2014/07/27

Event Reflexivity - A Design Pattern Pattern?

The previous series of posts on the topic of Event Reflexivity each posed a question about what the general shape of the design pattern actually is. Over the following months after the posts I thought about the pattern a lot more and eventually came to the conclusion that a lot of the questions aren't a simple matter of choosing what's correct - the reason these questions are hard to answer is that both options could be correct. What I have here is in fact not a single unclear design pattern, but instead a whole family of possible choices on a design pattern, with different specifics being more useful to specific cases.

The specific design choices that a particular implementation takes should answer such questions as:

  • Ordering control: Are subscribers of some named action invoked in a controllable fashion?
  • Invocation functions: What kinds of actions or ways of invoking them are actually provided?
  • Heirarchial actions: Do the types of actions occupy a simple flat namespace, or is there some heirarchial structure to them? Can a subscriber catch an entire subspace of actions at once?
  • Explicit or implicit subscription: Do subscribers have to explicitly list every action (or action subspace) they wish to receive?
  • Filterable arguments: Does the implementation offer a built-in way to filter specific values of arguments by some pre-declared pattern?

Perhaps the only real design question for the Event-Reflexive Design Pattern Pattern is to decide on some neat concise language that implementations can use to explain their specific choices on these issues.

2 comments:

  1. Sure, that Design pattern is named FRP (Functional Reflective Programming). Some years ago Microsoft created the "Rx" (Reactive Extensions) library that is an implementation in more imperative languages like Perl, C#, C++ and so on.

    Today that Rx library was nearly ported to every language. Microsoft itself suport .Net, C++, Ruby, Python and JavaScript, and there exists a lot of ports for other languages too.

    In Perl i found Rx.pl (https://github.com/eilara/Rx.pl) but i don't knew how much already was implemented. Anyway nobody worked on it since a year, so could be that it is already dead. But "Rx" is really, really, realllllly awesome!

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