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Re2: MOPT-04 - identification, part 2

by mstone (Deacon)
on Jan 08, 2003 at 01:52 UTC ( #225140=note: print w/ replies, xml ) Need Help??


in reply to Re: MOPT-04 - identification, part 2
in thread MOPT-04 - identification, part 2

  • Actually you can redefine subroutine by using eval "sub func {'...'}"

That would create a new function at runtime, but it wouldn't follow the rules of dynamic scoping (see MOPT-03 for details).

As you've used it, the eval() just binds a new function to the name 'func'. To be dynamic, the new version of func() would have to be restricted to the evaluation context of the function that defined it:

$VAR = "old VAR"; sub func {return ("old func()")} sub show {printf "VAR='%s', func()='%s'\n", $VAR, func()} sub redefine { eval 'sub func {return ("new func()")}'; local ($VAR) = "new VAR"; show(); } show(); redefine(); show();

returns:

VAR='old VAR', func()='old func()' VAR='new VAR', func()='new func()' VAR='old VAR', func()='new func()'

which shows the difference between redefining a globally-scoped function and re-binding a dynamically-scoped variable.

  • and better yet is just to use anonymous subroutines and use ordinary variables to switch between different ones:

Yes, that would work:

$VAR = "old VAR"; $FUNC = sub {return ("old func()")}; sub show {printf "VAR='%s', func()='%s'\n", $VAR, &$FUNC()} sub redefine { local ($FUNC) = sub {return ("new func()")}; local ($VAR) = "new VAR"; show(); } show(); redefine(); show();

returns:

VAR='old VAR', func()='old func()' VAR='new VAR', func()='new func()' VAR='old VAR', func()='old func()'

which does the right thing. But to get the effect of a dynamically-scoped function, you have to use a dynamically scoped (aka: local) variable, and I did say that in the original article.

HOWEVER.. check out the next reply to see how I was, in fact, wrong.


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