a dive into clojure

A DIVE INTO CLOJURE | April 6, 2012 | @skuro

Carlo Sciolla, Product Lead @ BackbaseA dive into Clojure

Friday, April 6, 2012


About meNext gen portal software for the Bank 2.0, a recent history of Alfresco and ECM specialist, Meetup organizer.

Amsterdam Clojurians

http://www.backbase.com http://bit.ly/amsclj

Carlo Sciolla

http://skuro.tk@skuro

Product Lead


http://www.backbase.com


http://bit.ly/amsclj


http://skuro.tk

http://skuro.tk


Motivationala.k.a. the towel they forgot to bring


A DIVE INTO CLOJURE | April 6, 2012 | @skuroA DIVE INTO CLOJURE | April 6, 2012 | @skuro

I've seen things you people wouldn't believeProgramming languages evolved throughout half a century. Paradigms rose and faded, with OOP and Java™ now the de-facto standards. Yet there’s more to programming than what objects have to offer.



The standard packageThere are a number of design choices behind the Java™ programming languages, and the ecosystem around it, that are quickly taken for granted. This effectively creates a horizon that can limit your sight.

* Single inheritance* [Almost] everything is an Object* Single dispatch* Non-covariant generics* Type erasures* C-like syntax* Frameworks composition



A paradigm shiftAs much as they’ve disappeared from mainstream development, they’re now regaining momentum. Functional languages such as Scala and Clojure are getting the spots on.

f (x)Friday, April 6, 2012


?The rocky road to LISPThere are more than fifty years of history behind any modern Lisp, and while most of them have greatly influenced other languages, there’s a unique core bulk of concepts that only finds home in a Lisp.

cons

gensym trampoline

lambda

macro

recur

let

monad

zipper

seq




“”A DIVE INTO CLOJURE | April 6, 2012 | @skuro

When you finally get itThe very idea that you have to get Lisp is fascinating. As a matter of facts, every Lisp developer experienced a moment of illumination, after which all those parens finally fit together.

Lisp is worth learning for the profound enlightenment experience you will have when you finally get it; that experience will make you a better programmer for the rest of your days

Eric S. Raymond

“



Open the parensto never look at programs the same way



Double rainbowsAs much as it might seem gibberish at first sight, at its very core there’s little to know about Lisp syntax. Clojure adds some syntactic sugar on top of standard Lisp, but the basics are all there.

(defn hello [name] (println (str “Hello, ” name “!”)))




Function callsThe first symbol after an open parenthesis is interpreted as the name of the function to call, with all the other members passed as parameters.

function calls





Evaluation strategy


user> (hello “World”)




Evaluation strategyVariables binding follow lexicographical order.






first invocation




Evaluation strategyVariables binding follows lexicographical order. Then start from the innermost list.



second invocation

(defn hello [name] (println “Hello, World!”))



Evaluation strategyVariables binding follows lexicographical order. Then start from the innermost list. Substitute it with its yielded value, then repeat.



done

(defn hello [name] nil)

user> (hello “World”)Hello, World!


Evaluation strategyVariables binding follows lexicographical order. Then start from the innermost list. Substitute it with its yielded value, then repeat. When we have a single value, we’re done and can yield a result.



user> (hello “World”)Hello, World!niluser>


Read, Eval, Print LoopYou just saw the REPL in action. More than a simple function execution tool, it’s

Read

Eval

Print



by any other interface it wouldn’t smell the sameWhatʼs in a seq



Much more than a collectionA sequential access data structure, most of the Clojure standard library functions are able to process a sequence.


clojure.core/seq([coll]) Returns a seq on the collection. If the collection is empty, returns nil. (seq nil) returns nil. seq also works on Strings, native Java arrays (of reference types) and any objects that implement Iterable.



(seq '(1 "test" :odd true)) => (1 "test" :odd true)(seq [1 "test" :odd true]) => (1 "test" :odd true)(seq {1 "test" :odd true}) => ([1 "test"] [:odd true])(seq #{1 "test" :odd true}) => (1 :odd true "test")(seq “test”) => (\t \e \s \t)


More than a collectionA sequential access data structure, most of the Clojure standard library functions are able to process a sequence. Wrapping a collection in a sequence is just one function call away.



Automatic conversionYou don’t even have to bother converting your data structure, as the standard library will do that for you.


(map identity [1 2 3 4]) ; (1 2 3 4)(filter odd? ‘(1 2 3 4)) ; (1 3)(reduce str “test”) ; “test”



‘cause Java beans give you gasImmutability



On state and identityWhen using a POJO to model an identity, you fail to keep time into consideration. Mutable state makes it hard to reason about code execution in a concurrent environment.

t

??

Horse horse = new Horse(14);horse.getPosition();



Persistent data structuresIn Clojure, data structures are immutable. There’s no “add to map”, you rather have to create a new map from the previous one, plus one element.

(let [x {:one 1}] (assoc x :two 2) (println x) ; {:one 1} (println (assoc x :two 2))) ; {:one 1 :two 2}



Doing real workImmutability doesn’t get in the way of solving programming problems. If anything, it makes it easier to reason about execution!

(defn letter-count [string] (let [reduce-fn (fn [counts letter] (let [count (or (counts letter) 0)] (assoc counts letter (inc count))))] (reduce reduce-fn {} string)))




user> (letter-count “Amsterdam”)





(reduce-fn {} \A)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)(reduce-fn {\A 1 \m 1 \s 1 \t 1} \e)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)(reduce-fn {\A 1 \m 1 \s 1 \t 1} \e)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1} \r)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)(reduce-fn {\A 1 \m 1 \s 1 \t 1} \e)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1} \r)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1} \d)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)(reduce-fn {\A 1 \m 1 \s 1 \t 1} \e)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1} \r)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1} \d)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1 \d 1} \a)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)(reduce-fn {\A 1 \m 1 \s 1 \t 1} \e)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1} \r)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1} \d)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1 \d 1} \a)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1 \d 1 \a 1} \m)






(reduce-fn {} \A)(reduce-fn {\A 1} \m)(reduce-fn {\A 1 \m 1} \s)(reduce-fn {\A 1 \m 1 \s 1} \t)(reduce-fn {\A 1 \m 1 \s 1 \t 1} \e)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1} \r)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1} \d)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1 \d 1} \a)(reduce-fn {\A 1 \m 1 \s 1 \t 1 \e 1 \r 1 \d 1 \a 1} \m){\A 1 \m 2 \s 1 \t 1 \e 1 \r 1 \d 1 \a 1}






Software transactional memoryWhen shared mutable state is really required, Clojure offers a number of constructs to handle it in a inherently thread safe manner, which frees you from resource locking.

clojure.core/ref([x] [x & options])

clojure.core/atom([x] [x & options])

clojure.core/agent([state & options])

sync coord



let your code write itselfMacro advantage



The classic exampleCan you tell what’s wrong with this code?

(defn unless “Executes body only if pred is false” [pred body] (if (not pred) body)

(unless (odd? 11) (println “woot?”))(unless (odd? 12) (println “woot?”))

user code



Eager evaluation kills itWhen implementing unless as a function, its arguments are evaluated before unless itself is executed. This effectively prevents you from getting it right.

(unless (odd? 11) (println “woot?”))=> “woot?”

(unless (odd? 12) (println “woot?”))=> “woot?”



Macros FTWMacros look like functions, but are treated in a slightly different way: their arguments are passed unevaluated, and they’re supposed to return code!

(defmacro unless [pred body] (list 'if (list 'not pred) body))



Reflection on steroidsYou can inspect your macro with macroexpand and macroexpand-1. The result now looks good: treating code as data, we can shuffle all the pieces and put them in the order we want. This is LISP.

(macroexpand '(unless (odd? 12) (println "woot?")))

(if (not (odd? 12)) (println "woot?"))

=>



Syntactic sugarThe syntax quote reader macro helps you write macros, as you end up writing a “template” for the code you want to generate.

(defmacro unless [pred body] `(if (not ~pred) ~body))



Programmable programming languageMacros are a powerful tool, and with great power comes great responsibility: use with care. That said, macros alone puts the whole concept of DSL just a built in of the language.

; automatic resource freeing(with-open [f (java.io.FileInputStream. "foo.txt")] (loop [c (.read f)] (when (not (= -1 c)) (println (char c)) (recur (.read f)))))

; threading macro(-> c char println)



when you ain’t gonna need it allLazy as a sloth



Infinite sequencesIn presence of strict (eager) evaluation, a collection of infinite elements will blow up your heap.

List<Integer> allInts = new ArrayList<Integer>();for (Integer i = 0; ; ++i) { allInts.add(i);}



Lazy sequences to the rescueA call to lazy-seq will put evaluation on hold, so that only the requested elements are “realized”. Also on the plus side, if you don’t hold a reference on the head the unused elements are garbage collected!

(defn all-ints [from] (cons from (lazy-seq (all-ints (inc from)))))

(take 10 (all-ints 0))=> (0 1 2 3 4 5 6 7 8 9)



The upsides and the flipsideMost of the Clojure sequence library is made of lazy functions. While this is handy in most cases, you must be aware of what’s lazy in your code, as you might have to force the full realization of a seq.

(with-open [session (hibernate-session)] (let [names [“carlo” “thomas” “luca” “nol”] results (map #(find-user % session) names)] (reset! users results))

(take 1 @users) ; org.hibernate.SessionException: Session was already closed



The programmer in controlYou can choose whether is nice to have a lazy sequence, and when it’s best to have all the elements available at once.

user=> (doc doall)-------------------------clojure.core/doall([coll] [n coll]) When lazy sequences are produced via functions that have side effects, any effects other than those needed to produce the first element in the seq do not occur until the seq is consumed. doall can be used to force any effects. Walks through the successive nexts of the seq, retains the head and returns it, thus causing the entire seq to reside in memory at one time.



Whatʼs next



Some drops in the seaDespite being so young a language, the Clojure ecosystem is already huge. These are only a few ideas on where to start if you want to learn more about Clojure.

* http://clojure.org* ClojureScript* Overtone* core-logic

* http://4clojure.com* #clojure on Freenode


http://clojure.org

http://clojure.org


Always open for businessIn two and a half year, we never missed a meetup. Come along and enjoy the Dutch Clojure community!

Amsterdam Clojurians


April 18th

hosted by:





Q/A



Thanks @skuro http://skuro.tk http://www.backbase.com

http://imgs.xkcd.com/comics/lisp.jpg


http://skuro.tk

http://skuro.tk





a dive into clojure

Technology

user hello world hello

println hello

user hello world defn

scalaand clojure

println str hello

backbasea dive

itsa dive

samea dive