Partially Implemented Protocols

Last time I discussed using a hash maps to improve code reuse with protocols. While we produced a set of good solutions, I couldn't help but feel like we aren't quite there. There still is a fair amount of chatter on the list about how to implement a default protocol, for example.

I started experimenting with an idea I'm calling a Partially Implemented Protocol (PIP). The idea is that you can use a PIP like you would a BaseClass in Java. After playing around for a little bit, I settled on the following solution:

	(in-ns 'clojure.core)
	(defrecord PIP [:protocol :impl])
	(defn pip? [x] (instance? clojure.core.PIP x))
	(defn extract-pair [p m]
	(if (pip? p)
	[(:protocol p) (merge (:impl p) m)]
	[p m]))
	(defn pip [p m]
	(if (or (protocol? p) (pip? p))
	(let [[extracted-p extracted-m] (extract-pair p m)]
	(PIP. extracted-p extracted-m))
	(throw (Exception. "Protocol not provided"))))
	(defn merge-pip
	[& pips]
	(let [pips (map #(pip % {}) pips)]
	(if (not (apply = (map :protocol pips)))
	(throw (Exception. "Trying to merge different protocols."))
	(pip (:protocol (first pips)) (apply merge (map :impl pips))))))
	(defmacro extend+ [atype & proto+maps]
	`(extend ~atype
	~(apply concat
	(map extract-pair
	(partition 2 proto+maps)))))

view raw protocol-util.clj hosted with ❤ by GitHub

The two major components is the PIP record and extend+ macro. Let me re-write yesterday's example with the new expand+ syntax.

(defprotocol UIProtocol
 (render [])
 (action [evt])
 (button-down [evt])
 (button-up [evt]))

(def BaseUIProtocol
 (pip UIProtocol
 {:action (fn [this evt] ...)
   :button-down (fn ([this evt]...)
   :button-up (fn ([this evt]...)})

(extend+ TerranUIType
  BaseUIProtocol
  {:render an-existing-fn})

(extend+ ProtossUIType
  BaseUIProtocol
  {:render a-different-fn})

What I really like about the use of a PIP is that it allows reference to the protocol & partial implementation with one record. You can get at the internal simply by using keyword lookup, and you can also derive a PIP from another PIP like so:

(def up-down-override
  :button-down button-down-2
  :button-up button-up-2})

(extend+ ZergUIType
  (pip BaseUIProtocol up-down-override)
  {:render a-buggy-fn})

I'm also working on versions of reify+ proxy+, etc. that all work with PIPs, assuming this idea is worth exploring.

Protocol Implementation Awesomeness

I recently read a post about Racket & Clojure, and there was a question about protocols raised. I wanted to take the time to discuss composition of implementations with Protocols. Of course, you probably know this term better as inheritance.

Personally, my main driver for multiple inhertiance is being able to re-use different partial implementations of a class. There are a lot a times I would want to mix & match implementations of this protocol, such as UI code. Consider the following protocol

(defprotocol UIProtocol
 (render [])
 (action [evt])
 (button-down [evt])
 (button-up [evt]))

Now, we can extend the protocol as such

(extend UIType
  UIProtocol
  {:render an-existing-fn
   :action (fn [this evt] ...)
   :button-down (fn ([this evt]...)
   :button-up (fn ([this evt]...)})

So far so good. However, suppose we have another UI type we want to define

(extend AnotherUIType
  UIProtocol
  {:render a-different-fn
   :action (fn [this evt] ...)
   :button-down (fn ([this evt]...)
   :button-up (fn ([this evt]...)})

The only thing that is different between the implementations is the rendering logic (similar behavior, different skin? What sort of problem domain has that?). Our implementation works, but it's repetitive.

In order to clean it up, we'll need to use a defining feature of Lisp: code is data. Remember we are building a data structure to define our type. As such, we can use every DRY trick we know. In our specific example, we take advantage of the fact that the extend macro is expecting a hash-map, and it doesn't care how it gets there. So, here's a new take on the code.

(def base-impl
  {:action (fn [this evt] ...)
   :button-down (fn ([this evt]...)
   :button-up (fn ([this evt]...)})

(extend TerranUIType
  UIProtocol
  (merge base-impl {:render an-existing-fn}))

(extend ProtossUIType
  UIProtocol
  (merge base-impl {:render a-different-fn})

BAM! I've overridden render with my specific implementation. We just simulated the abstract base class pattern. Now, let's see how flexible this really lets us be.

(def up-down-override
  :button-down button-down-2
   :button-up button-up-2})

(extend ZergUIType
  UIProtocol
  (merge base-impl up-down-override {:render a-buggy-fn});-p

What this let me do is define my own inheritance rules a la carte. No implicit order of operations, no compiler limitations, no confusing precedence rules. If you can figure out the map merging, you can do it. Period. That's why I think Clojure Protocols are so awesome!

Steve Jobs just ruined the iPhone for Clojure

Recently Apple released new terms of service with their  iPhone OS.  By now most of you have seen the following section.  I've copied this from Daring Fireball

3.3.1 — Applications may only use Documented APIs in the manner prescribed by Apple and must not use or call any private APIs. Applications must be originally written in Objective-C, C, C++, or JavaScript as executed by the iPhone OS WebKit engine, and only code written in C, C++, and Objective-C may compile and directly link against the Documented APIs (e.g., Applications that link to Documented APIs through an intermediary translation or compatibility layer or tool are prohibited).

I'm pissed off.  I believe the hype, and there's a lot of HCI experiments I'd like to do with the iPhone & iPad.  There's a ton of opportunities & ideas, waiting to be discovered.  Being able to do these experiments in Clojure would be some much fun, that it's worth the $500 investment.

If I'm reading the TOS, we can't have a version of Clojure on this platform.  Any ideas I have - no, we have - now can't be shared with the world.

This is NOT thinking differently Apple.

New Getting started page

Clojure's getting started information has moved.  You can check it out here:

Clojure Getting Started

Too much Clojure when...

I can no long spell the following words properly. This is really annoying when talking to people outside of this community :)

• Closure/Clojure
• Definitely/Defnitely
• Disclosure/Disclojure (self inflicted)
• Composure/Compojure

Thanks for nothing Rich :-p

What're your experiences?

Thoughts on namespace management

Konrad Hinsen recently posted some ideas for handling namespaces

http://onclojure.com/2010/02/17/managing-namespaces

There are a lot of good ideas in there, and right now I'd like to talk specifically about the proposed :like & :clone clauses.  I have a bit of a different approach.

Supposed we add a new project file, namespace_config.clj.  It would be a basic rules engine for configuring namespaces.  I'm thinking there would be a macro, extend-ns, that is roughly defined as follows. The exact implementation needs work.

	(defmacro extend-ns
	[pred & ns-clauses]
	(if (pred a-ns)
	(let [clauses (apply hash-map ns-clauses)]
	`(do
	(import ~@(:import clauses))
	(use ~@(:use clauses))
	(require ~@(:require ))))))

view raw ns-rules.clj hosted with ❤ by GitHub

This lets us set up a series of predicates & resulting actions that would be applied to each ns in the project. Everything would be done in one central location, so (hopefully) that would cut down on maintenance.

Code Kata: A data sifter

Here's a problem inspired by a scheme example I discovered another day.

http://programming-musings.org/2006/02/07/scheme-code-kata/

Write a data sifter, sift, that partitions a string into a list of lists.  Start with the case of using letters as a delimiter, and numbers as data.  There can be any number of repetitions of numbers & letters.

user=>(sift "a1b2cd34")
(("a" ("1")) ("b" ("2")) ("c" ()) ("d" ("3" "4")))

Next, add the ability to your sift function to accept a list as input, as well as a string.

user=>(sift ("a" "1" "b" "2" "c" "d" "3" "4"))

(("a" ("1")) ("b" ("2")) ("c" ()) ("d" ("3" "4")))

After that, add the ability to take a vector/array as an input

user=>(sift ["a" "1" "b" "2" "c" "d" "3" "4"])

(("a" ("1")) ("b" ("2")) ("c" ()) ("d" ("3" "4")))

Finally, let your sift accept a collection of any object, and an arbitrary predicate.  If the predicate is true, the object is a delimiter (e.g. a String).  If the predicate is false, the object is data (e.g. a Number).

user=>(sift string? ["a" 1 "b" 2 "c" "d" 3 4])

(("a" (1)) ("b" (2)) ("c" ()) ("d" (3 4)))

I'll be posting my solution in about a week.