Clojure and Swing – a new productivity sweet spot? discusses how Clojure, a Lisp dialect that runs on the JVM, can be used to develop graphical user interfaces (GUIs) using Java Swing in a more productive way than plain Java. It presents several advantages of Clojure for Swing development, such as reducing boilerplate code, easier definition of actions and event bindings, and increased flexibility and reusability through functional programming techniques. Examples are provided of common Swing programming tasks implemented more concisely and readably in Clojure compared to Java. The document concludes that Clojure is a powerful and flexible language that offers opportunities to apply Lisp concepts to Swing GUI development

1. Clojureand Swing – a new productivity sweet spot? Simon White

2. Skills Matter The UK Open Source Training Company Training on: Agile Development, Test Driven Development, Java, Eclipse, Spring, Hibernate, Ajax, Ruby on Rails, Struts Ti, Wicket, JavaServer Faces, Tapestry, Beehive, AOP, RIFE and more! Training straight from the Source Experts write and teach our courses: Adrian Colyer, Chad Fowler, Howard M. Lewis Ship Craig Larman, Dave Crane, Kevlin Henney, Rob Harrop, Kito Mann, Rod Johnson and many more Partners with leading edge companies BEA, IBM, Interface21, Sun, Tangosol, wso2 © Catalysoft Ltd, 2010

3. Speaker Qualifications Simon White, IndependentJava developer Games published 1985 PhD Artificial Intelligence Written Scientific Software for: Remote Sensing Drug Discovery Medical & Genetics Research Swing Developer since 2000 Author of JIDE Charts SkillsMatter Author & Trainer © Catalysoft Ltd, 2010

4. What is Clojure? A functional language that runs on the JVM A new dialect of LISP Not Closure Not Clozure MYTH: LISP is slow MYTH: Therefore Clojure is slow

5. Functional Languages What is a function? a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)) [Wordnet] Focus on the input/output relation e.g. length of a string maps a string to an integer Immutability Good; Side-effects Bad

6. LISP Syntax (functor param1 param2 ... paramn) for example, (+ 1 2) Brackets denote evaluation A single quote protects from evaluation so '(x y z) does not use x as a function With nested expressions, evaluate inner expression first: (+ (+ 2 3) (* 2 3)) is 11 Special forms like let, cond deviate from the syntax and must be learned

7. Hello Factorial n! = n × (n-1) × (n-2) × ... × 1 Also 0! = 1 (defnfac “Computes the factorial of n” [n] (if (= n 0) 1 (* n (fac (- n 1)))))

8. REPLRead-Evaluate-Print-Loop user=> (defn add-one [n] (+ 1 n)) #'user/add-one user=> (add-one 6) 7 user=>

9. Java vs. LISP

10. Why mix Java and LISP? To get the best of both: Speedy development of custom code in LISP Speedy development through library reuse in Java Speedy development of applications with concurrent processing

11. LISP Promotes Agility LISP was agile long before agility was respected Easy to mix and match function application Accommodate changing requirements Many small functions => very reusable code REPL Encourages ad-hoc testing Functional style makes it easy to write unit tests

12. Clojureas a LISP dialect Simplified syntax compared to Common LISP cond, let Not object-oriented But you can define structures and multi-methods No multiple value returns Arguably syntactic sugar anyway Lazy evaluation of sequences Destructuring Structural pattern matching on function parameters Concurrency

13. Data Structures 1: Lists (list 'a 'b 'c) -> (a b c) (first '(a b c)) -> a (rest '(a b c)) -> (b c) (nth '(a b c) 1) -> b (cons 'a '(b c)) -> (a b c) (concat '(a b) '(c d)) -> (a b c d)

14. Data Structures 2: Vectors (vector 'a 'b 'c) -> [a b c] (first '[a b c]) -> a (rest '[a b c]) -> (b c) (nth '[a b c] 1) -> b (cons 'a '[b c]) -> (a b c) (conj '[a b] 'c) -> [a b c] (concat '[a b] '[c d]) -> (a b c d)

15. Data Structures 3: Maps Key-Value Pairs (get '{a 1, b 2} 'b) -> 2 (assoc '{a 1, b 2} 'c 3) -> {c 3, a 1, b 2} (dissoc '{a 1, b 2} 'b) -> {a 1} (defstruct book :title :author) (struct-map book :title "Jungle Book" :author "Rudyard Kipling") (bean obj)

16. Everything is a Sequence Lists, Vectors and Maps are all sequences first, rest & cons always return a sequence (range 5) -> (0 1 2 3 4) (take 2 (range 5)) -> (0 1) (take 3 (repeat 'x)) -> (x xx)

17. Predicates (Boolean-Valued Tests) (= a 10) (identical? a b) (even? a) (odd? a) (integer? a)

18. Decisions (if (= a 1) 5 0) (cond (= a 1) 5 (= a 2) 10 true 0) (when (= a 1) 5) a == 1 ? 5 : 0 Java

19. "Iteration" (for [a '(1 2 3)] (+ a 1)) -> (2 3 4) (doseq [a (range 5)] a) -> nil (doseq [a (range 3)] (println a)) -> 0 1 2 nil

20. Anonymous Functions user=> (fn [a b] (+ a b 1)) #<user$eval__101$fn__103 ...@a613f8> user=> ((fn [a b] (+ a b 1)) 3 4) 8 user=> #(+ %1 %2) #<user$eval__121$fn__123 ...@56f631>

21. Higher Order Functions (inc 1)->2 (map inc '(1 2 3))->(2 3 4) (map + '(1 2 3) '(10 11 12))->(11 13 15) (max 1 2 3)->3 (apply max '(1 2 3))->3 (filter even? (range 10))->(0 2 4 6 8)

22. Partial Functions Partial returns a function user=> (def add-two (partial + 2)) #'user/add-two user=> (add-two 5) 7

23. Java Interoperability

24. A GUI in a functional language? Functional languages use functions as mathematical models of computation: f(x1, x2, ...) -> x’ Great for factorials, but how does it work with user-interfaces? Forms? Keyboard & Mouse? Events?

25. Need to Think Differently Values of functions ‘computed’ as the result of a user-interaction Consider y-or-n-p CL> (y-or-n-p "Do you really want to quit?") T

26. Modelling Change of State f(x) -> x’ Name: “George” DOB: “11-Jul-73” Address: “16 Goldman Square” Name: “George” DOB: “11-Jul-73” Address: “3 Elm Avenue” This is a new (immutable) value, not a modified one

27. Java Swing: Create a JFrame import javax.swing.JFrame; public class MyClass { ... public static void main(String[] args) { JFrame frame = new JFrame("My Frame"); frame.setBounds(300, 300, 600, 400); frame.setVisible(true); } }

28. Clojure Swing: Create a JFrame (ns user (:import (javax.swingJFrame))) (defn make-frame [] (let [f (JFrame. "My Frame")] (.setBounds f 300 300 600 400) (.setVisible f true) f ) ) f is the return value

29. Alternative: Use doto (defn make-frame [] (let [f (JFrame. "My Frame")] (doto f (.setBounds 300 300 600 400) (.setVisible true))))

30. Create a Panel & Button (ns user (:import java.awt.FlowLayout (javax.swingJButtonJPanel))) (defn make-panel [] (let [panel (JPanel. (FlowLayout.)) button (JButton. "Press Me")] (doto panel (.add button))))

31. Make a Button do Something (defn make-panel2 [] (let [panel (JPanel. (FlowLayout.)) button (JButton. "Press Me")] (.addActionListener button (proxy [ActionListener] [] (actionPerformed [e] (println e)))) (doto panel (.add button)) ) ) #<ActionEventjava.awt.event.ActionEvent[ACTION_PERFORMED,cmd=Press Me...

32. Clojure Custom Component (defn make-component [msg] (proxy [javax.swing.JComponent] [] (paintComponent [g] (let [height (.getHeight this) width (.getWidth this)] (doto g ... ))))) Detail on next slide

33. Custom Component (cont/d) (doto g (.setColorColor/gray) (.fillOval 20 20 (- width 20) (- height 20)) (.setColorColor/yellow) (.fillOval 0 0 (- width 20) (- height 20)) (.setFont (.deriveFont (.getFont g) (float 50))) (.setColorColor/black) (.drawStringmsg (int (/ (- width (.stringWidth (.getFontMetrics g) msg)) 2)) (int (/ height 2))))

34. Three ways in which Clojure can help Swing Reducing boiler-plate code Easy to write code that generates the required patterns Definitions of actions and easier binding Easier to separate configuration (name, icon, keyboard shortcut, ...) from code hook Flexibility and Reusability Using functions as parameters for user customisation of behaviour

36. Use the same options a lot of the time

38. 2. Defining Swing Actions Swing Actions are a nice idea, but have problems. Typically, you might have: Lots of inner classes with repeated boiler plate code, or: Separate classes that extend AbstractAction but are too tightly coupled to a main class so that they have the necessary context for the actionPerformed() method.

39. Clojure Actions: Defer binding for actionPerformed method With an Action in Clojure it’s possible to preset the ‘constant’ variables like Name, Icon, Accelerator ; but defer the binding for the handler. This means we can easily separate the creation of an action according to some template from its binding to a response function.

40. Create an Action when the handler function is known (defmacro copy-action [handler] `(make-action {:name "Copy" :command-key "copy" :icon (ImageIcon. (fetch-image "copy.png")) :handler ~handler :mnemonic (mnemonic ) :accelerator (accelerator "ctrl C")}))

41. 3. Flexibility and Reusability (def *tracing-actions* true) (defntrace-action [handler] (fn [#^ActionEvent e] (try (when *tracing-actions* (print "Doing" (.getActionCommand e))) (handler e) (finally (when *tracing-actions* (println "Done"))))))

42. Swing Worker & Busy Cursor (defmacro with-busy-cursor [component f] `(proxy [SwingWorker] [] (doInBackground [] (.setCursor ~component (Cursor/getPredefinedCursor Cursor/WAIT_CURSOR)) ~f) (done [] (.setCursor ~component (Cursor/getDefaultCursor))))) (with-busy-cursor chart (load-file data-file))

43. Clojure/Swing in the Real World

44. Creating a Chart Model (defn make-model "Create and return a ChartModel using the supplied rows and picking out the x and y columns" [model-name rows #^String x-col #^String y-col] (let [model (DefaultChartModel. model-name)] (doseq [row rows] (let [x (get row x-col) y (get row y-col)] (when (and (number? x) (number? y)) (.addPoint model (double x) (double y)))) ) model))

45. Summary Clojureis Powerful and Flexible Excellent Java Interoperability Opportunities to apply LISP power to Swing GUI development Can be used for Real World Applications A Secret Weapon for Productivity?

46. Closure Example (defnplusn [x] (fn [y] (+ x y))) (def plus5 (plusn 5)) (plus5 3) >> 8