NAME

Return::MultiLevel - return across multiple call levels

SYNOPSIS

  use Return::MultiLevel qw(with_return);

  sub inner {
    my ($f) = @_;
    $f->(42);  # implicitly return from 'with_return' below
    print "You don't see this\n";
  }

  sub outer {
    my ($f) = @_;
    inner($f);
    print "You don't see this either\n";
  }

  my $result = with_return {
    my ($return) = @_;
    outer($return);
    die "Not reached";
  };
  print $result, "\n";  # 42

DESCRIPTION

This module provides a way to return immediately from a deeply nested call stack. This is similar to exceptions, but exceptions don't stop automatically at a target frame (and they can be caught by intermediate stack frames using "eval"). In other words, this is more like setjmp(3)/longjmp(3) than "die".

Another way to think about it is that the "multi-level return" coderef represents a single-use/upward-only continuation.

Functions

The following functions are available (and can be imported on demand).

with_return BLOCK

Executes BLOCK, passing it a code reference (called $return in this description) as a single argument. Returns whatever BLOCK returns.

If $return is called, it causes an immediate return from "with_return". Any arguments passed to $return become "with_return"'s return value (if "with_return" is in scalar context, it will return the last argument passed to $return).

It is an error to invoke $return after its surrounding BLOCK has finished executing. In particular, it is an error to call $return twice.

DEBUGGING

This module uses "unwind" from "Scope::Upper" to do its work. If "Scope::Upper" is not available, it substitutes its own pure Perl implementation. You can force the pure Perl version to be used regardless by setting the environment variable "RETURN_MULTILEVEL_PP" to 1.

If you get the error message "Attempt to re-enter dead call frame", that means something has called a $return from outside of its "with_return { ... }" block. You can get a stack trace of where that "with_return" was by setting the environment variable "RETURN_MULTILEVEL_DEBUG" to 1.

BUGS AND LIMITATIONS

You can't use this module to return across implicit function calls, such as signal handlers (like $SIG{ALRM}) or destructors ("sub DESTROY { ... }"). These are invoked automatically by perl and not part of the normal call chain.

SUPPORT AND DOCUMENTATION

After installing, you can find documentation for this module with the "perldoc" command.

    perldoc Return::MultiLevel

You can also look for information at <https://metacpan.org/pod/Return::MultiLevel>.

To see a list of open bugs, visit <https://rt.cpan.org/Public/Dist/Display.html?Name=Return-MultiLevel>.

To report a new bug, send an email to "bug-Return-MultiLevel [at] rt.cpan.org".

AUTHOR

Lukas Mai, "<l.mai at web.de>"

COPYRIGHT & LICENSE

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See <http://dev.perl.org/licenses/> for more information.