Hi Itay,
Thanks for posting this example. Being new to Clojure it's a nice example
to study since it solves a very realistic problem that many new to
functional programming will face.
I think I've unraveled most of how the code is working but there's one
function I'm not particularly clear about.
(defn- run-observers-till-fixpoint [prev next]
(let [observers (next :observers)
new-next (run-observers prev next observers)]
(if (= new-next next)
new-next
(recur next new-next) )))
While I understand run-observers, run-observers-till-fixpoint has be
baffled. Why is this required?
2009/2/27 Itay Maman <itay.ma...@gmail.com>
>
> Some of the reaction for Waterfront was related to the Application
> Context Pattern (ACP) - The pattern that allows most of Waterfront's
> code to be purely functional. I'll try to explain the basics in this
> post. Let me start with the motivation: the reason why FP is at odds
> with GUI code.
>
> (Pure) Functional code has no side effects, which implies immutability
> of state. There are no fields nor global variables that can be
> assigned to. Thus, one function can affect the computation carried out
> by another function only by the passing of parameters. Most GUI
> systems are built around the notion of event handlers which are
> invoked by a message processing loop. There is no chain of calls from
> one event handler to another.
> In particular, if handler "A" computed some new value it cannot pass
> it on to handler "B" because the system will call "B" only after "A"
> returns. That's the predicament.
>
> ACP overcomes this by capturing the applications current state in an
> immutable map. All event handlers receive a single parameter which is
> the "current" context and compute the "new" context. A typical handler
> (henceforth: "context processing function") will carry out these
> activities: (a) Examine the current context; (b) Perform some GUI
> operations (setSize, setText, etc.); (c) Compute a new context based
> on the current context and on information obtained from the GUI
> (getText, etc.). The caller (henceforth: "dispatcher") takes the
> returned context and will use it as the new current context, the next
> time a context processing function is invoked.
>
> This means that when you register event handler with a Swing widget
> the handler needs to to call the ACP dispatcher passing it a context
> processing function.
>
> The net effect of this approach is that only the dispatcher has to
> deal with mutable state. The context processors are functional: they
> merely compute the new state from the current.
>
> application-context-pattern.clj (http://groups.google.com/group/
> clojure/web/application-context-pattern.clj) shows a concrete example.
> It's about 140 LOC (ripped off from the real Waterfront codebase)
> structured as follows:
> Lines 1..40: General-purpose helpers.
> Lines 40..90: The ACP infrastructure
> Lines 90..140: A quick sample, built around ACP.
>
> The sample program opens a JFrame with two buttons: Input and Output.
> A click on the input button will pop-up an input dialog box. A click
> on the output button will pop-up a message box showing the last value
> entered into the input box. There's also a JLabel showing the length
> of the input, but let's ignore it for the moment.
>
> The entry point into the ACP world is the bootstrap function. It takes
> two parameters: a context processing function and an initial context.
> In the example, this is carried out at the bottom of the run-it
> function:
>
> (defn run-it []
> (let [build-ui (fn [ctx]
> (let [f (javax.swing.JFrame. "Frame")
> b-in (javax.swing.JButton. "Input")
> b-out (javax.swing.JButton. "Output")]
>
> (.addActionListener b-in (new-action-listener (fn [event]
> ((ctx :dispatch) get-input))))
>
> (.addActionListener b-out (new-action-listener (fn [event]
> ((ctx :dispatch) show-output))))
>
> (.setLayout f (java.awt.FlowLayout.))
> (doseq [x [b-in b-out]]
> (.add f x) )
>
> (doto f
> (.setSize 500 300)
> (.setDefaultCloseOperation javax.swing.JFrame/
> DISPOSE_ON_CLOSE)
> (.setVisible true))
>
> (assoc ctx :frame f) ))]
>
> (invoke-later #(bootstrap build-ui {})) ))
>
>
> invoke-later is a utility function that is mapped to SwingUtilities/
> invokeLater.
>
> Let's drill down into the build-ui function: It takes the current
> context (ctx parameter). Then it creates the frame and the buttons. It
> uses new-action-listener (another utility) to register an action
> listener with the buttons. The first listener looks like this:
> ((ctx :dispatch) get-input))))
>
> It uses (ctx :dispatch) to obtain the dispatcher from which ctx was
> obtained, and evaluates it passing get-input as the context processing
> function. The call to bootstrap initialized this dispatcher and added
> the :dispatch mapping to the initial context.
>
> get-input looks like this:
> (defn- get-input [ctx]
> (let [reply (javax.swing.JOptionPane/showInputDialog nil "Type in
> something")]
> (assoc ctx :user-input reply) ))
>
> It pops-up an input box, and returns a new context which is the same
> as the current context except that :user-input is now mapped to value
> returned from the input box.
>
> show-output is the context processing function for the output button:
> (defn- show-output [ctx]
> (javax.swing.JOptionPane/showMessageDialog nil (ctx :user-
> input)) )
>
> Note that show-output returns nil which the dispatcher interprets as
> "no change to ctx".
>
> What we have is that get-input communicates with show-output by
> returning a new context. There's no assignment into atoms or the
> likes. The mutable state is encapsulated within the dispatcher.
>
> It is now time for the JLabel to step into the plate. We now want to
> add a JLabel that will show the length of the user input. We want this
> label to be updated on the fly, with no explicit user request. This
> type of behavior is common in GUI applications. To this end, the
> dispatcher also supports the notion of observers. In ACP an observer
> is a function takes two contexts: old-ctx and new-ctx.
>
> An observer typically compares the contexts. If the mappings it is
> interested in were changed, it carries out these activities:
> (a) Updates the GUI (setText, setEnabled, etc.); (b) Computes a new
> context (that is: a context that is even newer than new-ctx). After a
> context processing function was invoked, the dispatcher will run all
> observers in a loop until a steady state was reached.
>
> Here's the observer that takes care of updating the label:
> (defn- input-observer [old-ctx new-ctx]
> (when-not (= (old-ctx :user-input) (new-ctx :user-input))
> (.setText (new-ctx :label) (str "Input length: " (count (new-
> ctx :user-input)))) ))
>
> One register an observer by adding it to (ctx :observers):
> (assoc ctx :frame f :label label :observers (cons input-
> observer (ctx :observers)))
>
> There all sort of variations on these theme and some subtleties, but I
> want to keep this post coherent so I'll skip these for now. Anyway,
> all the fundamentals are here. As you can see this pattern is quite
> powerful. Almost all functionality of Waterfront is built on top of
> this.
>
> For instance, the menu system is described as a DSL (a vector of maps)
> mapped to the :menu key of the context. An observer keeps an eye on
> the :menu mapping and rebuilds the JMenuBar whenever it changes. Also,
> the plugin-loader is an observer that watches the :plugins list. When
> a new entry is found, it loads this plugin. In fact, in Waterfront,
> the startup function simply registers the plugin observer and loads
> the list of plugins from a file.
>
>
> (file:
> http://groups.google.com/group/clojure/web/application-context-pattern.clj
> )
>
> >
>
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