2014/03/06

Hierarchies of Actions in Event-Reflexive Programming

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So far this series we have seen the introduction to event-reflexive programming, and a couple of use-cases it would appear in. This time our story continues in chronological fashion, following the development of various IRC-related systems.

The first attempt at an IRC bot was a large soup of various event-reflexive concepts, and was the experimentation bench for a lot of my first ideas about it. One pattern I found very useful was to include partly-dynamic data in with action names. That is, to use information at runtime to direct the flow of event handling. In particular in IRC, the most obvious one comes from considering the command name in incoming IRC messages.

The simplest implementation of this could be expressed something like the following, presuming our underlying IRC implementation gives us simple objects to represent each message:

my $message = ...;
my $command = $message->command;

run_plugins "on_message_$command", $message;

With this simple mechanism we now have a way for each plugin to react to specifically-named IRC events, without them having to capture all the events and filter for only the ones they care about.

However, it turns out that in a number of places we actually want to capture all the messages (for example, debugging and logging). No great problem here; we can simply make a second call to a generic on_message instead and pass in the command string itself as the first argument:

my $message = ...;
my $command = $message->command;

run_plugins "on_message_$command", $message;
run_plugins "on_message", $command, $message;

A pattern seems to be emerging here. We can extend this further, for example to handle the specific CTCP message verbs in IRC CTCP messages (for now, don't worry if you don't know what CTCP means; just consider that it's a second sub-hierarchy of messages):

sub on_message_PRIVMSG {
  my ($message) = @_;

  if(message is CTCP) {
    my $verb = ...;

    run_plugins "on_message_ctcp_$verb", $message;
    run_plugins "on_message_ctcp", $verb, $message;
    run_plugins "on_message", "ctcp", $verb, $message;
  }
}

However, the mechanism we've built here still seems a little unsatisfactory. Any given plugin could handle more than one of these cases, meaning it would be called multiple times. Maybe it would be better to build it such that we only call the most-specific event handler that each plugin defines. To do that we would have to build that logic right in to the basic definition of run_plugins.

One possible idea would be to pass an array reference containing pieces of event name, which should be joined by underscores (_) until a suitable handling method is found, and the remaining pieces would be passed as the first positional arguments. Thus a call such as:

run_plugins [ "on_message", "ctcp", $verb ], $message;

would invoke handlers similarly to the previous example, except that it will call at-most one action handler per plugin, meaning that specific handlers "override" more generic ones that plugin provides.

The main question of this post is therefore

To what extent should arguments be interpreted as part of the dispatch of action handlers themselves? Should some arguments be allowed to take part in forming the action name itself, to allow a degree of override-like dispatch logic on a per-plugin basis?

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