Does Perl monkey-patching allow you to see the patched package's scope?

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+3 Q:

Does Perl monkey-patching allow you to see the patched package's scope?

I'm monkey patching a package using a technique given at the beginning of "How can I monkey-patch an instance method in Perl?". The problem that I'm running into is that the original subroutine used a package-level my variable which the patched subroutine appears not to have access to, either by full path specification or implicit use.

Is there any way to get at the data scoped in this way for use in the patched subroutine?

Thanks in advance!

+5 A:

No. The thing you're mistaken in is that they are not package scoped. A lexical variable is by definition limited to its lexical scope, in other words, the block it is in.

Leon Timmermans 2009-01-23 22:13:51

Ah, good point -- so if you can't modify the original source file, you're SOL unless some method returns it?

cdleary 2009-01-23 22:47:27

That's right. And if you're thinking, "Why have two different ways to subdivide a namespace -- by package and by lexical scope? What a godawful way to run a language!", you're also right.

j_random_hacker 2009-01-24 00:02:44

I disagree. Both work in different ways, and have different uses. Lexicals are the best solution for most problems but there are situations where dynamic scoping better.

Leon Timmermans 2009-01-24 01:54:23

@Leon: It seems (from this run-in) that lexicals go against the idea of "gentleman's privacy". I'd like to get at those values and have to hack the interpreter to do so. :-(

cdleary 2009-01-24 02:10:31

There is a reason all common programing languages have lexical scope (I think emacs lisp is the most common exception). Dynamic scope causes major headaches when not used properly (which is hard). Look up dynamic scope on Wikipedia for more information on the issue.

Leon Timmermans 2009-01-24 02:57:06

Lexicals are also scoped to their file, so you're out of luck.

brian d foy 2009-01-24 03:25:12

@Leon: I wasn't talking about lexical scoping itself, but the idea that Perl's lexically scoped variables are not visible outside the current package. "our" doesn't make a variable dynamically scoped (that's an incorrect use of the term "dynamic scoping").

cdleary 2009-01-24 04:14:08

@cdleary: I think your statement that "lexically scoped variables are not visible outside the current package" is a bit confusing. Lexicals aren't affected by the current package as far as I'm aware.

converter42 2009-01-26 21:05:21

@converter42: Does it make more sense to say that lexicals aren't visible outside the package in which they're declared? "current package" seems like bad terminology on my part.

cdleary 2009-01-28 22:29:50

@cdleary: No. lexicals have nothing to do with packages at all; packages are only relevant for resolving unqualified use of undeclared globals and for determining the package of something you are declaring. A lexical has scope to the end of a block (and each file is an implicit block).

ysth 2010-08-08 19:58:17

+6 A:

You can obtain lexicals with the PadWalker module. Evil, but it works.

jrockway 2009-01-23 23:11:17

http://search.cpan.org/perldoc?PadWalker

Brad Gilbert 2009-01-23 23:14:36

Wow, that's interesting. Filthy, but interesting.

j_random_hacker 2009-01-24 00:03:52

Lexicals (ie: declared with 'my') are not visible outside the lexical scope (file or block) in which they are declared. That's the whole point of lexical variables.

If there is a subroutine/method which is in the same scope as the lexical var, then it can return the value of the lexical and that can allow indirect access to the var from outside its scope.

There is no such thing as a 'full path specification' for lexical variables. That's for package variables. If the var was declared with 'our' instead of 'my' you could do that.

noswonky 2009-01-29 03:42:51

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Does Perl monkey-patching allow you to see the patched package's scope?

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