Real World Applications/Event Driven Applications

Introduction

An event driven application is an application that reacts to external events.

Examples of events would be:

• A loan application has been accepted/rejected (commercial business).
• A new rostering schedule is ready for distribution to crew (airline system).
• A simulated car has hits another simulated car (commercial racing game).
• A HTML message has been received (web server).
• A key has been pressed (text editor).

The examples demonstrate that events can be anything from high level business events ("A loan application accepted/rejected") to low level events ("User pressed key").

In the following, I will show one way to architecture a Haskell system so that it can scale from small "toy" applications (dealing with low level IO events) to large scale "Enterprise" applications (dealing with high level business events).

Please note that the following is not the only way to attack the problem. So please contribute your (clearly superior of course) alternative way to do it here: [[1] Real World Applications]

Events in Haskell

In the following, I define an "Event" to be a value describing something that has happened in the past. And yes this should really be called an "Event Notification" but life is too short :-)

Here is a straightforward way to define Events in Haskell:

data Event =
    EventUserExit            -- User wants to exit
  | EventUserSave            -- User wants to save
  | EventUserSaveAs String
  | EventUserUndo            -- User wants to undo
  | EventUserRedo            -- User wants to redo
  deriving(Eq,Show)

Events can be high level or low level depending on how "low level" in the system you are operating. Within a UI sub-system the events are typically low level (key pressed, window closed). In a large scale distributed system the events are typically high level business events (EventCustomerCreated <details>).

A Tiny Event Driven Haskell Application

Let's begin with a simple event driven Haskell application:

module Main where

import Domain
import Event
import UI

main :: IO ()
main = run newDomain

run :: Domain -> IO ()
run dm = do
  event <- uiUpdate dm
  when (event != EventExit) $
    run $ domainUpdate dm event

Growing the Application

• Crash Recovery
• Undo/Redo
• Time
• UI
• Databases
• Client/Server

Questions and feedback

If you have any questions or suggestions, feel free to mail me.