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# Generated by Pod::WikiDoc version 0.18 =pod =head1 NAME Sub::Uplevel - apparently run a function in a higher stack frame =head1 VERSION This documentation describes version 0.2002 =head1 SYNOPSIS use Sub::Uplevel; sub foo { print join " - ", caller; } sub bar { uplevel 1, \&foo; } #line 11 bar(); # main - foo.plx - 11 =head1 DESCRIPTION Like Tcl's uplevel() function, but not quite so dangerous. The idea is just to fool caller(). All the really naughty bits of Tcl's uplevel() are avoided. B<THIS IS NOT THE SORT OF THING YOU WANT TO DO EVERYDAY> =over 4 =item B<uplevel> uplevel $num_frames, \&func, @args; Makes the given function think it's being executed $num_frames higher than the current stack level. So when they use caller($frames) it will actually give caller($frames + $num_frames) for them. C<uplevel(1, \&some_func, @_)> is effectively C<goto &some_func> but you don't immediately exit the current subroutine. So while you can't do this: sub wrapper { print "Before\n"; goto &some_func; print "After\n"; } you can do this: sub wrapper { print "Before\n"; my @out = uplevel 1, &some_func; print "After\n"; return @out; } C<uplevel> will issue a warning if C<$num_frames> is more than the current call stack depth. =begin _private So it has to work like this: Call stack Actual uplevel 1 CORE::GLOBAL::caller Carp::short_error_loc 0 Carp::shortmess_heavy 1 0 Carp::croak 2 1 try_croak 3 2 uplevel 4 function_that_called_uplevel 5 caller_we_want_to_see 6 3 its_caller 7 4 So when caller(X) winds up below uplevel(), it only has to use CORE::caller(X+1) (to skip CORE::GLOBAL::caller). But when caller(X) winds up no or above uplevel(), it's CORE::caller(X+1+uplevel+1). Which means I'm probably going to have to do something nasty like walk up the call stack on each caller() to see if I'm going to wind up before or after Sub::Uplevel::uplevel(). =end _private =begin _dagolden I found the description above a bit confusing. Instead, this is the logic that I found clearer when CORE::GLOBAL::caller is invoked and we have to walk up the call stack: * if searching up to the requested height in the real call stack doesn't find a call to uplevel, then we can return the result at that height in the call stack * if we find a call to uplevel, we need to keep searching upwards beyond the requested height at least by the amount of upleveling requested for that call to uplevel (from the Up_Frames stack set during the uplevel call) * additionally, we need to hide the uplevel subroutine call, too, so we search upwards one more level for each call to uplevel * when we've reached the top of the search, we want to return that frame in the call stack, i.e. the requested height plus any uplevel adjustments found during the search =end _dagolden =back =head1 EXAMPLE The main reason I wrote this module is so I could write wrappers around functions and they wouldn't be aware they've been wrapped. use Sub::Uplevel; my $original_foo = \&foo; *foo = sub { my @output = uplevel 1, $original_foo; print "foo() returned: @output"; return @output; }; If this code frightens you B<you should not use this module.> =head1 BUGS and CAVEATS Well, the bad news is uplevel() is about 5 times slower than a normal function call. XS implementation anyone? Sub::Uplevel overrides CORE::GLOBAL::caller temporarily for the scope of each uplevel call. It does its best to work with any previously existing CORE::GLOBAL::caller (both when Sub::Uplevel is first loaded and within each uplevel call) such as from Contextual::Return or Hook::LexWrap. However, if you are routinely using multiple modules that override CORE::GLOBAL::caller, you are probably asking for trouble. As of version 0.20, Sub::Uplevel requires Perl 5.6 or greater. =head1 HISTORY Those who do not learn from HISTORY are doomed to repeat it. The lesson here is simple: Don't sit next to a Tcl programmer at the dinner table. =head1 THANKS Thanks to Brent Welch, Damian Conway and Robin Houston. =head1 AUTHORS David A Golden E<lt>dagolden@cpan.orgE<gt> (current maintainer) Michael G Schwern E<lt>schwern@pobox.comE<gt> (original author) =head1 LICENSE Original code Copyright (c) 2001 to 2007 by Michael G Schwern. Additional code Copyright (c) 2006 to 2008 by David A Golden. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See http://www.perl.com/perl/misc/Artistic.html =head1 SEE ALSO PadWalker (for the similar idea with lexicals), Hook::LexWrap, Tcl's uplevel() at http://www.scriptics.com/man/tcl8.4/TclCmd/uplevel.htm