From Java to Parellel Clojure - Clojure South 2019

:clojure/south - São Paulo, 2019
From Java to parallel Clojure
Leonardo Borges

@leonardo_borges

www.leonardoborges.com

www.recordpoint.com

A bit about me
• Head of Engineering at RecordPoint

• Founder of the Sydney Clojure User Group

• Open-source contributor

• Author of bouncer and imminent

• Author of Clojure Reactive Programming

A bit about me
• 2nd edition is out now!

What we’ll talk about
• Being a Lisp on the JVM

• Functional Programming strengths

• Concurrency and parallelism

Java has come a long way…
• ForkJoin;

• Lambda Expressions;

• Method References;

• CompletableFutures;

• JShell;

• Reactive Streams / Stream API;

• …and more!

So why would you invest in
Clojure?

Here’s a few reasons
Classes and Interfaces

• Minimize the accessibility of
classes and members

• In public classes, use
accessor methods, not public
ﬁelds

• Minimize mutability

Immutability
(def ages [10 20 30])
(def names ["Leo" "Liv" "Bruce"])

Immutability
(def ages [10 20 30])
(defrecord Person [fname age])
(def leo (->Person "Leo" 10))
;; {:fname "Leo", :age 10}

Immutability
(def ages [10 20 30])
(map (fn [fname age]
(->Person fname age)) names ages)
;; ({:fname "Leo", :age 10}
;; {:fname "Liv", :age 20}
;; {:fname "Bruce", :age 30})

Immutability
(def ages [10 20 30])
(map ->Person names ages)
;; ({:fname "Leo", :age 10}
;; {:fname "Liv", :age 20}
;; {:fname "Bruce", :age 30})

What if we want to add new
ages and names?

Adding new elements to vectors
(def new-ages (conj ages 40)) ;; [10 20 30 40]
ages ;; [10 20 30]
(def new-names (conj names "Gwen")) ;; ["Leo" "Liv"
"Bruce" "Gwen"]
names ;; ["Leo" "Liv" "Bruce"]

Persistent data structures
(def xs ‘(0 1 2))
(def ys ‘(3 4 5))

Persistent data structures
(def xs ‘(0 1 2))
(def ys ‘(3 4 5))
(def zs (concat xs ys))

Lambdas and Streams

• Prefer lambdas to anonymous
classes

• Prefer method references to
lambdas

• Favor the use of standard
functional interfaces

Prefer lambdas to anonymous classes
Collections.sort(names, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o1.compareTo(o2);
}
});

Prefer lambdas to anonymous classes
Collections.sort(names, (o1, o2) -> o1.compareTo(o2));
Collections.sort(names, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o1.compareTo(o2);
}
});

Prefer method references to lambdas
Collections.sort(names, String::compareTo);
Collections.sort(names, (o1, o2) -> o1.compareTo(o2));

The Clojure way
(sort names) ;; ("Bruce" "Leo" “Liv”)
(sort-by #(count %) names) ;; ("Bruce" "Leo" “Liv")
(sort-by count names) ;; ("Bruce" "Leo" "Liv")

Anonymous functions
(sort-by (fn [s] (count s)) names) ;; ("Bruce" "Leo" "Liv")
(sort-by #(count %) names) ;; ("Bruce" "Leo" "Liv")
(sort-by count names) ;; ("Bruce" "Leo" "Liv")

Concurrency

• Synchronize access to shared
mutable data

• Avoid excessive
synchronization

Synchronise access to shared mutable data
class StopThread {
private static boolean stopRequested;
private static synchronized void requestStop() {
stopRequested = true;
}
private static synchronized boolean stopRequested() {
return stopRequested;
}
public static void example3() throws InterruptedException {
Thread backgroundThread = new Thread(() -> {
while (!stopRequested())
System.out.println("going....");
});
backgroundThread.start();
TimeUnit.SECONDS.sleep(1);
requestStop();
}
}

Synchronise access to shared mutable references
(def stop-requested (atom false))
(defn request-stop! []
(reset! stop-requested true))
(defn stop-requested? []
@stop-requested)
(defn example-3 []
(let [background-thread (java.lang.Thread. (fn []
(while (not (stop-requested?))
(prn "going..."))))]
(.start background-thread)
(Thread/sleep 1000)
(request-stop!)))

What about multiple shared
references?

STM - Software Transactional Memory
(def account-a (ref 100))
(def account-b (ref 250))
(defn transfer [amount from to]
(dosync
(alter from #(- % amount))
(alter to #(+ % amount))))
(transfer 25 account-a account-b)
@account-a ;; 75
@account-b ;; 275

Clojure makes it easy to do the
right thing

(defn example-3 []
(let [background-thread (java.lang.Thread. (fn []
(while (not @stop-requested)
(prn "going..."))))]
(.start background-thread)
(Thread/sleep 1000)
(request-stop!)))

(defn example-3[]
(future
(while (not @stop-requested)
(prn "going...")))
(Thread/sleep 1000)
(request-stop!))

Concurrency with futures
(def doubler (partial * 2))
(defn service-a [n]
(future
(Thread/sleep 1000)
n))
(defn service-b [n]
(future
(Thread/sleep 1000)
(Math/pow n 2)))
(defn service-c [n]
(future
(Thread/sleep 1000)
(Math/pow n 3)))
(defn service-d [n]
(future
(Thread/sleep 1000)
(Math/pow n 4)))
(let [doubled (doubler @(service-a 10))]
(+ @(service-b doubled)
@(service-c doubled)
@(service-d doubled)))
;; Elapsed time: 4013.746558 msecs
(let [a (service-a 10)
doubled (doubler @a)
b (service-b doubled)
c (service-c doubled)
d (service-d doubled)]
(+ @b @c @d))

Concurrency with futures
(def doubler (partial * 2))
(defn service-a [n]
(future
(Thread/sleep 1000)
n))
(defn service-b [n]
(future
(Thread/sleep 1000)
(Math/pow n 2)))
(defn service-c [n]
(future
(Thread/sleep 1000)
(Math/pow n 3)))
(defn service-d [n]
(future
(Thread/sleep 1000)
(Math/pow n 4)))
(let [doubled (doubler @(service-a 10))]
(+ @(service-b doubled)
@(service-c doubled)
@(service-d doubled)))
(let [a (service-a 10)
doubled (doubler @a)
(+ @b @c @d))
Blocks main thread!

Concurrency with CompletableFutures
static Integer doubler(Integer n) {
return 2 * n;
}
static CompletableFuture<Integer> serviceA(Integer n) {
return CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
}
return n;
});
}
static CompletableFuture<Integer> serviceB(Integer n) {
try {
Thread.sleep(1000);
}
return Double.valueOf(Math.pow(n, 2)).intValue();
});
}
static CompletableFuture<Integer> serviceC(Integer n) {
try {
Thread.sleep(1000);
}
return Double.valueOf(Math.pow(n, 3)).intValue();
});
}

Concurrency with CompletableFutures
final CompletableFuture<Integer> doubled = serviceA(10).thenApply(CompletableFutures::doubler);
final CompletableFuture<Integer> resultB = doubled.thenCompose(CompletableFutures::serviceB);
final CompletableFuture<Integer> resultC = doubled.thenCompose(CompletableFutures::serviceC);
CompletableFuture<Void> allFutures = CompletableFuture.allOf(resultB, resultC);
allFutures.whenComplete((v, ex) -> {
try {
System.out.println("Result: " + resultB.get() + " - " + resultC.get());
} catch (Exception e) {
}
});

What if?
;; Wouldn't it be great to be able to write:
(let [doubled (doubler (service-a 10))
;; then once that's all done concurrently...
(+ b c d))

Concurrency with imminent
(require '[imminent.core :as i])
(defn service-a [n]
(i/future
(Thread/sleep 1000)
n))
(defn service-b [n]
(i/future
(Thread/sleep 1000)
(Math/pow n 2)))
(defn service-c [n]
(i/future
(Thread/sleep 1000)
(Math/pow n 3)))
(defn service-d [n]
(i/future
(Thread/sleep 1000)
(Math/pow n 4)))

(let [doubled (i/map (service-a 10) doubler)
b (i/bind doubled service-b)
c (i/bind doubled service-c)
d (i/bind doubled service-d)
result (i/sequence [b c d])]
(i/map result
(fn [[b c d]]
(+ b c d))))

(defn f-doubler [n]
(i/const-future (* n 2)))
(i/mdo [a (service-a 10)
doubled (f-doubler a)
(i/return (+ b c d)))

(bind
(service-a 10)
(fn* ([a]
(bind
(f-doubler a)
(fn* ([doubled]
(bind
(service-b doubled)
(fn* ([b]
(bind
(service-c doubled) (fn* ([c]
(bind
(service-d doubled)
(fn* ([d]
(i/return (+ b c d))))))))))))))))))

(def a+ (i/alift +))
(i/mdo [a (service-a 10)
doubled (f-doubler a)]
(a+ (service-b doubled)
(service-c doubled)
(service-d doubled)))

What’s with map, bind and
alift?

The algebra of library design
i/map => Functor
i/bind => Monad
i/alift => Applicative

References
• Clojure Reactive Programming - http://bit.ly/cljRp
• Imminent - http://bit.ly/immi-clj
• The Algebra of Library Design - http://bit.ly/2HBBJwJ
• Purely Functional Data Structures - https://amzn.to/2zGZizS
• Java 8 CompletableFuture - http://bit.ly/j8Future
• Java 8 Streams - http://bit.ly/j8stream
• Category Theory - http://amzn.to/1NfL08U

Thank you!
Leonardo Borges

@leonardo_borges

www.leonardoborges.com

www.recordpoint.com
:clojure/south - São Paulo, 2019

From Java to Parellel Clojure - Clojure South 2019

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