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Man Pages


Manual Reference Pages  -  RETURN::MULTILEVEL (3)

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NAME

Return::MultiLevel - return across multiple call levels

CONTENTS

SYNOPSIS



  use Return::MultiLevel qw(with_return);

  sub inner {
    my ($f) = @_;
    $f->(42);  # implicitly return from with_return below
    print "You dont see this\n";
  }

  sub outer {
    my ($f) = @_;
    inner($f);
    print "You dont 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.

AUTHOR

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

COPYRIGHT & LICENSE

Copyright 2013-2014 Lukas Mai.

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.

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perl v5.20.3 RETURN::MULTILEVEL (3) 2014-09-07

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