Event-Reflexivity in Static Languages

<< First | < Prev

In the previous article, we looked at the idea of explicit registration of handlers for events in an event-reflexive system, and touched on the idea that it may be more useful (or in fact, required) when dealing with a static language like C, rather than a dynamic langauge like Perl. Today's story will look at the different requirements for static languages in more detail.

In the examples in previous posts we have been able to use dynamic language features to easily implemented named actions by simply creating functions of the right name, and dispatch to them simply by passing that string name to the central dispatch functions.

# In a handler module
sub add_user
{ ... }

# In dispatching code
run_events "add_user", @args;

This became especially useful when creating dynamic action names in the IRC cases

# In a handler module
sub on_message_JOIN
{ ... }

sub on_message
{ ... }

# In dispatching code
run_events [ "on_message", "JOIN" ], @args;

To be clear here, we have used the following abilities of Perl (though similar should apply to most dynamic languages) in order to easily implement this system:

• The ability to invoke a function in a module based on a dynamic string at runtime
• The ability to pass a variable list of arguments to a function as a simple list, without the intermediate dispatcher function having to understand them
• The ability to return any kind of value or list of values from a function

In a static language, this simply isn't going to work. We'll need something much stronger to bind all these pieces together. We'll need a way to more strongly identify the named actions as hook points, some way to pass the arguments around between them, and some way to interpret the return values for possible methods of combination or short-circuit return

The first idea to handle this is simply to number the events in some sort of globally-defined enumeration. But this of course creates a single global numbering, and half of the point of event-reflexivity in the first place was to avoid this kind of centralisation - a central numbering would mean that plugin modules couldn't themselves create and emit new events. They can't just invent new numbers because they might collide with existing ones.

Perhaps what is required here is that the event-reflexivity core can allocate contiguous blocks of numbered events, and allow some kind of association between event numbers and friendly string names for the convenience of programmers and users. When a new module wishes to allocate some events, it can request a block from the core, and be given its starting number. While that number would be dynamically allocated between different instances of the system, or even different runs on the same machine, it would at least be constant throughout one program run, allowing other modules to bind or invoke them. The friendly naming system would then exist to allow programs to look up the current number for a known event name, to bind or invoke it.

typedef ERcore_event_id int;

ERcore_event_id ercore_allocate_events(int n_events,
                                       const char *evname[]);
ERcore_event_id ercore_lookup_event(const char *name);
const char *ercore_event_name(ERcore_event_id id);

Having a way to create the identity of named events, we next need a way to register a function to actually handle them. This is where our second problem arises - the problem of how to pass event arguments. Perhaps the simplest is simply to allow a single void * pointer argument, on the basis that the named event would document somewhere what this was supposed to point at - likely a structure of some kind. Because C lacks the ability to create true closures, it may be necessary to pass a second pointer argument at binding time, and passing both that and the event argument to the invoked functions at dispatch time.

void ercore_bind_event(ERcore_event_id id,
                            void (*fn)(ERcore_event_id id,
                                       void *args,
                                       void *data),
                            void *data);
void ercore_run_event(ERcore_event_id,
                      void *args);

Here we've only defined the simplest of the invocation functions, run_events, because any of the others would require some consideration of the return value as well. Here we start to run into problems of needing to know how to interpret the meanings of these values. This means we can't implement any of the more interesting invocation methods as seen in the second post (Kinds of Invocation in Event-Reflexive Programming).

Our handling of arguments isn't very satisfactory, forcing invoking code to always pack their arguments into a structure, and all the handling code to unpack them from it again. We've also made our arguments totally opaque to the actual dispatch system, meaning we can't do any of the interesting tricks we could do in dynamic languages where these are visible (such as those seen in the third post, Instrumentation and Logging in Event-Reflexive Programming).

Finally, by making the event identity a simple ID number and having opaque argument structures that the event dispatch core cannot inspect, we have lost our ability to perform dynamic dispatch based on some of the arguments, as seen in the fourth post (Hierarchies of Actions in Event-Reflexive Programming).

We have seen in previous posts that all of these abilities are useful things to have, in combination they define the essential nature of what Event-Reflexivity is really all about. It would be nice if we could have these things in static languages as well as dynamic ones.

This then leads to the final, and most expansive question of this series of posts:

How do you create a powerful system of event-reflexivity in a static language such as C? How do you cope with combining return values and short-circuit evaluation in those dispatch modes that require them? How do you pass different kinds of arguments to invoked functions in a useful and simple way? And how do you perform dynamic multiple dispatch on of pieces of the event identity?