won't be in the symbol table; but then again, you shouldn't be able to call them from Perl, either, so finding them with ->can is simply wrong. If this is the case, you'll need to elaborate more as to what's going on.	[reply] [d/l]
Re^2: Workings of UNIVERSAL::can by Anonymous Monk on Jul 06, 2011 at 23:37 UTC
The algorithm is a simple symbol table search in method resolution order. Hi :D Actually :) Method resolution order is not an algorithm (or an actual order) or any more than symbol table search is an algorithm. Or put another way, a sort-algorithm is not an actual algorithm, but a bubble-sort-algorithm is an sort-algorithm; is an algorithm. Perl's default method-resolution-order, algorithm , is a linear-depth-first-search. You can change that in modern perls with the mro pragma to C3 linearization	[reply]
Re^3: Workings of UNIVERSAL::can by Somni (Friar) on Jul 07, 2011 at 00:12 UTC
The fact that mro is changeable is why I said method resolution order, and not a specific order. Taken individually, each of the items is not an algorithm. Taken together (symbol table search in method resolution order), it is an algorithm, though ill-specified due to the variable of which mro is in effect.	[reply]
Re^4: Workings of UNIVERSAL::can by Anonymous Monk on Jul 08, 2011 at 08:38 UTC