NAME
    Callback::Frame - Preserve error handlers and "local" variables across
    callbacks

SYNOPSIS
        use Callback::Frame;

        my $callback;

        frame(name => "base frame",
              code => sub {
                        $callback = frame(name => "frame #1",
                                          code => sub {
                                                    die "some error";
                                                  });
                      },
              catch => sub {
                         my $stack_trace = shift;
                         print $stack_trace;
                         ## Also, $@ is set to "some error at ..."
                       }
        )->();

        $callback->();

    This will print something like:

        some error at synopsis.pl line 9.
        ----- Callback::Frame stack-trace -----
        synopsis.pl:10 - frame #1
        synopsis.pl:17 - base frame

BACKGROUND
    When programming with callbacks in perl, you create anonymous functions
    with "sub { ... }". These functions are especially useful because when
    they are called they will preserve their surrounding lexical
    environment.

    In other words, the following bit of code

        my $callback;
        {
          my $var = 123;
          $callback = sub { $var };
        }
        print $callback->();

    will print 123 even though $var is no longer in scope when the callback
    is invoked.

    Sometimes people call these anonymous functions that reference variables
    in their surrounding lexical scope "closures". Whatever you call them,
    they are essential for convenient and efficient asynchronous
    programming.

DESCRIPTION
    The problem that this module solves is that although closures preserve
    their lexical environment, they don't preserve their dynamic
    environment.

    Consider the following piece of broken code:

        use AnyEvent;

        eval { ## broken!
          $watcher = AE::timer 0.1, 0,
            sub {
              die "some error";
            };
        };

        if ($@) { ## broken!
          print STDERR "Oops: $@";
        }

        AE::cv->recv;

    The intent behind the "eval" above is obviously to catch any exceptions
    thrown inside the callback. However, this will not work because the
    "eval" will only be in effect while installing the callback in the event
    loop, not while running it. When the event loop calls the callback, it
    will probably wrap its own "eval" around the callback and you will see
    something like this:

        EV: error in callback (ignoring): some error at broken.pl line 6.

    (The above applies to EV which is a well-designed event loop. Other
    event loops may fail more catastrophically.)

    So the root of the problem is that the dynamic environment has not been
    preserved. In this case it is the dynamic exception handlers that were
    not preserved. In some other cases we would like to preserve dynamically
    scoped (aka "local") variables (see below).

    By the way, "lexical" and "dynamic" are the lisp terms and I use them
    because they actually make sense. When it applies to variables, perl
    confusingly calls dynamic scoping "local" scoping.

    Here is how we could fix this code using Callback::Frame:

        use AnyEvent;
        use Callback::Frame;

        frame(
          code => sub {
            $watcher = AE::timer 0.1, 0,
              frame(code => sub {
                              die "some error";
                            });
          }, catch => sub {
            print STDERR "Oops: $@";
          }
        )->();

        AE::cv->recv;

    Now we see the desired error message:

        Oops: some error at fixed.pl line 8.

    We created two framed to accomplish this: A root frame to contain the
    exception handler, and another frame to use as a callback. Because the
    callback frame is created while the root frame is running, the callback
    will preserve the dynamic environment of the root frame.

USAGE
    This module exports two functions, "frame" and "fub".

    "frame" is the general interface and requires at least a "code" argument
    which should be a coderef (a function or a closure). It will return
    another coderef that "wraps" the coderef you passed in. When this codref
    is run, it will re-instate the dynamic environment that was present when
    the frame was created, and then run the coderef that you passed in as
    "code".

    IMPORTANT NOTE: All callbacks that may be invoked outside the dynamic
    environment of the current frame should be wrapped in a new "frame" or
    "fub" call so that the dynamic environment will be correctly re-applied
    when the callback is invoked.

    "fub" is a wrapper around frame and exists to simplify converting
    existing callbacks into Callback::Frame enabled code. For example, given
    the following AnyEvent statement:

        $watcher = AE::io $sock, 0, sub { do_stuff() };

    In order for the callback to have its dynamic environment maintained,
    you just need to change it to this:

        $watcher = AE::io $sock, 0, fub { do_stuff() };

    "frame" and "fub" both also accept "catch" and "local" parameters which
    are described in detail below.

    If you wish to run a coderef inside an existing frame's dynamic
    environment, when creating a frame you can pass in an existing frame as
    the "existing_frame" parameter. When this frame is executed, the "code"
    of the frame will be run inside "existing_frame"'s dynamic environment.
    This is useful for throwing exceptions from within some given callback's
    environment (timeouts for example) and for extracting/setting a
    callback's local variables.

    Libraries that wrap callbacks in frames can use the
    "Callback::Frame::is_frame()" function to determine if a given callback
    is already wrapped in a frame. It returns true if the callback is
    wrapped in a frame and is therefore suitable for use with
    "existing_frame".

    You should never need to, but the internal frame stack can be accessed
    at $Callback::Frame::top_of_stack. When this variable is defined, a
    frame is currently being executed.

NESTING AND STACK-TRACES
    Frames can be nested. When an exception is raised, the most deeply
    nested "catch" handler is invoked. If this handler itself throws an
    error, the next most deeply nested handler is invoked with the new
    exception but the original stack trace. If the last "catch" handler
    re-throws the error, the error will simply be thrown to your application
    from wherever the callback happened to be called (probably not what you
    want).

    When a "catch" handler is called, not only is $@ set, but also a
    stack-trace string is passed in as the first argument. All frames will
    be listed in this stack-trace, starting with the most deeply nested
    frame.

    If you want you can use simple frame names like "accepted" but if you
    are recording error messages in a log you might find it useful to name
    your frames things like "accepted connection from $ip:$port at $time"
    and "connecting to $host (timeout = $timeout seconds)".

    All frames you omit the name from will be shown as "ANONYMOUS FRAME" in
    stack-traces.

    One important thing to keep in mind is that frames aren't necessarily
    always arranged in terms of a stack. Really, frames are more of a tree
    data structure (known in lisp as a "spaghetti stack"). This occurs most
    often when two asynchronous request frames are started up concurrently
    while the same frame is in effect. At this point the "stack" has
    essentially branched. If you are ever surprised by an exception handler
    being called twice, this is probably what is happening.

"LOCAL" VARIABLES
    In the same way that using "catch" as described above preserves the
    dynamic environment of error handlers, "local" preserves the dynamic
    environment of variables. Of course, the scope of these bindings is not
    actually local in the real sense of the word, only in the perl sense.

    Technically, "local" maintains the dynamic environment of bindings.
    Although perl coders often ignore the distinction, lisp coders are
    careful to distinguish between variables and bindings. See, when a
    lexical binding is created, it is there "forever" -- or at least until
    it is no longer reachable by your program according to the rules of
    lexical scoping. Therefore, lexical variables are statically mapped to
    bindings and it is redundant to distinguish between lexical variables
    and bindings.

    However, with dynamic variables the same variable can refer to different
    bindings at different times. That's why they are called "dynamic"
    variables and lexical variables are called "static" variables.

    Because any code in any file, function, or package can access a dynamic
    variable, they are the opposite of local. They are global. However, the
    bindings are only global for a little while at a time. After a while
    they will go out of scope and then they are no longer visible at all. Or
    sometimes they will get "shadowed" by some other binding and will come
    back again later.

    OK, to make this concrete, after running this bit of code the binding
    containing 2 is lost forever:

        our $foo = 1;
        my $cb;

        {
          local $foo = 2;
          $cb = sub {
            return $foo;
          };
        }

        say $foo;     # 1
        say $cb->();  # 1  <- not 2!
        say $foo;     # 1

    Here's a way to "fix" that using Callback::Frame:

        our $foo = 1;
        my $cb;

        frame(local => __PACKAGE__ . '::foo',
              code => sub {
          $foo = 2;
          $cb = frame(code => sub {
            return $foo;
          });
        })->();

        say $foo;     # 1
        say $cb->();  # 2  <- hooray!
        say $foo;     # 1

    Don't be fooled into thinking that this is a lexical binding though.
    While the variable $foo points to the binding containing 2, all parts of
    the program will see this binding as demonstrated in the following
    example:

        our $foo = 1;
        my $cb;

        sub global_foo_getter {
          return $foo;
        }

        frame(local => __PACKAGE__ . '::foo',
              code => sub {
          $foo = 2;
          $cb = frame(code => sub {
            return global_foo_getter();
          });
        })->();

        say $foo;     # 1
        say $cb->();  # 2  <- still 2
        $foo;         # 1

    You can install multiple local variables in the same frame:

        frame(local => __PACKAGE__ . '::foo',
              local => 'main::bar',
              ... );

    Note that if you have both "catch" and "local" elements in a frame, in
    the event of an error the local bindings will not be present inside the
    "catch" handler (use a nested frame if you need this).

    All variable names must be fully package qualified. The easiest way to
    do this for variables in your current package is to use the ugly
    "__PACKAGE__ ." technique.

    Objects stored in local bindings managed by Callback::Frame will not be
    destroyed until all references to the frame-wrapped callback that
    contains the binding, along with all references to any deeper frames,
    are freed.

SEE ALSO
    <The Callback::Frame github repo>

    The Callback::Frame philosophy is that we actually like callback style
    and just want to simplify the reseting of dynamic environments as
    necessary. This module should make adding error handling support to an
    existing asynchronous application as easy as possible (it shouldn't
    force you to pass extra parameters around). Finally, this module should
    make lifer easier because as a side effect of adding error checking it
    should also produce detailed and useful stack traces.

    This module's "catch" syntax is of course modeled after "normal
    language" style exception handling as implemented by Try::Tiny &c.

    This module depends on "Guard" to maintain the
    $Callback::Frame::active_frames datastructure, and to ensure that even
    when exceptions are thrown, "local" binding updates aren't lost.

    AnyEvent::Debug is a very interesting and useful module that provides an
    interactive debugger for AnyEvent applications and uses some of the same
    techniques that Callback::Frame does. AnyEvent::Callback and
    AnyEvent::CallbackStack sort of solve the dynamic error handler problem.
    Unlike these modules, Callback::Frame is not related at all to AnyEvent,
    except that it happens to be useful in AnyEvent libraries and
    applications (among other things).

    Miscellaneous other modules: IO::Lambda::Backtrace,
    POE::Filter::ErrorProof

    Python Tornado's <StackContext> and "async_callback"

    <Let Over Lambda, Chapter 2>

    <UNWIND-PROTECT vs. Continuations>

BUGS
    For now, "local" bindings can only be created in the scalar namespace.
    Also, none of the other nifty things that local can do (like localising
    a hash table value) are supported yet.

    The "local" implementation is currently pretty inefficient. It evals a
    string that returns a sub whenever a frame is entered. Fortunately,
    perl's compiler is really fast. Also, this overhead is not there at all
    when you are only using the "catch" functionality which I anticipate to
    be the most common usage pattern.

AUTHOR
    Doug Hoyte, "<doug@hcsw.org>"

COPYRIGHT & LICENSE
    Copyright 2012 Doug Hoyte.

    This module is licensed under the same terms as perl itself.

