NAME
Return::MultiLevel - Return across multiple call levels
VERSION
version 0.08
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
$return
is called, it causes an immediate return fromwith_return
. Any arguments passed to$return
becomewith_return
's return value (ifwith_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.
CAVEATS
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.
AUTHORS
Lukas Mai
Graham Ollis <plicease@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2013,2014,2021 by Lukas Mai.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.