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From: tchrist@convex.COM (Tom Christiansen)
Newsgroups: comp.lang.perl
Subject: Re: package switching
Message-ID: <1991May01.170306.891@convex.com>
Date: 1 May 91 17:03:06 GMT
References: <DNB.91May1115254@meshugge.media.mit.edu>
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Reply-To: tchrist@convex.COM (Tom Christiansen)
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Organization: CONVEX Software Development, Richardson, TX
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From the keyboard of dnb@meshugge.media.mit.edu (David N. Blank):
:Howdy-
:   I've been having a great deal of fun with eval lately (as Tom and
:Randall both know) in an effort to make something I am writing really
:generic.  I've hit another stumper on the subject: "package".  If I
:want to be able to switch packages in a very generic way:
:
:sub blah{
: package @_[0];
:&blah("bleh");

If that were to work, you would really mean $_[0] -- don't want
to be supplying array contexts willy-nilly.

:How do I do it?  I have seen Randall use this idiom:
:
:>  $ret = eval "package main; $action"
:
:but I would prefer not to stick the entire contents of a subroutine
:into one scalar variable.  Is there some magic way to expand the
:scope of the package commands?  Thanks.

Magic way?  Nay.  A package declaration only affects what default
package identifiers get compiled into.  That means it doesn't 
affect the run time scoping at all,unlike local().   Thus simply 
doing 

    package main;
    eval "package $package";
    $foo = 1;

won't do you any good, as you're not compiling anything: foo will still be
in main, not $package.  You've little choice but to put your subroutine in
a variable an eval it so it gets compiled.  if that's what you're trying
to do.  This is expensive.

Now, the only time I really care about other things' packages is
when I get a filehandle passed into me as a variable.  For example,
in my open2() routine in the FAQ, I do this:

    package open2;
    # blah blah
    # force unqualified filehandles into callers' package
    local($package) = caller;
    $dad_rdr =~ s/^[^']+$/$package'$&/;
    $dad_wtr =~ s/^[^']+$/$package'$&/;

Other than that, I'm not sure why you want to dynamically switch
packages at run time instead of compile time.

--tom