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.\" ========================================================================
.\"
.IX Title "Return::MultiLevel 3pm"
.TH Return::MultiLevel 3pm "2014-09-07" "perl v5.20.1" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
Return::MultiLevel \- return across multiple call levels
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  use Return::MultiLevel qw(with_return);
\&
\&  sub inner {
\&    my ($f) = @_;
\&    $f\->(42);  # implicitly return from \*(Aqwith_return\*(Aq below
\&    print "You don\*(Aqt see this\en";
\&  }
\&
\&  sub outer {
\&    my ($f) = @_;
\&    inner($f);
\&    print "You don\*(Aqt see this either\en";
\&  }
\&
\&  my $result = with_return {
\&    my ($return) = @_;
\&    outer($return);
\&    die "Not reached";
\&  };
\&  print $result, "\en";  # 42
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This module provides a way to return immediately from a deeply nested call
stack. This is similar to exceptions, but exceptions don't stop automatically
at a target frame (and they can be caught by intermediate stack frames using
\&\f(CW\*(C`eval\*(C'\fR). In other words, this is more like
\&\fIsetjmp\fR\|(3)/\fIlongjmp\fR\|(3) than \f(CW\*(C`die\*(C'\fR.
.PP
Another way to think about it is that the \*(L"multi-level return\*(R" coderef
represents a single\-use/upward\-only continuation.
.SS "Functions"
.IX Subsection "Functions"
The following functions are available (and can be imported on demand).
.IP "with_return \s-1BLOCK\s0" 4
.IX Item "with_return BLOCK"
Executes \fI\s-1BLOCK\s0\fR, passing it a code reference (called \f(CW$return\fR in this
description) as a single argument. Returns whatever \fI\s-1BLOCK\s0\fR returns.
.Sp
If \f(CW$return\fR is called, it causes an immediate return from \f(CW\*(C`with_return\*(C'\fR. Any
arguments passed to \f(CW$return\fR become \f(CW\*(C`with_return\*(C'\fR's return value (if
\&\f(CW\*(C`with_return\*(C'\fR is in scalar context, it will return the last argument passed to
\&\f(CW$return\fR).
.Sp
It is an error to invoke \f(CW$return\fR after its surrounding \fI\s-1BLOCK\s0\fR has finished
executing. In particular, it is an error to call \f(CW$return\fR twice.
.SH "DEBUGGING"
.IX Header "DEBUGGING"
This module uses \f(CW\*(C`unwind\*(C'\fR from
\&\f(CW\*(C`Scope::Upper\*(C'\fR to do its work. If
\&\f(CW\*(C`Scope::Upper\*(C'\fR is not available, it substitutes its own pure
Perl implementation. You can force the pure Perl version to be used regardless
by setting the environment variable \f(CW\*(C`RETURN_MULTILEVEL_PP\*(C'\fR to 1.
.PP
If you get the error message \f(CW\*(C`Attempt to re\-enter dead call frame\*(C'\fR, that means
something has called a \f(CW$return\fR from outside of its \f(CW\*(C`with_return { ... }\*(C'\fR
block. You can get a stack trace of where that \f(CW\*(C`with_return\*(C'\fR was by setting
the environment variable \f(CW\*(C`RETURN_MULTILEVEL_DEBUG\*(C'\fR to 1.
.SH "BUGS AND LIMITATIONS"
.IX Header "BUGS AND LIMITATIONS"
You can't use this module to return across implicit function calls, such as
signal handlers (like \f(CW$SIG{ALRM}\fR) or destructors (\f(CW\*(C`sub DESTROY { ... }\*(C'\fR).
These are invoked automatically by perl and not part of the normal call chain.
.SH "AUTHOR"
.IX Header "AUTHOR"
Lukas Mai, \f(CW\*(C`<l.mai at web.de>\*(C'\fR
.SH "COPYRIGHT & LICENSE"
.IX Header "COPYRIGHT & LICENSE"
Copyright 2013\-2014 Lukas Mai.
.PP
This program is free software; you can redistribute it and/or modify it
under the terms of either: the \s-1GNU\s0 General Public License as published
by the Free Software Foundation; or the Artistic License.
.PP
See http://dev.perl.org/licenses/ for more information.