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.

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.