Books I read 2016

January
- El Eternauta, Héctor Germán Oesterheld and Francisco Solano López
- Barcelona. Los vagabundos de la chatarra, Jorge Carrión and Sagar Fornies
- Rip Van Winkle, Washington Irving
- La guerra interminable (The Forever War), Joe Haldeman
- Maintanable JavaScript, Nicholas C. Zakas
- Ve y pon un centinela (Go Set a Watchman), Harper Lee
- El nombre del viento (The Name of the Wind), Patrick Rothfuss
- You Don't Know JS: Async & Performance, Kyle Simpson
- Sapiens: A Brief History of Humankind, Yuval Noah Harari
- The Principles of Object Oriented JavaScript, Nicholas C. Zakas

February
- The Leprechauns of Software Engineering, Laurent Bossavit
- The Wise Man's Fear, Patrick Rothfuss
- Fundamentals of Object-Oriented Design in UML, Meilir Page-Jones
- Old Man's War, John Scalzi
- Cevdet Bey e hijos (Cevdet Bey ve Oğulları), Orhan Pamuk
- Ringworld, Larry Niven
- Why Nations Fail: The Origins of Power, Prosperity, and Poverty, Daron Acemoglu and James A. Robinson

March
- The Grumpy Programmer's Guide To Building Testable PHP Applications, Chris Hartjes
- The Grapes of Wrath, John Steinbeck
- The Coding Dojo Handbook, Emily Bache
- Fahrenheit 451, Ray Bradbury

April
- Implementation Patterns, Kent Beck (3rd time)
- The Power of Habit: Why We Do What We Do in Life and Business, Charles Duhigg
- How to Win Friends and Influence People, Dale Carnegie
- Understanding the Four Rules of Simple Design, Corey Haines (2nd time)
- La llamada de Cthulhu (The Call of Cthulhu), El horror de Dunwich (The Dunwich Horror) and Las ratas en las paredes (The Rats in the Walls), H. P. Lovecraft
- The Tales of Beedle the Bard, J. K. Rowling

May
- The Slow Regard of Silent Things, Patrick Rothfuss
- Building Microservices, Designing Fine-Grained Systems, Sam Newman
- Man's Search for Meaning, Viktor Frankl
- Manifiesto del Partido Comunista (Das Kommunistiche Manifest), K. Marx and F. Engels
- Web Development with Clojure, Build Bulletproof Web Apps with Less Code, Dimitri Sotnikov
- Patterns of Enterprise Application Architecture, Martin Fowler
- The Checklist Manifesto: How to Get Things Right, Atul Gawande
- How to Fail at Almost Everything and Still Win Big: Kind of the Story of My Life, Scott Adams

June
- Pragmatic Thinking and Learning, Refactor Your Wetware, Andy Hunt
- Testable JavaScript, Mark Ethan Trostler
- The Secrets of Consulting: A Guide to Giving & Getting Advice Successfully, Gerald M. Weinberg
- La desfachatez intelectual. Escritores e intelectuales ante la política, Ignacio Sánchez-Cuenca
- REST in Practice, Jim Webber, Savas Parastatidis and Ian Robinson
- Mindset, Carol Dweck

July
- The Call of the Wild, Jack London
- Weinberg on Writing: The Fieldstone Method, Gerald M. Weinberg
- Dinner at the Homesick Restaurant, Anne Tyler
- How to Solve It: A New Aspect of Mathematical Method, G. Polya
- Object Oriented Software Engineering: A Use Case Driven Approach, Ivar Jacobson
- A Far Cry from Kensington, Muriel Spark

August
- The Difference Engine, William Gibson and Bruce Sterling
- Clojure Applied, From Practice to Practitioner, Ben Vandgrift and Alex Miller
- Vampir, Joann Sfar
- Los mares del Sur, Manuel Vázquez Montalbán
- El océano al final del camino (The Ocean at the End of the Lane), Neil Gaiman
- La Guerra Civil Española, Paul Preston and José Pablo García
- Wiser: Getting Beyond Groupthink to Make Groups Smarter, Cass R. Sunstein and Reid Hastie

September
- And The Weak Suffer What They Must?: Europe, Austerity and the Threat to Global Stability, Yanis Varoufakis
- Clojure Reactive Programming, Leonardo Borges
- Designing Object-Oriented Software 1st ed., Rebecca Wirfs-Brock, Brian Wilkerson and Lauren Wiener
- Divina Comedia (Divina Commedia), Dante Alighieri

October
- David Copperfield, Charles Dickens
- Utopía (Utopia), Thomas More
- La pulga de acero (Сказ о тульском косом Левше и о стальной блохе), Nikolái Leskov
- Living Clojure, An Introduction and Training Plan for Developers, Carin Meier
- Buddha in Blue Jeans: An Extremely Short Zen Guide to Sitting Quietly and Being Buddha, Tai Sheridan

November
- De tu tierra (Paesi tuoi), El Camarada (Il Compagno), Cesare Pavese
- A Passage to India, E. M. Forster
- React: Up & Running. Building Web Applications, Stoyan Stefanov
- El relato de Arthur Gordon Pym (The Narrative of Arthur Gordon Pym of Nantucket),Edgar Allan Poe.

December
- The Remains of the Day, Kazuo Ishiguro
- Quiet: The Power of Introverts in a World That Can't Stop Talking, Susan Cain
- Robinson Crusoe, Daniel Defoe
- All you Zombies, Robert A. Heinlein

TDD Training at Gradiant (in spanish)

La semana pasada Luis Rovirosa y yo impartimos una formación de TDD en las oficinas de Gradiant en Vigo.

Gradiant es el acrónimo en inglés del Centro Tecnolóxico de Telecomunicacións de Galicia (Galician Research and Development Center in Advanced Telecomunications) y utilizan los conocimientos y experiencia de sus profesionales de investigación para aportar valor a empresas y organizaciones mediante la transferencia de conocimiento y el diseño de productos especializados.

La naturaleza del centro y su ubicación en el campus de la Universidad de Vigo me recordó bastante a la época en que trabajé en el BSC.

Este fue el segundo curso de TDD de Codesai que imparto con Luis, y era todo un reto, debido al número de asistentes, 18 personas, y, sobretodo, a la gran variedad de tecnologías y tipos de sistemas que desarrollan, que iban desde C empotrado para procesado de señales o IOT, hasta APIs REST con node.js, pasando por diversas aplicaciones en Java, C++ o Scala.

Creo que hacer los cursos en pareja es una gran ventaja. Por un lado, porque podemos atender mucho mejor, sobre todo si el grupo es numeroso, a las parejas durante las sesiones prácticas dándoles feedback y contestando sus dudas, para ayudar a que asimilen mejor los conceptos que persigue trabajar cada kata. Por otro lado, porque nos permite asumir roles diferentes, parecidos a los de conductor y navegador en pair-programming, para percibir mejor el pulso de nuestra audiencia y modular mejor nuestro mensaje. En este sentido, Luis y yo, nos complementamos bastante bien porque, al tener backgrounds bastante distintos y personalidades diferentes, podemos enriquecer tanto las explicaciones teóricas como las prácticas, con un rango de experiencias y perpectivas más amplio, lo que considero que fue muy útil, dada la diversidad del grupo.

Algunos de los desarrolladores que asistieron al curso trabajaban en sistemas empotrados haciendo IOT o procesado de señales en los que cuentan con recursos muy limitados y en entornos en los que testear es bastante complicado. Para tratar de ayudarlos, aparte de responder dudas concretas y de las conversaciones de pasillo, reunimos una compilación de recursos interesantes para hacer testing y TDD con C empotrado.

Nos gustó muchísimo la experiencia de trabajar con las personas que asistieron al curso. Era un grupo muy agradable y con muchísimas ganas de aprender. Además, durante el curso, surgieron preguntas y debates muy interesantes debido a la gran diversidad de tipos de proyecto desarrollados por los asistentes.

Hemos acabado reventados después de dos días intensos... Pero también motivadísimos #tdd #craftmanship https://t.co/eXtEBVssmZ

— Javi (@jsrois) December 20, 2016

Mega cursazo! Gracias @trikitrok @luisrovirosa y gracias @Gradiant por traerlos! A ver si vuelven 😉https://t.co/pMuI7aZl2E

— Fernando Vilas (@fer_vilas) December 20, 2016

Como siempre acabamos cansadísimos pero muy contentos porque creo que conseguimos nuestro objetivo de iniciar al grupo en testing y TDD, y de ilusionarlos para empezar a aventurarse en una forma de trabajo diferente.

Antes de terminar quería darle las gracias especialmente a Javi Sánchez Rois y a Fernando Vilas por acogernos, llevarnos a sitios ricos para comer y presentarnos al Dinoseto 😉.

Publicado originalmente en el blog de Codesai.

An example of introducing symmetry to enable duplication removal

Symmetry is a subtle concept that may seem only related to code aesthetics. However, as Kent Beck states in Implementation Patterns,

"...finding and expressing symmetry is a preliminary step to removing duplication. If a similar thought exists in several places in the code, making them symmetrical to each other is a first good step towards unifying them"

In this post we'll look at an example of expressing symmetry as a way to make duplication more visible. This is the initial code of a version of a subset of the Mars Rover kata that Álvaro García and I used in a refactoring workshop some time ago:

	public class Rover {
	private String direction;
	private int y;
	private int x;
	public Rover(int x, int y, String direction) {
	this.direction = direction;
	this.y = y;
	this.x = x;
	}
	public void receive(String commandsSequence) {
	for (int i = 0; i < commandsSequence.length(); ++i) {
	String command = commandsSequence.substring(i, i + 1);
	if (command.equals("l") || command.equals("r")) {
	// Rotate Rover
	if (direction.equals("N")) {
	if (command.equals("r")) {
	direction = "E";
	} else {
	direction = "W";
	}
	} else if (direction.equals("S")) {
	if (command.equals("r")) {
	direction = "W";
	} else {
	direction = "E";
	}
	} else if (direction.equals("W")) {
	if (command.equals("r")) {
	direction = "N";
	} else {
	direction = "S";
	}
	} else {
	if (command.equals("r")) {
	direction = "S";
	} else {
	direction = "N";
	}
	}
	} else {
	// Displace Rover
	int displacement1 = -1;
	if (command.equals("f")) {
	displacement1 = 1;
	}
	int displacement = displacement1;
	if (direction.equals("N")) {
	y += displacement;
	} else if (direction.equals("S")) {
	y -= displacement;
	} else if (direction.equals("W")) {
	x -= displacement;
	} else {
	x += displacement;
	}
	}
	}
	}
	}

view raw initial_mars_rover_code.java hosted with ❤ by GitHub

This code is using conditionals to express two consecutive decisions:

1. Which command to execute depending on a command code.
2. How to execute the command depending on the direction the rover faces.

These decisions are repeated for different responsibilities, displacing the rover and rotating the rover, so the code presents a Case Statements smell. Since those decisions are completely independent, we could mechanically refactor the code to start using polymorphism instead of conditionals. This way we'll end having two consecutive single dispatches, one for each decision.

If we start applying Replace Type Code with State/Strategy refactoring to the code as it is now, there is a subtle problem, though.

Looking carefully more carefully, we can observe that in the case of the rover's displacement, the sequence of two decisions is clearly there:

However, that sequence is not there in the case of the rover's rotations. In this case there's a third decision (conditional) based on the command type which as, we'll see, is not necessary:

This difference between the two sequences of decisions reveals a lack of symmetry in the code. It's important to remove it before starting to refactor the code to substitute the conditionals with polymorphism.

I've seen many cases in which developers naively start extracting methods and substituting the conditionals with polymorphism starting from asymmetrical code like this one. This often leads them to create entangled and leaking abstractions.

Particularly, in this case, blindly applying Replace Type Code with State/Strategy refactoring to the left and right rotations can very likely lead to a solution containing code like the following:

	public enum Direction {
	private final String value;
	Direction(String value) {
	this.value = value;
	}
	public static Direction from(String value) {
	for (Direction current : values()) {
	if (current.value.equals(value)) {
	return current;
	}
	}
	throw new RuntimeException("Passed value is not correct: " + value);
	}
	public Direction rotate(Command command) {
	if (Command.RIGHT == command) {
	return rotateRight();
	} else {
	return rotateLeft();
	}
	}
	protected abstract Direction rotateRight();
	protected abstract Direction rotateLeft();
	NORTH("N") {
	@Override
	protected Direction rotateRight() {
	return EAST;
	}
	@Override
	protected Direction rotateLeft() {
	return WEST;
	}
	},
	SOUTH("S") {
	@Override
	protected Direction rotateRight() {
	return (WEST);
	}
	@Override
	protected Direction rotateLeft() {
	return (EAST);
	}
	},
	WEST("W") {
	@Override
	protected Direction rotateRight() {
	return (NORTH);
	}
	@Override
	protected Direction rotateLeft() {
	return (SOUTH);
	}
	},
	EAST("E") {
	@Override
	protected Direction rotateRight() {
	return (SOUTH);
	}
	@Override
	protected Direction rotateLeft() {
	return (NORTH);
	}
	};
	}

view raw direction_knowing_about_commands.java hosted with ❤ by GitHub

	public enum Command {
	RIGHT("r"),
	LEFT("l");
	private String value;
	Command(String value) {
	this.value = value;
	}
	public static Command from(String value) {
	for (Command current : values()) {
	if (current.value.equals(value)) {
	return current;
	}
	}
	throw new RuntimeException("Passed value is not correct: " + value);
	}
	}

view raw command_leaking_encoding_knowledge.java hosted with ❤ by GitHub

Notice how the Direction class contains a decision based on the encoding of the command. To be able to take that decision, Direction needs to know about the encoding, which is why it's been made public in the Command class.

This is bad... Some knowledge about the commands encoding has leaked from the Command class into the Direction class. Direction shouldn't be taking that decision in the first place. Neither should it know how commands are encoded. Moreover, this is a decision that was already taken and doesn't need to be taken again.

How can we avoid this trap?

We should go back to the initial code and instead of mechanically applying refactoring, we should start by removing the asymmetry from the initial code.

You can see one way of doing it in this video:



After this refactoring all cases are symmetrical (they present the same sequence of decisions)

We've not only removed the asymmetry but we've also made more explicit the case statements (Case Statements smell) on the command encoding and the duplicated switches on the direction the rover is facing:

	public void receive(String commandsSequence) {
	for (int i = 0; i < commandsSequence.length(); ++i) {
	String command = commandsSequence.substring(i, i + 1);
	if (command.equals("l")) {
	// Rotate Rover to the left
	if (direction.equals("N")) {
	direction = "W";
	} else if (direction.equals("S")) {
	direction = "E";
	} else if (direction.equals("W")) {
	direction = "S";
	} else {
	direction = "N";
	}
	} else if (command.equals("r")) {
	// Rotate Rover to the right
	if (direction.equals("N")) {
	direction = "E";
	} else if (direction.equals("S")) {
	direction = "W";
	} else if (direction.equals("W")) {
	direction = "N";
	} else {
	direction = "S";
	}
	} else {
	// Displace Rover
	int displacement1 = -1;
	if (command.equals("f")) {
	displacement1 = 1;
	}
	int displacement = displacement1;
	if (direction.equals("N")) {
	y += displacement;
	} else if (direction.equals("S")) {
	y -= displacement;
	} else if (direction.equals("W")) {
	x -= displacement;
	} else {
	x += displacement;
	}
	}
	}
	}

view raw mars_rover_symmetric.java hosted with ❤ by GitHub

Now it's clear that the third decision (third nested conditional) in the original rover's rotations code was unnecessary.

If we start the Replace Type Code with State/Strategy refactoring now, it's more likely that we'll end with a code in which Direction knows nothing about how the commands are encoded:

	public enum Direction {
	NORTH {
	@Override
	public Coordinates move(Coordinates coordinates, int displacement) {
	return coordinates.incrementY(displacement);
	}
	@Override
	public Direction rotateRight() {
	return EAST;
	}
	@Override
	public Direction rotateLeft() {
	return WEST;
	}
	}, SOUTH {
	@Override
	public Coordinates move(Coordinates coordinates, int displacement) {
	return coordinates.incrementY(-displacement);
	}
	@Override
	public Direction rotateRight() {
	return WEST;
	}
	@Override
	public Direction rotateLeft() {
	return EAST;
	}
	}, EAST {
	@Override
	public Coordinates move(Coordinates coordinates, int displacement) {
	return coordinates.incrementX(displacement);
	}
	@Override
	public Direction rotateRight() {
	return SOUTH;
	}
	@Override
	public Direction rotateLeft() {
	return NORTH;
	}
	}, WEST {
	@Override
	public Coordinates move(Coordinates coordinates, int displacement) {
	return coordinates.incrementX(-displacement);
	}
	@Override
	public Direction rotateRight() {
	return NORTH;
	}
	public Direction rotateLeft() {
	return SOUTH;
	}
	};
	public static Direction pointingTo(String directionCode) {
	if (directionCode.equals("N")) {
	return NORTH;
	} else if (directionCode.equals("S")) {
	return SOUTH;
	} else if (directionCode.equals("E")) {
	return EAST;
	} else {
	return WEST;
	}
	}
	abstract public Coordinates move(Coordinates coordinates, int displacement);
	abstract public Direction rotateRight();
	abstract public Direction rotateLeft();
	}

view raw direction.java hosted with ❤ by GitHub

and the encoding of each command is known in only one place:

	public class CommandCodeInterpreter {
	private static final int DISPLACEMENT_LENGTH = 1;
	private static final String FORWARD_MOVEMENT = "f";
	private static final String BACKWARD_MOVEMENT = "b";
	private static final String RIGHT_ROTATION = "r";
	private static final String LEFT_ROTATION = "l";
	private static Map<String, Command> knownCommands = knownCommands();
	public static Command interpret(String commandCode) {
	return getCommandFor(commandCode);
	}
	private static Command getCommandFor(String commandCode) {
	return knownCommands.get(commandCode);
	}
	private static Map<String, Command> knownCommands() {
	Map<String, Command> knownCommands = new HashMap<>();
	knownCommands.put(FORWARD_MOVEMENT, new Movement(DISPLACEMENT_LENGTH));
	knownCommands.put(BACKWARD_MOVEMENT, new Movement(-DISPLACEMENT_LENGTH));
	knownCommands.put(RIGHT_ROTATION, new RightRotation());
	knownCommands.put(LEFT_ROTATION, new LeftRotation());
	return Collections.unmodifiableMap(knownCommands);
	}
	}

view raw command_factory.java hosted with ❤ by GitHub

As we said at the beginning, symmetry is a very useful concept that can help you guide refactoring. Detecting asymmetries and thinking why they happen, can help you to detect hidden duplication and, as in this case, sometimes entangled dimensions of complexity ^[1].

Then, by removing those asymmetries, you can make the duplication more visible and disentangle the entangled dimensions of complexity. The only thing you need is to be patient and don't start "obvious" refactorings before thinking a bit about symmetry.

^[1] Dimension of Complexity is a term used by Mateu Adsuara in a talk at SocraCan16 to name an orthogonal functionality. In that talk he used dimensions of complexity to group the examples in his test list and help him choose the next test when doing TDD. He talked about it in this three posts: Complexity dimensions - FizzBuzz part I, Complexity dimensions - FizzBuzz part II and Complexity dimensions - FizzBuzz part III. Other names for the same concept that I've heard are axes of change, directions of change or vectors of change.

Recorded talk about functions in Clojure (in Spanish)

Last Wednesday we did our third remote talk about Clojure as part of our small Clojure/ClojureScript study group.

This time we talked a bit about functions and we recorded it as well.

This is the resulting video:

You'll find the examples we used here.

Again I'd like to especially thank Ángel and Magento Barcelona for letting me do the talk from their office.

I hope you find it useful.

Interesting Talk: "GOOS style TDD by Example"

I've just watched this interesting talk by Sagy Rozman

• GOOS style TDD by Example

Interesting Talk: "Neurobollocks: el nuevo aceite de serpiente"

I've just watched this interesting talk by Marta Ferrero and Pablo Garaizar

• Neurobollocks: el nuevo aceite de serpiente

TDD Training at Ve Interactive (in spanish)

Hace unas semanas Luis Rovirosa y yo impartimos una formación de TDD en las oficinas de Ve Interactive en Bilbao.

Luis lleva bastante tiempo impartiendo el curso de TDD de Codesai, tanto en solitario como junto con Carlos Blé, pero para mi era la privera vez. Me gustó mucho la estructura y el ritmo que le da Luis al curso.

Se trata un curso muy intenso y eminentemente práctico que dura dos días, cada uno de los cuales se divide en cuatro bloques de dos horas. Al comienzo de cada bloque presentamos una serie de conceptos teóricos, que comentamos y debatimos con el grupo, y rápidamente pasamos a ponerlos en práctica mediante una serie de katas de complejidad y dificultad creciente, en las que acompañamos continuamente a las parejas dándoles feedback y contestando sus dudas, para que los conceptos acaben de ser asimilados.

Fue todo un placer trabajar con los 14 desarrolladores que asistieron al curso. Era un grupo con una actitud genial y con un nivel muy bueno. A pesar de lo intenso del curso, trabajaron con mucho entusiasmo cada uno de los ejercicios y se produjeron debates muy interesantes y constructivos.

Nos fuimos de Bilbao exhaustos, pero muy contentos, tanto por las personas que conocimos como por el feedback que recibimos. Personalmente, me encanto trabajar con Luis y aprendí muchísimo de él.

Publicado originalmente en el blog de Codesai.

Interesting Talk: "Overcoming the Challenges of Mentoring"

I've just watched this great talk by Kim Crayton

• Overcoming the Challenges of Mentoring

Interesting Talk: "Code Symbiosis, a technical keynote"

I've just watched this wonderful talk by Michael Feathers

• Code Symbiosis, a technical keynote

Recorded talk about destructuring in Clojure (in Spanish)

I recently started to work at Codesai.

Some members of the team wanted to learn Clojure, so we started a small Clojure/ClojureScript study group.

We created a slack called clojuresai where I'm posting some readings (we're reading Clojure for the Brave and True) and exercises (we're working through a selection of exercises from 4Clojure) each week and where they can ask doubts about the exercises and readings, and comment things they find interesting.

Some colleagues from the Clojure Developers Barcelona meetup that are beginning to learn clojure have also joined the study group.

Now and then, we do introductory talks using Google Hangouts (Codesai's team is distributed) to go deeper in some of the topics they've just read in the book.

So far we've done two talks. The first one was about Clojure collections. Unfortunately, we didn't record it, but you can find the examples we used here.

Today we did a second remote talk. This time it was about destructuring in Clojure, and since some of the members of the study group members weren't able to make it, we decided to record it.

This is the resulting video:

You'll find the examples we used in this previous post.

I'd like to especially thank Ángel and Magento Barcelona for letting me do the talk today from their office (their internet connection is much better than mine).

Recently, Miguel de la Cruz (thanks a million Miguel!!) has joined our study group as a tutor. He knows a lot of Clojure so he will help answering doubts and giving some talks that we'll hopefully share with you soon, as well.

Interesting Talk: "Testing on the Toilet"

I've just watched this very interesting talk by Alfredo Casado and Laura Morillo

• Testing on the Toilet

Interesting Talk: "3X"

I've just watched this wonderful talk by Kent Beck

• 3X

Interesting Talk: "Productivity is Killing Us"

I've just watched this wonderful talk by Adam Yuret

• “Productivity” is Killing Us

Interesting Talk: " Punishment Driven Development"

I've just watched this very interesting talk by Louise Elliott

• Punishment Driven Development

Creating custom effects in re-frame

In previous posts, we learned how to use coeffects and effects in re-frame to have effectful event handlers that are pure functions.

The example with effects we saw, used two of re-frame's built-in effect handlers (dispatch and dispatch-later). Since the set of possible side-effects is open-ended, re-frame gives you a way to define your own effects.

In this post, we'll show how to create your own effect by defining one that writes to the local storage.

Let's have a look at an event handler using that effect that writes to the local storage:

	(ns visual-spelling.play.answer.handlers
	(:require
	[visual-spelling.db :as db]))
	(defn word-checked-handler [{:keys [db]} _]
	(let [db (db/update-on-word-checked db)]
	{:db db
	:write-localstore {:visited-words (:visited-words db)
	:results (:results db)}}))

view raw handlers_using_effect.cljs hosted with ❤ by GitHub

Notice the a key-value pair having the key :write-localstore in the effects map returned by the event handler. This key-value pair is telling re-frame to do a side-effect, which is uniquely identified by that key and requires the data in the corresponding value. But, how does re-frame know how to action on the :write-localstore effect?

We have to use re-frame's reg-fx function to associate the effect key, (:write-localstore), with an effect handler, (local-store/write function):

	(ns visual-spelling.register-effects
	(:require
	[re-frame.core :as re-frame]
	[visual-spelling.local-store :as local-store]))
	(re-frame/reg-fx
	:write-localstore local-store/write)

view raw register_effects.cljs hosted with ❤ by GitHub

reg-fx receives two arguments: the key that identifies the effect and the function which actions the side-effect, the effect handler.

When an event handler returns an effects map which contains a given effect key, the effect handler associated with it using reg-fx will be called to action on the effect, passing it the value associated to the effect key in the effects map.

The local-store/write function is passed a map containing the key-value pairs it has to write to the local store.

	(ns visual-spelling.local-store)
	(defn- write-key
	[ls-key data]
	(.setItem js/localStorage ls-key (str data)))
	(defn- load-key [ls-key]
	(some->> (.getItem js/localStorage ls-key)
	(cljs.reader/read-string)))
	(defn write [data-kv]
	(doseq [[k v] data-kv]
	(write-key k v)))
	(defn load [ls-keys]
	(reduce
	#(assoc %1 %2 (load-key %2))
	{}
	ls-keys))

view raw local_store.cljs hosted with ❤ by GitHub

Finally, this is how we'd test the event handler using it:

	(ns visual-spelling.play.answer.word-checked-handler-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[visual-spelling.test-helpers.builders
	:refer [make-word make-raw-word raw-parts parts]]
	[visual-spelling.play.answer.handlers :refer [word-checked-handler]]
	[visual-spelling.test-helpers.db-builder
	:refer [make-db]]
	[visual-spelling.test-helpers.handlers-checkers
	:refer [check-writing-to-local-storage-contains
	check-writing-to-local-storage]]))
	(deftest test-right-and-wrong-word-handlers
	(let [answer-timestamp :some-ts
	correct-word-text "ess"
	initial-current-word-index 1
	db (make-db
	:language :catalan
	:current-word (make-word
	:status :all-right-answers
	:correct-word correct-word-text
	:answer-ts answer-timestamp
	:parts (parts "e" "s"
	{:correct-answer "s"
	:user-answer "s"}))
	:current-word-index initial-current-word-index
	:words {"first-word" (make-raw-word)
	"current-word" (make-raw-word
	:description "current-word"
	:parts (raw-parts
	"e"
	{:correct-answer "ss"
	:options ["c", "ç", "k", "s", "ss", "z", "q"]}))
	"next-word" (make-raw-word
	:description "next-word"
	:parts (raw-parts
	"e"
	{:correct-answer "ss"
	:options ["c", "ç", "k", "s", "ss", "z", "q"]}))})
	co-fx {:db db}
	resulting-fx (word-checked-handler co-fx :no-event)]
	(testing "it saves the visited words in the local storage"
	(check-writing-to-local-storage-contains
	resulting-fx :visited-words correct-word-text))
	(testing "that right word handler saves the user answer to results"
	(check-writing-to-local-storage resulting-fx :results))))

view raw word_checked_handler_test.cljs hosted with ❤ by GitHub

being check-writing-to-local-storage-contains and check-writing-to-local-storage:

	;; some more code
	;;.....
	(defn- extract-from-writing-to-local-storage
	[fx saved-thing-key]
	(-> fx :write-localstore saved-thing-key))
	(defn check-writing-to-local-storage
	[fx saved-thing-key]
	(is (= (extract-from-writing-to-local-storage fx saved-thing-key)
	(extract-from-db-in-fx-or-cofx fx saved-thing-key))))
	(defn check-writing-to-local-storage-contains
	[fx saved-thing-key expected-content]
	(is (contains? (extract-from-writing-to-local-storage
	fx saved-thing-key)
	expected-content)))
	;;.....
	;; some more code

view raw some_checkers.cljs hosted with ❤ by GitHub

And that's all we need to know in order to create a custom effect in re-frame.

Interesting Talk: "No, Really, Learning Clojure Was Hard"

I've just watched this great talk by Steve Shogren

• No, Really, Learning Clojure Was Hard

Interesting Talk: "Time to grow up"

I've just watched this great talk by Jorge Barroso

• Time to grow up

Interesting Talk: "Arquitecturas funcionales: más allá de las lambdas"

I've just watched this wonderful talk by Juan Manuel Serrano Hidalgo

• Arquitecturas funcionales: más allá de las lambdas

Interesting Podcast: "Cognicast with Paul Stadig"

I've just listened to this great Cognicast podcast with Paul Stadig talking about about Polymorphism and other interesting things:

• Cognicast with Paul Stadig

Quotes from "Why should physicists study history?"

My top highlights from the wonderful paper Why should physicists study history? by Matthew Stanley:

"History is not a list of names and dates. It's a way of thinking that can be powerful and illuminating."

"Social interactions really do influence what scientist produce."

"How a community tells its history changes the way it thinks about itself."

"A historical perspective on science can help physicists understand what is going on when they practice their craft, and it provides numerous tools that are useful for physicists themselves."

"Physics is a social endeavor."

"Research is done by people. And people have likes and dislikes, egos and prejudices. Physicists, like everyone else get attached to their favorite ideas and hang on them perhaps long after they should let them go."

"The history of science can help dismantle the myth of the purely rational genius living outside the everyday world. It makes physics more human."

"And a more human physics is a good thing. For starters, it makes physics more accessible."

"A field in which people are aknowledged as people is much more appealing than one in which they are just calculating machines."

"Physics only work when people talk to each other and communication is not always easy."

"Everything seems obvious in retrospect."

"The history of physics can remind us how difficult is to justify ideas that now seem obvious."

"Complexity, not simplicity , has ruled the practice of science."

"Every discovery has come out of a messy mix of people, ideas, accidents and arguments."

"Students and young researchers are often heartened to learn that physics is hard work and that it is ok for their own efforts not to look like a text-boo presentation. Messiness is a standard. Mistakes are normal. The results of physics are not self-evident."

"The history of physics suggests that there are usually several ways to approach a problem."

"Turning complexity into good physics requires creativity. You can never tell what weird idea will help clarify a confusing observation or provide the key to interpreting an equation. History uncovers the strange stew of concepts that were necessary for the development of physics."

"The interplay of various approaches is what brought us a modern view."

"Strange but ultimately useful perspectives come from fields and disciplines apparently distant from the problem at hand."

"The history of science shows how important it is for scientists across different fields to talk to each other. Conversations among separate groups are healthy. Apparently isolated problems are often closedly tied together, and you never know where you will find the weird idea that solves your difficulty."

"The best strategy for encouraging diverse ideas is to cultivate a diverse community."

"Underrepresented groups that offer different ways of thingking are often the source of fresh insights and novel methods."

"Underrepresented groups are usually marginalized because of cultural inertia or deliberate decisions made long ago."

"The diversity of ideas and interpretations serves as a reminder that physics is a work in progress. Knowledge is provisional. There are always new ways to tackle a problem, and there is always more to be learned."

"Accepting uncertainty would require changes in how science is taught."

"Curiosity should be revered, and everyone should be encouraged to ask, what else?"

"If scientist are not explicit and honest about their doubts, a crisis of confidence arises when that uncertainty is revealed."

"Physics wasn't always as it is."

"The flip side of accepting physics will be different in the future is accepting that it was different in the past. Everyone has a tendency to assume that the way things are now is the norm. But history makes it clear that things were not always this way. An understanding of why people used to think differently is a powerful tool for understanding people today. By drawing attention to older, usnpoken assumptions, history shows us how to start paying attention to our own."

"A knowledge of the historic and physical background, gives the kind of independence from prejudices of his generation from which most scientists are suffering" Einstein

"..they should study the history of those ideas and understand the circumstances in which they were justified and found useful."

"History trains you to think critically about received ideas. History provides evidence of roads not taken."

"Science's plurality of interpretation can make the history of science a resource for modern scientific research." Hasok Chang

"Complementary Science -> recovering forgotten and unsolved puzzles from the past."

"Putting complementary science into practice demands difficult self-examination. Thinking deeply about assumptions and accepted knowledge can be hard to do in professional scientific contexts, but history is a mode in which it's encouraged."

"The simple realization that people used to think differently can be quite powerful."

"Physics doesn't have rigid rules."

"Scientist simply don't follow a rigid, linear problem-solving system. Sometimes they start with a hypothesis, sometimes with a strange observation, sometimes with a weird anomaly in an otherwise straightforward experiment."

"... a scientist must be an "unscrupulous opportunist", adopting and adapting various approaches as new challenges arise."

"History teaches that knowledge is not fixed."

"Engaging with history will teach you to understand ideas on their own terms."

"Historical thinking makes its subject dynamic. It helps you think about science as a series of questions rather than a series of statements. Those questions will continue into the future, and it is helpful to know what has been asked so far." "In the end, history of science exposes scientists to new ways of thinking and forces them to reexamine what is already known. Such intellectual flexibility is essential for any discipline, but it's particularly important for fields as influential and authoritative as physics and other sciences."

Interesting Talk: "So Long, and Thanks for All the Tests"

I've just watched this great talk by Seb Rose

• So Long, and Thanks for All the Tests

Using effects in re-frame

In re-frame, we'd like to use pure event handlers because they provide some important advantages, (mentioned in a previous post about coeffects in re-frame): local reasoning, easier testing, and events replay-ability.

However, as we said, to build a program that does anything useful, it's inevitable to have some side-effects and/or side-causes. So, there will be many cases in which event handlers won't be pure functions.

We also saw how using coeffects in re-frame allows to have pure event handlers in the presence of side-causes.

In this post, we'll focus on side-effects:

If a function modifies some state or has an observable interaction with calling functions or the outside world, it no longer behaves as a mathematical (pure) function, and then it is said that it does side-effects.

Let's see some examples of event handlers that do side-effects (from a code animating the evolution of a cellular automaton):

	(ns cellular-animation.handlers
	(:require
	[cellular-animation.evolution :as evolution]
	[re-frame.core :as re-frame]))
	(defn evolve [db _]
	(if (:evolving db)
	(let [db (update
	db :automaton-states
	(partial
	evolution/evolve (:rule db)))]
	(js/setTimeout
	(fn [] (re-frame/dispatch [:evolve]))
	100) ; <- side-effect!
	db)
	db))
	(defn start-stop-evolution [db _]
	(let [db (update db :evolving not)]
	(re-frame/dispatch [:evolve]) ; <- side-effect!
	db))

view raw impure_untestable_handlers.cljs hosted with ❤ by GitHub

	(ns cellular-animation.register-handlers
	(:require
	[re-frame.core :as re-frame]
	[cellular-animation.db :as db]
	[cellular-animation.handlers :as handlers]))
	(re-frame/reg-event-db
	:initialize-db
	(fn [_ _]
	db/default-db))
	(re-frame/reg-event-db
	:evolution-started-or-stopped
	handlers/start-stop-evolution)
	(re-frame/reg-event-db
	:evolve
	handlers/evolve)

view raw register_impure_untestable_handlers.cljs hosted with ❤ by GitHub

These event handlers, registered using reg-event-db, are impure because they're doing a side-effect when they dispatch the :evolve event. With this dispatch, they are performing an action at a distance, which is in a way a hidden output of the function.

These impure event handlers are hard to test. In order to test them, we'll have to somehow spy the calls to the function that is doing the side-effect (the dispatch). Like in the case of side-causes from our previous post, there are many ways to do this in ClojureScript, (see Isolating external dependencies in Clojure), only that, in this case, the code required to test the impure handler will be a bit more complex, because we need to keep track of every call made to the side-effecting function.

In this example, we chose to make explicit the dependency that the event handler has on the side-effecting function, and inject it into the event handler which becomes a higher order function. Actually, we injected a wrapper of the side-effecting function in order to create an easier interface.

Notice how the event handlers, evolve and start-stop-evolution, now receive as its first parameter the function that does the side-effect, which are dispatch-later-fn and dispatch, respectively.

	(ns cellular-animation.handlers
	(:require
	[cellular-animation.evolution :as evolution]))
	(defn evolve [dispatch-later-fn db _]
	(if (:evolving db)
	(let [db (update
	db :automaton-states
	(partial evolution/evolve (:rule db)))]
	(dispatch-later-fn :evolve 100)
	db)
	db))
	(defn start-stop-evolution [dispatch-fn db _]
	(let [db (update db :evolving not)]
	(dispatch-fn :evolve)
	db))

view raw testable-impure-handlers.cljs hosted with ❤ by GitHub

When the event handlers are registered with the events they handle, we partially apply them, in order to pass them their corresponding side-effecting functions, dispatch-later for evolve and dispatch for start-stop-evolution:

	(ns cellular-animation.register-handlers
	(:require
	[re-frame.core :as re-frame]
	[cellular-animation.db :as db]
	[cellular-animation.handlers :as handlers]
	[cellular-animation.dispatchers :as dispatchers]))
	(re-frame/reg-event-db
	:initialize-db
	(fn [_ _]
	db/default-db))
	(re-frame/reg-event-db
	:evolution-started-or-stopped
	(partial
	handlers/start-stop-evolution
	dispatchers/dispatch))
	(re-frame/reg-event-db
	:evolve
	(partial
	handlers/evolve
	dispatchers/dispatch-later))

view raw register-testable-impure-handlers.cljs hosted with ❤ by GitHub

These are the wrapping functions:

	(ns cellular-animation.dispatchers
	(:require
	[re-frame.core :as re-frame]))
	(defn dispatch [event]
	(re-frame/dispatch [event]))
	(defn dispatch-later [event ms]
	(js/setTimeout
	(fn [] (dispatch event))
	ms))

view raw dispatchers.cljs hosted with ❤ by GitHub

Now when we need to test the event handlers, we use partial application again to inject the function that does the side-effect, except that, in this case, the injected functions are test doubles, concretely spies which record the parameters used each time they are called:

	(ns cellular-animation.handlers-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[cellular-animation.handlers :as handlers]
	[cellular-animation.rules :as rules]))
	(defn- make-spy [args]
	(fn [& parameters] (swap! args conj parameters)))
	(deftest evolve-handler-test
	(testing
	"when the automaton is evolving it calls itself after changing its state"
	(let [initial-states [[1]]
	expected-states [[0 1 0] [1 0 1]]
	db {:automaton-states initial-states
	:rule rules/rule-90
	:evolving true}
	args (atom [])
	dispatch-later-fn (make-spy args)
	evolve (partial
	handlers/evolve dispatch-later-fn)
	resulting-db (evolve db :not-used-event)]
	(is (= resulting-db
	(merge db {:automaton-states expected-states})))
	(let [first-call-args (nth @args 0)]
	(is (= (count first-call-args) 2))
	(is (= (first first-call-args) :evolve))
	(is (= (second first-call-args) 100))))))
	(testing
	"when the automaton is not evolving it does not change its state"
	(let [db :some-db
	args (atom [])
	dispatch-later-fn (make-spy args)
	evolve (partial
	handlers/evolve dispatch-later-fn)
	resulting-db (evolve db :not-used-event)]
	(is (= resulting-db db))
	(is (zero? (count @args)))))
	(deftest start-stop-evolution-handler-test
	(testing
	"when the automaton is evolving it stops the evolution"
	(let [db {:evolving true}
	args (atom [])
	dispatch (make-spy args)
	start-stop-evolution (partial
	handlers/start-stop-evolution dispatch)
	resulting-db (start-stop-evolution db :not-used-event)]
	(is (= resulting-db (assoc db :evolving false)))
	(let [first-call-args (nth @args 0)]
	(is (= (count first-call-args) 1))
	(is (= (first first-call-args) :evolve)))))
	(testing
	"when the automaton is not evolving it starts the evolution"
	(let [db {:evolving false}
	args (atom [])
	dispatch (make-spy args)
	start-stop-evolution (partial
	handlers/start-stop-evolution dispatch)
	resulting-db (start-stop-evolution db :not-used-event)]
	(is (= resulting-db (assoc db :evolving true)))
	(let [first-call-args (nth @args 0)]
	(is (= (count first-call-args) 1))
	(is (= (first first-call-args) :evolve))))))

view raw testable-impure-handlers-test.cljs hosted with ❤ by GitHub

This is very similar to what we had to do to test event handlers with side-causes in re-frame before having effectful event handlers (see previous post). However, the code for spies is a bit more complex than the one for stubs.

Using test doubles makes the event handler testable again, but it's still impure, so we have not only introduced more complexity to test it, but also, we have lost the two other advantages cited before: local reasoning and events replay-ability.

Since re-frame's 0.8.0 (2016.08.19) release, this problem has been solved by introducing the concept of effects and coeffects.

Whereas, in our previous post, we saw how coeffects can be used to track what your program requires from the world (side-causes), in this post, we'll focus on how effects can represent what your program does to the world (side-effects). Using effects, we'll be able to write effectful event handlers that keep being pure functions.

Let's see how the previous event handlers look when we use effects:

	(ns cellular-animation.handlers
	(:require
	[cellular-animation.evolution :as evolution]))
	(defn evolve [{:keys [db]} _]
	(if (:evolving db)
	{:db (update db :automaton-states
	(partial evolution/evolve (:rule db)))
	:dispatch-later [{:ms 100 :dispatch [:evolve]}]}
	{:db db}))
	(defn start-stop-evolution [{:keys [db]} _]
	{:db (update db :evolving not)
	:dispatch [:evolve]})

view raw pure-effectful-handlers.cljs hosted with ❤ by GitHub

Notice how the event handlers are not side-effecting anymore. Instead, each of the event handlers returns a map of effects which contains several key-value pairs. Each of these key-value pairs declaratively describes an effect using data. re-frame will use that description to actually do the described effects. The resulting event handlers are pure functions which return descriptions of the side-effects required.

In this particular case, when the automaton is evolving, the evolve event handler is returning a map of effects which contains two effects represented as key/value pairs. The one with the :db key describes the effect of resetting the application state to a new value. The other one, with the :dispatch-later key describes the effect of dispatching the :evolve event after waiting 100 microseconds. On the other hand, when the automaton is not evolving, the returned effect describes that the application state will be reset to its current value.

Something similar happens with the start-stop-evolution event handler. It returns a map of effects also containing two effects. The one with the :db key describes the effect of resetting the application state to a new value, whereas the one with the :dispatch key describes the effect of immediately dispatching the :evolve event.

The effectful event handlers are pure functions that accept two arguments, being the first one a map of coeffects, and return, after doing some computation, an effects map which is a description of all the side-effects that need to be done by re-frame.

As we saw in the previous post about coeffectts, re-frame's effectful event handlers are registered using the reg-event-fx function:

	(ns cellular-animation.register-handlers
	(:require
	[re-frame.core :as re-frame]
	[cellular-animation.db :as db]
	[cellular-animation.handlers :as handlers]))
	(re-frame/reg-event-db
	:initialize-db
	(fn [_ _]
	db/default-db))
	(re-frame/reg-event-fx
	:evolution-started-or-stopped
	handlers/start-stop-evolution)
	(re-frame/reg-event-fx
	:evolve
	handlers/evolve)

view raw register-pure-effectfull-handlers.cljs hosted with ❤ by GitHub

These are their tests:

	(ns cellular-animation.handlers-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[cellular-animation.handlers :as handlers]
	[cellular-animation.rules :as rules]))
	(deftest evolve-handler-test
	(testing
	"when the automaton is evolving it calls itself after changing its state"
	(let [initial-states [[1]]
	expected-states [[0 1 0] [1 0 1]]
	db {:automaton-states initial-states
	:rule rules/rule-90
	:evolving true}]
	(is (= (handlers/evolve {:db db} [])
	{:db (merge db {:automaton-states expected-states})
	:dispatch-later [{:ms 100 :dispatch [:evolve]}]}))))
	(testing
	"when the automaton is not evolving it does not change its state"
	(let [db {:automaton-states :not-used-initial-states
	:rule :not-used-rule
	:evolving false}]
	(is (= (handlers/evolve {:db db} []) {:db db})))))
	(deftest start-stop-evolution-test
	(testing
	"when the automaton is evolving it stops the evolution"
	(let [db {:evolving true}]
	(is (= (handlers/start-stop-evolution {:db db} [])
	{:db (assoc db :evolving false)
	:dispatch [:evolve]}))))
	(testing
	"when the automaton is not evolving it starts the evolution"
	(let [db {:evolving false}]
	(is (= (handlers/start-stop-evolution {:db db} [])
	{:db (assoc db :evolving true)
	:dispatch [:evolve]})))))

view raw testing-pure-effectfull-handlers.cljs hosted with ❤ by GitHub

Notice how by using effects we don't need to use tests doubles anymore in order to test the event handlers. These event handlers are pure functions, so, besides easier testing, we get back the advantages of local reasoning and events replay-ability.

:dispatch and :dispatch-later are builtin re-frame effect handlers already defined. It's possible to create your own effect handlers. We'll explain how and show an example in a future post.

Interesting Paper: "Designing software for ease of extension and contraction"

I've just read this great paper by David L.Parnas

• Designing software for ease of extension and contraction

Interesting Talk: "A brief history and mishistory of modularity"

I've just watched this interesting talk by Ashley Williams

• A brief history and mishistory of modularity

Codesai

I'm very happy to announce that I've joined Codesai's team.

Many reasons made my path converge with Codesai's one.

First of all, the people in Codesai.
I met Carlos in 2011, because he was teaching the first TDD course I attended. Carlos is a humble and honest person that tries very hard to make sure his actions match what he believes. Later I've known him as a quiet, open-minded leader who is always willing to listen what other people have to say, and this permeates the whole Codesai team. During the last two years, I had the opportunity to meet most of Codesai's apprentices. They are very enthusiastic and eager to learn. I had great fun participating with them in some Software Craftsmanship Gran Canaria events and talking about code and life in general in Codesai's apartment.

I share Codesai's values and feel very confortable with their culture and communication style because the team creates a healthy and safe environment which respects the diversity of its members and the debate of ideas.

I met the rest of the team last week during Codesai's summit (aka #flejesai16) and it was a confirmation that I'm in the right place.

Second, the opportunity to learn from experienced XP practicioners.
I've been developing software for 6 years now. I've learned a lot, most of it by reading books, practising on my own, attending courses and from colleagues, both from the community and at work. However, I felt the need of having a mentor, because I think I'll learn more and better than if I keep working on my own. I've found some great mentors in Codesai who acknowledge that I need a safe environment to get experience step by step.

Finally, the last reason is more personal.
I live in Barcelona but I'm from Canary Islands. Codesai is a remote-first team but the core of Codesai's team is currently in my hometown in the Canary Islands. So being a part of Codesai also gives me the chance of visiting my family more often.

So far I'm enjoying the Codesai experience a lot.

I think we'll keep on evolving as a team and do great things.

Thanks a million for letting me in!

Interesting Paper: "Design Methodology for Reliable Software Systems"

I've just read this very interesting paper by Barbara Liskov

• Design Methodology for Reliable Software Systems

Using coeffects in re-frame

Event handlers, collectively, provide the control logic in a re-frame application. Since the mutation of the application state is taken care of by re-frame, these event handlers can be pure functions that given the current value of the application state as first argument and the event (with its payload) as second argument, provide a new value for the application state.

This is important because the fact that event handlers are pure functions brings great advantages:

• Local reasoning, which decreases the cognitive load required to understand the code.
• Easier testing, because pure functions are much easier to test.
• Events replay-ability, you can imagine a re-frame application as a reduce (left fold) that proceeds step by step. Following this mental model, at any point in time, the value of the application state would be the result of performing a reduce over the entire collection of events dispatched in the application up until that time, being the combining function for this reduce the set of registered event handlers.

However, to build a program that does anything useful, it's inevitable to have some side-effects and/or side-causes. So, there would be cases in which event handlers won't be pure functions.

In this post, we'll focus on side-causes.

"Side-causes are data that a function, when called, needs but aren't in its argument list. They are hidden or implicit inputs."

Let's see an example:

	(ns visual-spelling.play.answer.handlers
	(:require
	[re-frame.core :as re-frame]
	[visual-spelling.db :as db]
	[visual-spelling.words-in-use :as words-in-use]))
	(re-frame/reg-event-db
	:word-ready-to-check
	(fn [db _]
	(update
	(db/save-user-answer
	(js/Date.now) ;; <- side-cause!!
	db)
	:words-in-use
	words-in-use/check-current-word)))

view raw impure-handler-due-to-side-cause.cljs hosted with ❤ by GitHub

The event handler for the :word-ready-to-check event, that we are registering using reg-event-db, is not pure because it's using the value returned by JavaScript's Date.now function instead of getting it through its arguments. To make matters worse, in this particular case, this side-cause also makes the event handler untestable because JavaScript's Date.now function returns a different value each time it's called.

To be able to test this event handler we'll have to somehow stub the function that produces the side-cause. In ClojureScript, there are many ways to do this (using with-redefs, with-bindings, a test doubles library like CircleCI's bond, injecting and stubbing the dependency, etc.).

In this example, we chose to make the dependency that the handler has on a function that returns the current timestamp explicit and inject it into the event handler which becomes a higher order function.

	(ns visual-spelling.play.answer.handlers
	(:require
	[visual-spelling.db :as db]
	[visual-spelling.words-in-use :as words-in-use]))
	(defn word-ready-to-check-handler [time-fn db _]
	(update (db/save-user-answer (time-fn) db)
	:words-in-use
	words-in-use/check-current-word))

view raw handler-time-fn-injected.cljs hosted with ❤ by GitHub

In the previous code example, notice that the event handler now receives as its first parameter a function that returns the current timestamp, time-fn.

And this would be how before registering that event handler with reg-event-db, we perform a partial application to inject JavaScript's Date.now function into it:

	(ns visual-spelling.play.answer.register-handlers
	(:require
	[re-frame.core :as re-frame]
	[visual-spelling.play.answer.handlers :as handlers]))
	(re-frame/reg-event-db
	:word-ready-to-check
	(partial handlers/word-ready-to-check-handler js/Date.now))

view raw registering-handler-time-fn-injected.cljs hosted with ❤ by GitHub

Using the same technique, we can now inject a stub of the time-fn function into the event handler in order to test it:

	(deftest test-word-ready-to-check-handler
	(testing "when answer is correct, saves current word and updates status"
	(let [time-fn (stub-time-fn :any-timestamp)
	right-answered-word (make-word
	:correct-word "o"
	:status :all-gaps-answered
	:parts (parts {:correct-answer "o"
	:options ["a" "o" "e" "u"]
	:user-answer "o"}))
	db (build-db :current-word right-answered-word)
	resulting-db (answer-handlers/word-ready-to-check-handler
	time-fn db :no-event)]
	(check-status
	resulting-db :all-right-answers)
	(check-results-contains
	resulting-db (results/correct "o" :any-timestamp)))))

view raw testing-handler-stubbed-time-fn.cljs hosted with ❤ by GitHub

This is the code of the home-made factory to create stubs for time-fn that we used in the previous test:

	(ns visual-spelling.tests-helpers)
	(defn stub-time-fn [& timestamps]
	(let [a-ts (atom timestamps)]
	(fn []
	(let [ts (first @a-ts)]
	(swap! a-ts rest)
	ts))))

view raw stub-time-fn.cljs hosted with ❤ by GitHub

We've seen how using a test double makes the event handler testable again, but at the price of introducing more complexity.

The bottom line problem is that the event handler is not a pure function. This makes us lose not only the easiness of testing, as we've seen, but also the rest of advantages cited before: local reasoning and events replay-ability. It would be great to have a way to keep event handlers pure, in the presence of side-effects and/or side-causes.

Since re-frame's 0.8.0 (2016.08.19) release, this problem has been solved by introducing the concept of effects and coeffects. Effects represent what your program does to the world (side-effects) while coeffects track what your program requires from the world (side-causes). Now we can write effectful event handlers that keep being pure functions.

Let's see how to use coeffects in the previous event handler example. As we said, coeffects track side-causes, (see for a more formal definition Coeffects The next big programming challenge).

At the beginning, we had this impure event handler:

	(ns visual-spelling.play.answer.handlers
	(:require
	[re-frame.core :as re-frame]
	[visual-spelling.db :as db]
	[visual-spelling.words-in-use :as words-in-use]))
	(re-frame/reg-event-db
	:word-ready-to-check
	(fn [db _]
	(update
	(db/save-user-answer
	(js/Date.now) ;; <- side-cause!!
	db)
	:words-in-use
	words-in-use/check-current-word)))

view raw impure-handler-due-to-side-cause.cljs hosted with ❤ by GitHub

then we wrote a testable but still impure version:

	(ns visual-spelling.play.answer.handlers
	(:require
	[visual-spelling.db :as db]
	[visual-spelling.words-in-use :as words-in-use]))
	(defn word-ready-to-check-handler [time-fn db _]
	(update (db/save-user-answer (time-fn) db)
	:words-in-use
	words-in-use/check-current-word))

view raw handler-time-fn-injected.cljs hosted with ❤ by GitHub

Thanks to coeffects, we can eliminate the side-cause, passing the timestamp input through the argument list of the event handler. The resulting event handler is a pure function:

	(ns visual-spelling.play.answer.handlers
	(:require
	[visual-spelling.db :as db]
	[visual-spelling.words-in-use :as words-in-use]))
	(defn word-ready-to-check-handler [cofx _]
	(let [db (update (db/save-user-answer (:timestamp cofx) (:db cofx))
	:words-in-use
	words-in-use/check-current-word)]
	{:db db}))

view raw handler-using-timestamp-coeffect-no-destructuring.cljs hosted with ❤ by GitHub

This event handler receives, as its first parameter, a map of coeffects, cofx, which contains two coeffects, represented as key/value pairs. One of them is the current timestamp which is associated to the :timestamp key, and the other one is the application state associated to the :db key. The second argument is, as in previous examples, the event with its payload.

The map of coeffects, cofx, is the complete set of inputs required by the event handler to perform its computation. Notice how the application state is just another coeffect.

This is the same event handler but using destructuring (which is how I usually write them):

	(ns visual-spelling.play.answer.handlers
	(:require
	[visual-spelling.db :as db]
	[visual-spelling.words-in-use :as words-in-use]))
	(defn word-ready-to-check-handler [{:keys [db timestamp]} _]
	(let [db (update (db/save-user-answer timestamp db)
	:words-in-use
	words-in-use/check-current-word)]
	{:db db}))

view raw handler-using-timestamp-coeffect.cljs hosted with ❤ by GitHub

How does this work? How does the coeffects map get passed to the event handler?

We need to do two things previously:

First, we register the event handler using re-frame's reg-event-fx instead of reg-event-db.

When you use reg-event-db to associate an event id with the function that handles it, its event handler, that event handler gets as its first argument, the application state, db.

While event handlers registered via reg-event-fx also get two arguments, the first argument is a map of coeffects, cofx, instead of the application state. The application state is still passed in the cofx map as a coeffect associated to the :db key, it's just another coeffect. This is how the previous pure event handler gets registered:

	(ns visual-spelling.play.answer.register-handlers
	(:require
	[re-frame.core :as re-frame]
	[visual-spelling.play.answer.handlers :as handlers]))
	(re-frame/reg-event-fx
	:word-ready-to-check
	[(re-frame/inject-cofx :timestamp)] ;; <- interceptor injects coeffect
	handlers/word-ready-to-check-handler)

view raw reg-event-using-coeffect.cljs hosted with ❤ by GitHub

Notice the second parameter passed to reg-event-fx. This is an optional parameter which is a vector of interceptors. Interceptors are functions that wrap event handlers implementing middleware by assembling functions, as data, in a collection. They "can look after cross-cutting concerns helping to "factor out commonality, hide complexity and introduce further steps into the 'Derived Data, Flowing' story promoted by re-frame".

In this example, we are passing an interceptor created using re-frame's inject-cofx function which returns an interceptor that will load a key/value pair (coeffect id/coeffect value) into the coeffects map just before the event handler is executed.

Second, we factor out the coeffect handler, and then register it using re-frame's reg-cofx. This function associates a coeffect id with the function that injects the corresponding key/value pair into the coeffects map. This function is known as the coeffect handler. For this example, we have:

	(ns visual-spelling.play.answer.register-handlers
	(:require
	[re-frame.core :as re-frame]))
	(re-frame/reg-cofx
	:timestamp
	(fn [cofx _]
	(assoc cofx :timestamp (js/Date.now))))

view raw timestamp-coeffect.cljs hosted with ❤ by GitHub

Since the event handler is now a pure function, it becomes very easy to test it:

	(deftest test-word-ready-to-check-handler
	(testing "when answer is correct, saves current word and updates status"
	(let [right-answered-word (make-word
	:correct-word "o"
	:status :all-gaps-answered
	:parts (parts {:correct-answer "o"
	:options ["a" "o" "e" "u"]
	:user-answer "o"}))
	cofx {:db (build-db :current-word right-answered-word)
	:timestamp :any-timestamp}
	resulting-fx (answer-handlers/word-ready-to-check-handler
	cofx :no-event)]
	(check-status
	resulting-fx :all-right-answers)
	(check-results-contains
	resulting-fx (results/correct "o" :any-timestamp)))))

view raw test-word-ready-to-check-handler.cljs hosted with ❤ by GitHub

Notice how we don't need to use tests doubles anymore in order to test it. Thanks to the use of coeffects, the event handler is a pure function, and we can just pass any timestamp to it in its arguments.

More importantly, we've also regained the advantages of local reasoning and events replay-ability that comes with having pure event handlers.

In future posts, we'll see how we can do something similar using effects to keep events handlers pure in the presence of side-effects.

Interesting Talk: "ClojureScript Made Easy"

I've just watched this very interesting talk by Jonathan Boston

• ClojureScript Made Easy

Interesting Talk: "Leaving side effects aside"

I've just watched this wonderful talk by Juanma Serrano

• Leaving side effects aside

These are the slides.

Refactoring tests using builder functions in Clojure/ClojureScript

Using literals in your tests can have some advantages, such as, readability and traceability.

While this is true when the data are simple, it's less so when the data are nested, complex structures. In that case, using literals can hinder refactoring and thus become an obstacle to adapting to changes.

The problem with using literals for complex, nested data is that the knowledge about how to build such data is spread all over the tests. There are many tests that know about the representation of the data.

In that scenario, nearly any change in the representation of those data will have a big impact on the tests code because it will force us to change many tests.

This is an example of a test using literals, (from a ClojureScript application using re-frame, I'm developing with Francesc), to prepare the application state (usually called db in re-frame):

	(deftest test-right-and-wrong-word-handlers
	(let [co-fx {:db
	{:language :catalan
	:words-in-use {:current-word {:correct-word "ess"
	:answer-ts :any-ts
	:parts [(text/make "e")
	(tests-helpers/answered-gap "ss" [] 1 "ss")]}
	:current-word-index 1
	:visited-words {}
	:words {"first-word" {}
	"current-word" {:description "current-word"
	:parts ["e"
	{:correct-answer "ss"
	:options ["c", "ç", "k", "s", "ss", "z", "q"]}]}
	"next-word" {:description "next-word"
	:parts ["e"
	{:correct-answer "ss"
	:options ["c", "ç", "k", "s", "ss", "z", "q"]}]}}
	:words-keys ["first-word" "current-word" "next-word"]}}}]
	(testing "that right word handler selects the next word"
	(check-current-word-is-new
	(answer-handlers/right-word-handler co-fx :no-event)
	:description "next-word" :current-word-index 2))
	(testing "that wrong word handler repeats the same word"
	(check-current-word-is-new
	(answer-handlers/wrong-word-handler co-fx :no-event)
	:description "current-word" :current-word-index 1))))

view raw using-literals.cljs hosted with ❤ by GitHub

As you can see, this test knows to much about the structure of db.

There were many other tests doing something similar at some nesting level of the db.

To make things worse, at that moment, we were still learning a lot about the domain, so the structure of the db was suffering changes with every new thing we learned.

The situation was starting to be painful, since any refactoring provoke many changes in the tests, so we decided to fix it.

What we wanted was a way to place all the knowledge about the representation of the db in just one place (i.e., remove duplication), so that, in case we needed to change that representation, the impact of the change would be absorbed by changing only one place.

A nice way of achieving this goal in object-oriented code, and at the same time making your tests code more readable, is by using test data builders which use the builder pattern, but how can we do these builders in Clojure?

Option maps or function with keyword arguments are a nice alternative to traditional builders in dynamic languages such as Ruby or Python.

In Clojure we can compose functions with keyword arguments to get very readable builders that also hide the representation of the data.

We did TDD to write these builder functions. These are the tests for one of them, the db builder:

	(ns visual-spelling.test-helpers.db-builder-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[visual-spelling.test-helpers.db-helpers :as db-helpers]
	[visual-spelling.test-helpers.db-builder :refer [make-db]]
	[visual-spelling.db :as db]))
	(deftest test-db-builder
	(is (= (make-db) db/default-db))
	(is (= (make-db :results :some-results)
	(assoc db/default-db :results :some-results)))
	(is (= (make-db :language :some-language)
	(assoc db/default-db :language :some-language)))
	(let [db (make-db :words {:some-word {} :another-word {}})]
	(is (= (db-helpers/extract-from-db db :words)
	{:some-word {} :another-word {}}))
	(is (= (set (db-helpers/extract-from-db db :words-keys))
	(set [:some-word :another-word]))))
	(is (= (db-helpers/extract-from-db
	(make-db :current-word-index :some-index) :current-word-index)
	:some-index))
	(is (= (db-helpers/extract-from-db
	(make-db :current-word :some-word) :current-word)
	:some-word))
	(is (= (make-db :words-in-use :some-words-in-use)
	(assoc db/default-db :words-in-use :some-words-in-use)))
	(is (= (make-db :words-per-session :some-words-per-session)
	(assoc db/default-db :words-per-session :some-words-per-session)))
	(is (= (make-db :visited-words :some-:visited-words)
	(assoc db/default-db :visited-words :some-:visited-words))))

view raw db_builder_test.cljs hosted with ❤ by GitHub

and this is the db builder code:

	(ns visual-spelling.test-helpers.db-builder
	(:require
	[visual-spelling.db :as db]))
	(defn- property-used? [v]
	(not= v ::not-used))
	(defn- assoc-if-used [m k v]
	(if (property-used? v)
	(assoc m k v)
	m))
	(defn make-words-in-use
	[default-word-in-use words-in-use words current-word-index current-word]
	(let [words-keys (if (property-used? words) (keys words) ::not-used)]
	(if (property-used? words-in-use)
	words-in-use
	(-> default-word-in-use
	(assoc-if-used :words-keys words-keys)
	(assoc-if-used :words words)
	(assoc-if-used :current-word-index current-word-index)
	(assoc-if-used :current-word current-word)))))
	(defn make-db
	[& {:keys [language words current-word-index current-word
	visited-words results words-in-use words-per-session]
	:or {language ::not-used
	words ::not-used
	current-word-index ::not-used
	current-word ::not-used
	visited-words ::not-used
	results ::not-used
	words-in-use ::not-used
	words-per-session ::not-used}}]
	(let [words-in-use (make-words-in-use
	(:words-in-use db/default-db)
	words-in-use
	words current-word-index current-word)]
	(-> db/default-db
	(assoc :words-in-use words-in-use)
	(assoc-if-used :language language)
	(assoc-if-used :results results)
	(assoc-if-used :words-per-session words-per-session)
	(assoc-if-used :visited-words visited-words))))

view raw db_builder.cljs hosted with ❤ by GitHub

which is using associative destructuring on the function's optional arguments to have a function with keyword arguments, and creating proper default values using the :or keyword (have a look at the material of a talk about destructuring I did some time ago for Clojure Barcelona Developers community).

After creating builder functions for some other data used in the project, our test started to read better and to be robust against changes in the structure of db.

For instance, this is the code of the test I showed at the beginning of the post, but now using builder functions instead of literals:

	(deftest test-right-and-wrong-word-handlers
	(let [db (make-db
	:language :catalan
	:current-word (make-word
	:correct-word "ess"
	:answer-ts :any-ts
	:parts (parts "e"
	{:correct-answer "ss"
	:user-answer "ss"}))
	:current-word-index 1
	:words {"first-word" (make-raw-word)
	"current-word" (make-raw-word
	:description "current-word"
	:parts (raw-parts
	"e"
	{:correct-answer "ss"
	:options ["c", "ç", "k", "s", "ss", "z", "q"]}))
	"next-word" (make-raw-word
	:description "next-word"
	:parts (raw-parts
	"e"
	{:correct-answer "ss"
	:options ["c", "ç", "k", "s", "ss", "z", "q"]}))})
	co-fx {:db db}]
	(testing "that right word handler selects the next word"
	(check-current-word-is-new
	(right-word-handler co-fx :no-event) :description "next-word" :current-word-index 2))
	(testing "that wrong word handler repeats the same word"
	(check-current-word-is-new
	(wrong-word-handler co-fx :no-event) :description "current-word" :current-word-index 1))))

view raw using-builders.cljs hosted with ❤ by GitHub

which not only hides the representation of db but also eliminates details that are not used in particular tests, while still being as easy to read, if not easier, than the version using literals.

We have seen how, by composing builder functions and using them in our tests, we managed to reduce the surface of the impact that changes in the representation of data might have on our tests. Builder functions absorb the impact of those changes, and enable faster refactoring, and, by doing so, enable adapting to changes faster.

Interesting Talk: "Dependency management in Clojure"

I've just watched this great talk by Holger Schauer

• Dependency management in Clojure

Interesting Talk: "Growing Clojure systems. From functions to microservices"

I've just watched this very interesting talk by Marcin Bilski

• Growing Clojure systems. From functions to microservices

Interesting Interview: "Objects, Functions, Virtual Machines, IDEs and More"

I've just watched this very interesting interview with Dave Thomas and Erik Meijer

• Objects, Functions, Virtual Machines, IDEs and More

Interesting Talk: "Native mobile apps with ClojureScript"

I've just watched this very interesting talk by Viktor Eriksson

• Native mobile apps with ClojureScript

Kata: Variation on Cellular Automata, animating automaton evolution using re-frame

In the last Clojure Developers Barcelona meetup, we started to use re-frame to animate in the browser the code to evolve cellular automata that we wrote some weeks ago.

We managed to make a rough version but we didn't have time to test it and make it nice.

When I got home I redid the exercise from scratch using a mix of TDD and REPL Driven Development.

First, I coded a couple of evolution rules. These are their tests:

	(ns cellular-animation.rules-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[cellular-animation.rules :as rules]))
	(deftest rules-tests
	(testing "rule 90"
	;+-----------------------------------------------------------------+
	;| Neighborhood | 111 | 110 | 101 | 100 | 011 | 010 | 001 | 000 |
	;+-----------------------------------------------------------------+
	;| New Center Cell | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
	;+-----------------------------------------------------------------+
	(is (= (rules/rule-90 [1 1 1]) 0))
	(is (= (rules/rule-90 [1 1 0]) 1))
	(is (= (rules/rule-90 [1 0 1]) 0))
	(is (= (rules/rule-90 [1 0 0]) 1))
	(is (= (rules/rule-90 [0 1 1]) 1))
	(is (= (rules/rule-90 [0 1 0]) 0))
	(is (= (rules/rule-90 [0 0 1]) 1))
	(is (= (rules/rule-90 [0 0 0]) 0)))
	(testing "rule 30"
	;+-----------------------------------------------------------------+
	;| Neighborhood | 111 | 110 | 101 | 100 | 011 | 010 | 001 | 000 |
	;+-----------------------------------------------------------------+
	;| New Center Cell | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0 |
	;+-----------------------------------------------------------------+
	(is (= (rules/rule-30 [1 1 1]) 0))
	(is (= (rules/rule-30 [1 1 0]) 0))
	(is (= (rules/rule-30 [1 0 1]) 0))
	(is (= (rules/rule-30 [1 0 0]) 1))
	(is (= (rules/rule-30 [0 1 1]) 1))
	(is (= (rules/rule-30 [0 1 0]) 1))
	(is (= (rules/rule-30 [0 0 1]) 1))
	(is (= (rules/rule-30 [0 0 0]) 0))))

view raw cellular-animation.rules_test.cljs hosted with ❤ by GitHub

and this is their code:

	(ns cellular-animation.rules)
	(def rule-90
	{[1 1 1] 0
	[1 1 0] 1
	[1 0 1] 0
	[1 0 0] 1
	[0 1 1] 1
	[0 1 0] 0
	[0 0 1] 1
	[0 0 0] 0})
	(def rule-30
	{[1 1 1] 0
	[1 1 0] 0
	[1 0 1] 0
	[1 0 0] 1
	[0 1 1] 1
	[0 1 0] 1
	[0 0 1] 1
	[0 0 0] 0})

view raw cellular-animation.rules.cljs hosted with ❤ by GitHub

Next, I coded the cellular automata evolution, these are its tests:

	(ns cellular-animation.evolution-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[cellular-animation.evolution :as evolution]
	[cellular-animation.rules :as rules]))
	(deftest cellular-automaton-evolution
	(testing "evolution from an initial state"
	(is (= (evolution/evolve rules/rule-90 [[1]])
	[[0 1 0]
	[1 0 1]]))
	(is (= (evolution/evolve rules/rule-90 [[0 1 0] [1 0 1]])
	[[0 0 1 0 0]
	[0 1 0 1 0]
	[1 0 0 0 1]]))))

view raw cellular-animation.evolution_test.cljs hosted with ❤ by GitHub

and the resulting code:

	(ns cellular-animation.evolution)
	(defn- extract-neighborhoods [state]
	(partition 3 1 (repeat 0) (cons 0 state)))
	(defn- evolve-once [rule state]
	(mapv rule (extract-neighborhoods state)))
	(defn- pad-with-zeroes [state]
	(vec (cons 0 (conj state 0))))
	(defn- next-state [rule state]
	(->> state
	pad-with-zeroes
	(evolve-once rule)))
	(defn evolve [rule states]
	(->> (last states)
	(next-state rule)
	(conj (mapv pad-with-zeroes states))
	vec))

view raw cellular-animation.evolution.cljs hosted with ❤ by GitHub

Once I had the evolution working, I started to work on the view, using Figwheel and a hard-coded list of cellular automaton states on the db. This way I focused only on rendering the list of cellular automaton states.

With that working, I played on the REPL to create a subscriber that reacted to changes on the cellular automaton states making the view render.

This is the resulting code of the view:

	(ns cellular-animation.views
	(:require
	[re-frame.core :as re-frame]))
	(def ^:private cell-representations
	{0 "\u00A0" 1 "*"})
	(defn- render-state [state-index cell-index cell-state]
	(with-meta
	[:span.cell (cell-representations cell-state)]
	{:key (str "cell-state-" state-index "-cell-" cell-index)}))
	(defn- render-states [state-index state]
	(with-meta
	[:div
	(map-indexed (partial render-state state-index) state)]
	{:key (str "state-" state-index)}))
	(defn- automaton-component []
	(let [automaton-states (re-frame/subscribe [:automaton-states])]
	(fn []
	[:div
	{:on-click #(re-frame/dispatch [:evolution-started-or-stopped])}
	(map-indexed render-states @automaton-states)])))
	(defn main-panel []
	[automaton-component])

view raw cellular-animation.views.cljs hosted with ❤ by GitHub

the code of the subscriber:

	(ns cellular-animation.subs
	(:require-macros
	[reagent.ratom :refer [reaction]])
	(:require
	[re-frame.core :as re-frame]))
	(re-frame/reg-sub
	:automaton-states
	(fn [db]
	(:automaton-states db)))

view raw cellular-animation.subs.cljs hosted with ❤ by GitHub

and the default db:

	(ns cellular-animation.db
	(:require
	[cellular-animation.rules :as rules]))
	(def default-db
	{:automaton-states [[1]]
	:rule rules/rule-90
	:evolving false})

view raw cellular-animation.db.cljs hosted with ❤ by GitHub

Then, I used TDD to write the event handlers.

The use of effects keeps re-frame handlers pure. They allow us to avoid making side effects. We just have to describe as data the computation that will be made instead of doing it. re-frame takes care of that part.

These are the handlers tests:

	(ns cellular-animation.handlers-test
	(:require
	[cljs.test :refer-macros [deftest testing is]]
	[cellular-animation.handlers :as handlers]
	[cellular-animation.rules :as rules]))
	(deftest evolve-handler-test
	(testing "when the automaton is evolving it calls itself after changing its state"
	(let [initial-states [[1]]
	expected-states [[0 1 0] [1 0 1]]
	db {:automaton-states initial-states :rule rules/rule-90 :evolving true}]
	(is (= (handlers/evolve-handler {:db db} [])
	{:db (merge db {:automaton-states expected-states})
	:dispatch-later [{:ms 100 :dispatch [:evolve]}]}))))
	(testing "when the automaton is not evolving it does not change its state"
	(let [db {:automaton-states :not-used-initial-states :rule :not-used-rule :evolving false}]
	(is (= (handlers/evolve-handler {:db db} []) {:db db})))))
	(deftest start-stop-evolution-test
	(testing "when the automaton is evolving it stops the evolution"
	(let [db {:evolving true}]
	(is (= (handlers/start-stop-evolution {:db db} [])
	{:db (assoc db :evolving false)
	:dispatch [:evolve]}))))
	(testing "when the automaton is not evolving it starts the evolution"
	(let [db {:evolving false}]
	(is (= (handlers/start-stop-evolution {:db db} [])
	{:db (assoc db :evolving true)
	:dispatch [:evolve]})))))

view raw cellular-animation.handlers_test.cljs hosted with ❤ by GitHub

these are the handlers:

	(ns cellular-animation.handlers
	(:require
	[cellular-animation.evolution :as evolution]))
	(defn evolve-handler [{:keys [db]} _]
	(if (:evolving db)
	{:db (update db :automaton-states
	(partial evolution/evolve (:rule db)))
	:dispatch-later [{:ms 100 :dispatch [:evolve]}]}
	{:db db}))
	(defn start-stop-evolution [{:keys [db]} _]
	{:db (update db :evolving not)
	:dispatch [:evolve]})

view raw cellular-animation.handlers.cljs hosted with ❤ by GitHub

Notice, how in order to dispatch to another handler (a side effect) we just have to add a key-value pair to the map of effects returned by the handler. In this case, I used the :dispatch and :dispatch-later effects which are part of re-frame's built-in effects. You can also create and register your own effects (I will talk about it in some future post).

To see the whole picture this is the code that registers the handlers:

	(ns cellular-animation.register-handlers
	(:require
	[re-frame.core :as re-frame]
	[cellular-animation.db :as db]
	[cellular-animation.handlers :as handlers]))
	(re-frame/reg-event-db
	:initialize-db
	(fn [_ _]
	db/default-db))
	(re-frame/reg-event-fx
	:evolution-started-or-stopped handlers/start-stop-evolution)
	(re-frame/reg-event-fx
	:evolve handlers/evolve-handler)

view raw cellular-animation.register_handlers.cljs hosted with ❤ by GitHub

I think that effects and coeffects (I'll talk about them in a future post) are just great!

They make testing handlers logic very easy (since handlers keep being pure functions) and avoid having to use test doubles.

Finally, everything is put together in the core namespace:

	(ns cellular-animation.core
	(:require
	[reagent.core :as reagent]
	[re-frame.core :as re-frame]
	[devtools.core :as devtools]
	[cellular-animation.register-handlers]
	[cellular-animation.subs]
	[cellular-animation.views :as views]
	[cellular-animation.config :as config]))
	(defn dev-setup []
	(when config/debug?
	(enable-console-print!)
	(println "dev mode")
	(devtools/install!)))
	(defn mount-root []
	(reagent/render [views/main-panel]
	(.getElementById js/document "app")))
	(defn ^:export init []
	(re-frame/dispatch-sync [:initialize-db])
	(dev-setup)
	(mount-root))

view raw cellular-animation.core.cljs hosted with ❤ by GitHub

You can see the cellular automaton evolving in this video (I used garden library to add some CSS to make it look nicer):

You can find the code on this GitHub repository.

In this post I've tried to, somehow, describe the process I followed to write this code, but the previous gists just reflect the final version of the code. If you want to follow how the code evolved, have a look to all the commits on Github.

I really love using ClojureScript and re-frame with Figwheel.

Interesting Talk: "The Clockwork Gardener: Growing an Elm App With Templates"

I've just watched this interesting talk by Jessica Kerr

• The Clockwork Gardener: Growing an Elm App With Templates

Kata: Variation on Lights Out, a bit of FRP using reagi library

To finish with the series of exercises we've been doing lately around the Lights Out kata, I decided to use reagi library to raise the level of abstraction of the communications between the Lights and the LightsGateway components.

reagi is an FRP library for Clojure and ClojureScript which is built on top of core.async. I discovered it while reading Leonardo Borges' wonderful Clojure Reactive Programming book.

I started from the code of the version using the component library I posted recently about and introduced reagi. Let's see the code.

Let's start with the lights-gateway name space:

	(ns kata-lights-out.lights-gateway
	(:require
	[cljs-http.client :as http]
	[com.stuartsierra.component :as component]
	[reagi.core :as reagi]
	[cljs.core.async :as async])
	(:require-macros
	[cljs.core.async.macros :refer [go]]))
	(defn- extract-lights [response]
	(->> response
	:body
	(.parse js/JSON)
	.-lights
	js->clj))
	(defn- post [lights-stream uri params]
	(go
	(when-let [response (async/<!
	(http/post uri
	{:with-credentials? false
	:form-params params}))]
	(reagi/deliver lights-stream
	(extract-lights response)))))
	(defprotocol LightsGateway
	(reset-lights! [this m n])
	(flip-light! [this pos]))
	(defrecord ApiLightsGateway [config]
	component/Lifecycle
	(start [this]
	(println ";; Starting ApiLightsGateway component")
	(assoc this :lights-stream (reagi/events)))
	(stop [this]
	(println ";; Stopping ApiLightsGateway component")
	(reagi/dispose (:lights-stream this))
	this)
	LightsGateway
	(reset-lights! [this m n]
	(post (:lights-stream this)
	(:reset-lights-url config)
	{:m m :n n}))
	(flip-light! [this [x y]]
	(post (:lights-stream this)
	(:flip-light-url config)
	{:x x :y y})))
	(defn make-api-gateway [config]
	(->ApiLightsGateway config))

view raw reactive-kata-lights-out-lights_gateway.cljs hosted with ❤ by GitHub

The main change here is that the ApiLightsGateway component keeps a lights-stream which it's initialized in its start function and disposed of in its stop function using reagi.

I also use, reagi's deliver function to feed the lights-stream with the response that is taken from the channel that cljs-http returns when we make a post request to the server.

Next,the lights name space:

	(ns kata-lights-out.lights
	(:require
	[reagent.core :as r]
	[com.stuartsierra.component :as component]
	[kata-lights-out.lights-gateway :as lights-gateway]
	[reagi.core :as reagi]))
	(def ^:private light-off 0)
	(defn light-off? [light]
	(= light light-off))
	(defn- listen-to-lights-updates! [{:keys [lights-stream lights]}]
	(->> lights-stream
	(reagi/map #(reset! lights %))))
	(defprotocol LightsOperations
	(reset-lights! [this])
	(flip-light! [this pos]))
	(defrecord Lights [lights-gateway m n]
	component/Lifecycle
	(start [this]
	(println ";; Starting lights component")
	(let [this (assoc this
	:lights-stream (:lights-stream lights-gateway)
	:lights (r/atom []))
	this (assoc this :lights-gateway lights-gateway)]
	(listen-to-lights-updates! this)
	(reset-lights! this)
	this))
	(stop [this]
	(println ";; Stopping lights component")
	this)
	LightsOperations
	(reset-lights! [this]
	(lights-gateway/reset-lights! (:lights-gateway this) m n))
	(flip-light! [this pos]
	(lights-gateway/flip-light! (:lights-gateway this) pos)))
	(defn all-lights-off? [lights]
	(every? light-off? (flatten lights)))
	(defn make-lights [m n]
	(map->Lights {:m m :n n}))

view raw reactive-kata-lights-out-lights.cljs hosted with ❤ by GitHub

Notice how the dependency on core.async disappears and the code to update the lights atom is now a subscription to the lights-stream (look inside the listen-to-lights-updates! function). This new code is much easier to read and is at a higher level of abstraction than the one using core.async in previous versions of the exercise.

Now the lights-view name space:

	(ns kata-lights-out.lights-view
	(:require
	[reagent.core :as r]
	[kata-lights-out.lights :as lights]
	[reagi.core :as reagi]
	[com.stuartsierra.component :as component]))
	(defn- all-lights-off-message-content [config lights]
	(if (lights/all-lights-off? lights)
	(:success-message config)
	{:style {:display :none}}))
	(defn- all-lights-off-message-component [config lights]
	[:div#all-off-msg
	(all-lights-off-message-content config lights)])
	(defn- render-light [{:keys [light-on light-off]} light]
	(if (lights/light-off? light)
	light-off
	light-on))
	(defn- light-component [config clicked-light-positions i j light]
	^{:key (+ i j)}
	[:button
	{:on-click #(reagi/deliver clicked-light-positions [i j])}
	(render-light config light)])
	(defn- row-lights-component [config clicked-light-positions i row-lights]
	^{:key i}
	[:div (map-indexed (partial light-component config clicked-light-positions i) row-lights)])
	(defn- home-page [config clicked-light-positions lights-component]
	(fn []
	(let [lights (:lights lights-component)]
	[:div [:h2 (:title config)]
	(map-indexed (partial row-lights-component config clicked-light-positions) @lights)
	[all-lights-off-message-component config @lights]])))
	(defn- flip-light-when-clicked [lights-component clicked-light-positions]
	(->> clicked-light-positions
	(reagi/map #(lights/flip-light! lights-component %))))
	(defprotocol View
	(mount [this]))
	(defrecord LightsView [lights-component config]
	component/Lifecycle
	(start [this]
	(println ";; Starting LightsOutView component")
	(let [this (assoc this :clicked-light-positions (reagi/events))]
	(mount this)
	this))
	(stop [this]
	(println ";; Stopping LightsOutView component")
	(reagi/dispose (:clicked-light-positions this))
	this)
	View
	(mount [{:keys [clicked-light-positions]}]
	(flip-light-when-clicked
	lights-component clicked-light-positions)
	(r/render
	[home-page config clicked-light-positions lights-component]
	(.getElementById js/document "app"))))
	(defn make [config]
	(map->LightsView {:config config}))

view raw reactive-kata-lights-out-lights_view.cljs hosted with ❤ by GitHub

Here I also used reagi to create a stream of clicked-light-positions. Again the use of FRP makes the handling of clicks much simpler (in previous versions a callback was being used to do the same).

Another change to notice is that we made the view a component (LightsView component) in order to properly create and dispose of the clicked-light-positions stream.

Finally, this is the core name space where all the components are composed together:

	(ns kata-lights-out.core
	(:require
	[kata-lights-out.lights-view :as lights-view]
	[com.stuartsierra.component :as component]
	[kata-lights-out.lights :as lights]
	[kata-lights-out.lights-gateway :as lights-gateway]))
	(enable-console-print!)
	(defn init! [m n]
	(component/start
	(component/system-map
	:lights-gateway (lights-gateway/make-api-gateway
	{:reset-lights-url "http://localhost:3000/reset-lights"
	:flip-light-url "http://localhost:3000/flip-light"})
	:lights-component (component/using
	(lights/make-lights m n)
	[:lights-gateway])
	:lights-view (component/using
	(lights-view/make
	{:success-message "Lights out, Yay!"
	:light-on "1"
	:light-off "0"
	:title "Kata Lights Out"})
	[:lights-component]))))
	(init! 3 3)

view raw reactive-kata-lights-out-core.cljs hosted with ❤ by GitHub

This was a nice practice to learn more about doing FRP with reagi. I really like the separation of concerns and clarity that FRP brings with it.

You can find my code in these two GitHub repositories: the server and the client (see the master branch).

You can check the changes I made to use reagi here (see the commits made from the commit d51d2d4 (using a stream to update the lights) on).

Ok, that's all, next I'll start posting a bit about re-frame.

Kata: Variation on Lights Out, introducing component library

Recently at a Clojure Barcelona Developers event, we had a refactoring session in which we introduced Stuart Sierra's component library in a code that was already done: our previous solution to the Lights out kata.

First, we put the code in charge of talking to the back end and updating the lights atom in a separated component, ApiLightsGateway:

	(ns kata-lights-out.lights-gateway
	(:require
	[cljs-http.client :as http]
	[cljs.core.async :as async]
	[com.stuartsierra.component :as component]))
	(defn- extract-lights [response]
	(->> response
	:body
	(.parse js/JSON)
	.-lights
	js->clj))
	(defn- post [lights-channel uri params]
	(async/pipeline
	1
	lights-channel
	(map extract-lights)
	(http/post uri {:with-credentials? false :form-params params})
	false))
	(defprotocol LightsGateway
	(reset-lights! [this m n])
	(flip-light! [this pos]))
	(defrecord ApiLightsGateway [config]
	component/Lifecycle
	(start [this]
	(println ";; Starting ApiLightsGateway component")
	this)
	(stop [this]
	(println ";; Stopping ApiLightsGateway component")
	this)
	LightsGateway
	(reset-lights! [this m n]
	(post (:lights-channel this)
	(:reset-lights-url config)
	{:m m :n n}))
	(flip-light! [this [x y]]
	(post (:lights-channel this)
	(:flip-light-url config)
	{:x x :y y})))
	(defn make-api-gateway [config]
	(->ApiLightsGateway config))

view raw kata-lights-out-using-component-lights_gateway.cljs hosted with ❤ by GitHub

Then, we did the same for the lights code which was put in the Lights component:

	(ns kata-lights-out.lights
	(:require
	[reagent.core :as r]
	[cljs.core.async :as async]
	[com.stuartsierra.component :as component]
	[kata-lights-out.lights-gateway :as lights-gateway])
	(:require-macros
	[cljs.core.async.macros :refer [go-loop]]))
	(def ^:private light-off 0)
	(defn light-off? [light]
	(= light light-off))
	(defn- listen-to-lights-updates! [{:keys [lights-channel lights]}]
	(go-loop []
	(when-let [new-lights (async/<! lights-channel)]
	(reset! lights new-lights)
	(recur))))
	(defprotocol LightsOperations
	(reset-lights! [this m n])
	(flip-light! [this pos]))
	(defrecord Lights [lights-gateway]
	component/Lifecycle
	(start [this]
	(println ";; Starting lights component")
	(let [this (assoc this
	:lights-channel (async/chan)
	:lights (r/atom []))
	lights-channel (:lights-channel this)
	lights-gateway (assoc lights-gateway
	:lights-channel lights-channel)
	this (assoc this :lights-gateway lights-gateway)]
	(listen-to-lights-updates! this)
	this))
	(stop [this]
	(println ";; Stopping lights component")
	(async/close! (:lights-channel this))
	this)
	LightsOperations
	(reset-lights! [this m n]
	(lights-gateway/reset-lights! (:lights-gateway this) m n))
	(flip-light! [this pos]
	(lights-gateway/flip-light! (:lights-gateway this) pos)))
	(defn all-lights-off? [lights]
	(every? light-off? (flatten lights)))
	(defn make-lights []
	(map->Lights {}))

view raw kata-lights-out-using-component-lights.cljs hosted with ❤ by GitHub

This component provided a place to create and close the channel we used to communicate lights data between the Lights and the ApiLightsGateway components.

Next, we used the Lights component from the view code:

	(ns kata-lights-out.lights-view
	(:require
	[reagent.core :as r]
	[kata-lights-out.lights :as lights]))
	(def ^:private light-on "1")
	(def ^:private light-off "0")
	(defn- all-lights-off-message-content [lights]
	(if (lights/all-lights-off? lights)
	"Lights out, Yay!"
	{:style {:display :none}}))
	(defn- all-lights-off-message-component [lights]
	[:div#all-off-msg
	(all-lights-off-message-content lights)])
	(defn- on-light-click [lights-component pos]
	(lights/flip-light! lights-component pos))
	(defn- render-light [light]
	(if (lights/light-off? light)
	light-off
	light-on))
	(defn- light-component [lights-component i j light]
	^{:key (+ i j)}
	[:button
	{:on-click #(on-light-click lights-component [i j])} (render-light light)])
	(defn- row-lights-component [lights-component i row-lights]
	^{:key i}
	[:div (map-indexed (partial light-component lights-component i) row-lights)])
	(defn- home-page [lights-component]
	(fn []
	[:div [:h2 "Kata Lights Out"]
	(map-indexed (partial row-lights-component lights-component) @(:lights lights-component))
	[all-lights-off-message-component @(:lights lights-component)]]))
	(defn mount [lights-component]
	(r/render
	[home-page lights-component]
	(.getElementById js/document "app")))

view raw kata-lights-out-using-component-lights_view.cljs hosted with ❤ by GitHub

And, finally, we put everything together in the lights core name space:

	(ns kata-lights-out.core
	(:require
	[kata-lights-out.lights-view :as lights-view]
	[com.stuartsierra.component :as component]
	[kata-lights-out.lights :as lights]
	[kata-lights-out.lights-gateway :as lights-gateway]))
	(enable-console-print!)
	;; -------------------------
	;; Initialize app
	(defrecord MainComponent [lights-component m n]
	component/Lifecycle
	(start [this]
	(println ";; Starting main component")
	(lights/reset-lights! lights-component m n)
	(lights-view/mount lights-component)
	this)
	(stop [this]
	(println ";; Stopping lights component")
	this))
	(defn main-component [m n]
	(map->MainComponent {:n n :m m}))
	(defn init! [m n]
	(component/start
	(component/system-map
	:lights-gateway (lights-gateway/make-api-gateway
	{:reset-lights-url "http://localhost:3000/reset-lights"
	:flip-light-url "http://localhost:3000/flip-light"})
	:lights-component (component/using
	(lights/make-lights)
	[:lights-gateway])
	:main (component/using
	(main-component m n)
	[:lights-component]))))
	(init! 3 3)

view raw kata-lights-out-using-component-core.cljs hosted with ❤ by GitHub

This was a nice practice to learn more about component and what it might take to introduce it in an existing code base.

You can find the code we produced in these two GitHub repositories: the server and the client (see the componentization branch).

You can check the changes we made to componentize the code here (see the commits made on Aug 30, 2016).

As usual it was a great pleasure to do mob programming and learn with the members of Clojure Developers Barcelona.

Kata: Scrabble sets in Clojure

I recently did the Scrabble sets kata in Clojure.

I used a mix of a bit of TDD, a lot of REPL-driven development (RDD) following this cycle:

1. Write a failing test (using examples that a bit more complicated than the typical ones you use when doing only TDD).
2. Explore and triangulate on the REPL until I made the test pass with some ugly but complete solution.
3. Refactor the code to make it more readable.

I'm founding that this way of working in Clojure is very productive for me.

These are the tests I wrote using Midje:

	(ns scrabble-sets.core-test
	(:require
	[midje.sweet :refer :all]
	[scrabble-sets.core :as scrabble]))
	(facts
	"about scrabble"
	(fact
	"it shows the tiles that are left in the bag
	in descending order of the quantity of each tile left.
	In cases where more than one letter has the same quantity remaining,
	those letters appear in alphabetical order, with blank tiles at the end"
	(fact
	"when no tiles are in play"
	(scrabble/tiles-left
	"") => (str "12: E\n"
	"9: A, I\n"
	"8: O\n"
	"6: N, R, T\n"
	"4: D, L, S, U\n"
	"3: G\n"
	"2: B, C, F, H, M, P, V, W, Y, _\n"
	"1: J, K, Q, X, Z")))
	(fact
	"when some tiles are in play"
	(scrabble/tiles-left
	"PQAREIOURSTHGWIOAE_") => (str "10: E\n"
	"7: A, I\n"
	"6: N, O\n"
	"5: T\n"
	"4: D, L, R\n"
	"3: S, U\n"
	"2: B, C, F, G, M, V, Y\n"
	"1: H, J, K, P, W, X, Z, _\n"
	"0: Q")
	(scrabble/tiles-left
	"LQTOONOEFFJZT") => (str "11: E\n"
	"9: A, I\n"
	"6: R\n"
	"5: N, O\n"
	"4: D, S, T, U\n"
	"3: G, L\n"
	"2: B, C, H, M, P, V, W, Y, _\n"
	"1: K, X\n"
	"0: F, J, Q, Z"))
	(fact
	"when trying to put in play too many tiles of some kind"
	(scrabble/tiles-left
	"AXHDRUIOR_XHJZUQEE")
	=> "Invalid input. More X's have been taken from the bag than possible."))

view raw scrabble-sets.core-test.clj hosted with ❤ by GitHub

and this the resulting code:

	(ns scrabble-sets.core
	(:require
	[clojure.string :as string]))
	(def ^:private tiles-in-bag
	{"E" 12 "A" 9 "I" 9 "O" 8 "N" 6 "R" 6 "T" 6
	"L" 4 "S" 4 "U" 4 "D" 4 "G" 3 "_" 2 "B" 2
	"C" 2 "M" 2 "P" 2 "F" 2 "H" 2 "V" 2 "W" 2
	"Y" 2 "K" 1 "J" 1 "X" 1 "Q" 1 "Z" 1})
	(def ^:private group-by-frequency
	(partial group-by second))
	(defn- format-tile [[freq tiles]]
	(str freq ": " (string/join ", " (map str tiles))))
	(defn- format-tiles [sorted-tiles]
	(->> sorted-tiles
	(map format-tile)
	(string/join "\n")))
	(defn- sort-by-frequency [tiles-in-bag]
	(map (fn [[freq & [tiles]]]
	[freq (sort (map first tiles))])
	(sort-by key > (group-by-frequency tiles-in-bag))))
	(defn- consume-tile [tiles-in-bag tile-in-play]
	(update tiles-in-bag tile-in-play dec))
	(defn- consume [tiles-in-play tiles-in-bag]
	(reduce consume-tile tiles-in-bag tiles-in-play))
	(defn- format-error-message [consumed-tiles]
	(str "Invalid input. More "
	(string/join ", " (map first consumed-tiles))
	"'s have been taken from the bag than possible."))
	(defn- display-distribution [tiles-in-bag]
	(let [overconsumed-tiles (filter #(neg? (second %)) tiles-in-bag)]
	(if (empty? overconsumed-tiles)
	(format-tiles (sort-by-frequency tiles-in-bag))
	(format-error-message overconsumed-tiles))))
	(defn tiles-left [tiles-in-play]
	(->> tiles-in-bag
	(consume (map str tiles-in-play))
	display-distribution))

view raw scrabble-sets.core.clj hosted with ❤ by GitHub

See all the commits here if you want to follow the process.

You can find all the code on GitHub.

Kata: Parrot Refactoring in Clojure

I did Emily Bache's Parrot Refactoring Kata again (I did this kata in Java some time ago) but this time in Clojure. This is a very simple kata that is meant to practice the Replace Conditional with Polymorphism refactoring.

This is the initial code of the kata that have to be refactored:

	(ns parrot-refactoring.core)
	(def ^:private load-factor 9.0)
	(def ^:private base-speed 12.0)
	(defn- compute-base-speed-for-voltage [voltage]
	(min 24.0 (* voltage base-speed)))
	(defn speed [parrot]
	(case (:type parrot)
	:european-parrot
	base-speed
	:african-parrot
	(max 0.0 (- base-speed (* load-factor (:num-coconuts parrot))))
	:norwegian-blue-parrot
	(if (:nailed parrot)
	0.0
	(compute-base-speed-for-voltage (:voltage parrot)))
	(throw (Exception. "Should be unreachable!"))))

view raw initial-parrot-refactoring.core.clj hosted with ❤ by GitHub

Since Clojure provides several ways of achieving polymorphism, I did two versions of the kata:

- One using protocols:

	(ns parrot-refactoring.core)
	(def ^:private base-speed 12.0)
	(def ^:private minimum-speed 0.0)
	(defprotocol Parrot
	(speed [this]))
	(defrecord EuropeanParrot []
	Parrot
	(speed [_]
	base-speed))
	(defrecord AfricanParrot [num-coconuts]
	Parrot
	(speed [_]
	(let [load-factor 9.0]
	(max minimum-speed (- base-speed (* load-factor num-coconuts))))))
	(defrecord NorwegiaBlueParrot [voltage]
	Parrot
	(speed [_]
	(let [maximum-speed 24.0]
	(min maximum-speed (* voltage base-speed)))))
	(defrecord NailedNorwegiaBlueParrot []
	Parrot
	(speed [_] minimum-speed))
	(defn european []
	(->EuropeanParrot))
	(defn african [num-coconuts]
	(->AfricanParrot num-coconuts))
	(defn norwegian-blue-parrot [voltage]
	(->NorwegiaBlueParrot voltage))
	(defn nailed-norwegian-blue-parrot []
	(->NailedNorwegiaBlueParrot))

view raw parrot-refactoring-protocols.clj hosted with ❤ by GitHub

This refactoring was a bit more involved than the one I did for the next version, but I managed to get rid of the exception and the resulting tests were more readable.

If you feel like following the refactoring process, check the baby steps in the commits. You can find the code in this GitHub repository.

- And another one using multimethods:

	(ns parrot-refactoring.core)
	(def ^:private base-speed 12.0)
	(def ^:private minimum-speed 0.0)
	(defmulti speed :type)
	(defmethod speed :european-parrot [_]
	base-speed)
	(defmethod speed :african-parrot [{:keys [num-coconuts]}]
	(let [load-factor 9.0]
	(max minimum-speed (- base-speed (* load-factor num-coconuts)))))
	(defmethod speed :norwegian-blue-parrot [{:keys [nailed voltage]}]
	(let [maximum-speed 24.0]
	(if nailed
	minimum-speed
	(min maximum-speed (* voltage base-speed)))))
	(defmethod speed :default [_]
	(throw (Exception. "Should be unreachable!")))

view raw parrot-refactoring-multimethods.clj hosted with ❤ by GitHub

This version is much shorter and the refactoring is less involved than the previous one, but the resulting tests read much worse and the map representing the parrot has many "optional" entries.

Again, check the commits of the refactoring baby steps here, if you feel like following the refactoring process. You can find the code of this second version in this GitHub repository. I'd like to thank Emily Bache for sharing this kata.

Interesting Talk: "How the web is democratizing science (join in!)"

I've just watched this interesting talk by Abigail Cabunoc Mayes about Open Science

• How the web is democratizing science (join in!)

Mob Kata: Cellular Automata in Clojure

At this week's Clojure Developers Barcelona meetup, we did the Cellular Automata kata in Clojure.

We did mob programming and used a mix of TDD and RDD.

This is the code we wrote:

First, the tests using Midje for the rules 30 and 90:

	(ns cellular-automata.rules-test
	(:require
	[midje.sweet :refer :all]
	[cellular-automata.rules :as rules]))
	(facts
	"about rules"
	(fact
	;+-----------------------------------------------------------------+
	;| Neighborhood | 111 | 110 | 101 | 100 | 011 | 010 | 001 | 000 |
	;+-----------------------------------------------------------------+
	;| New Center Cell | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 0 |
	;+-----------------------------------------------------------------+
	"rule 90"
	(rules/rule-90 [1 1 1]) => 0
	(rules/rule-90 [1 1 0]) => 1
	(rules/rule-90 [1 0 1]) => 0
	(rules/rule-90 [1 0 0]) => 1
	(rules/rule-90 [0 1 1]) => 1
	(rules/rule-90 [0 1 0]) => 0
	(rules/rule-90 [0 0 1]) => 1
	(rules/rule-90 [0 0 0]) => 0)
	(fact
	;+-----------------------------------------------------------------+
	;| Neighborhood | 111 | 110 | 101 | 100 | 011 | 010 | 001 | 000 |
	;+-----------------------------------------------------------------+
	;| New Center Cell | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0 |
	;+-----------------------------------------------------------------+
	"rule 30"
	(rules/rule-30 [1 1 1]) => 0
	(rules/rule-30 [1 1 0]) => 0
	(rules/rule-30 [1 0 1]) => 0
	(rules/rule-30 [1 0 0]) => 1
	(rules/rule-30 [0 1 1]) => 1
	(rules/rule-30 [0 1 0]) => 1
	(rules/rule-30 [0 0 1]) => 1
	(rules/rule-30 [0 0 0]) => 0))

view raw ca_rules_test.clj hosted with ❤ by GitHub

and the tests for the evolution and rendering of automata:

	(ns cellular-automata.core-test
	(:require
	[midje.sweet :refer :all]
	[cellular-automata.core :as automata]
	[cellular-automata.rules :as rules]))
	(facts
	"an elementary cellular automaton"
	(facts
	"evolves from an initial state for some time steps
	following a given rule"
	(automata/evolve
	rules/rule-90
	[1 1 0 1 0 1 0]
	5) => [[1 1 0 1 0 1 0]
	[1 1 0 0 0 0 1]
	[1 1 1 0 0 1 0]
	[1 0 1 1 1 0 1]
	[0 0 1 0 1 0 0]
	[0 1 0 0 0 1 0]])
	(facts
	"can be rendered as text lines"
	(automata/render
	[[1 1 0 1 0 1 0]
	[1 1 0 0 0 0 1]
	[1 1 1 0 0 1 0]]) => "xx x x \nxx x\nxxx x "))

view raw ca_core_test.clj hosted with ❤ by GitHub

This is the corresponding code for the rules:

	(ns cellular-automata.rules)
	(def rule-90
	{[1 1 1] 0
	[1 1 0] 1
	[1 0 1] 0
	[1 0 0] 1
	[0 1 1] 1
	[0 1 0] 0
	[0 0 1] 1
	[0 0 0] 0})
	(def rule-30
	{[1 1 1] 0
	[1 1 0] 0
	[1 0 1] 0
	[1 0 0] 1
	[0 1 1] 1
	[0 1 0] 1
	[0 0 1] 1
	[0 0 0] 0})

view raw ca_rules.clj hosted with ❤ by GitHub

and for the automaton evolution and rendering:

	(ns cellular-automata.core
	(:require
	[clojure.string :as string]))
	(def ^:private representations
	{0 " "
	1 "x"})
	(defn- extract-neighborhoods [state]
	(partition 3 1 (repeat 0) (cons 0 state)))
	(defn- evolve-once [rule state]
	(mapv rule (extract-neighborhoods state)))
	(defn evolve [rule initial-state time-steps]
	(->> initial-state
	(iterate (partial evolve-once rule))
	(take (inc time-steps))))
	(defn- render-state [line]
	(apply str (map representations line)))
	(defn render [states]
	(->> states
	(map render-state)
	(string/join "\n")))

view raw ca_core.clj hosted with ❤ by GitHub

Printing on the REPL the result of rendering the Rule 90 automaton evolution during 15 steps, we obtain a nice Sierpinski triangle :

	(doseq
	[line (render
	(evolve
	rules/rule-90
	[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
	15))]
	(print line))
	x
	x x
	x x
	x x x x
	x x
	x x x x
	x x x x
	x x x x x x x x
	x x
	x x x x
	x x x x
	x x x x x x x x
	x x x x
	x x x x x x x x
	x x x x x x x x
	x x x x x x x x x x x x x x x x => nil

view raw sierpinsky_ca_clj_in_repl.clj hosted with ❤ by GitHub

You can find the code in this GitHub repository.

As usual, it was a pleasure to meet and program with Clojure Developers Barcelona members.

Kata: Varint in Clojure using Midje and test.check

In last week's Barcelona Software Craftsmanship Dojo we did the Varint kata created by Eric Le Merdy.

Since I was facilitating it, I couldn't code during the event.

Once I got home, I did it using Clojure.

These are the tests using Midje and Clojure test.check libraries:

	(ns varint.core-test
	(:require
	[varint.core :refer :all]
	[midje.sweet :refer :all]
	[clojure.test.check.clojure-test :refer [defspec]]
	[clojure.test.check.generators :as gen]
	[clojure.test.check.properties :as prop]))
	(facts
	"about varint"
	(facts
	"encoding numbers under 128"
	(encode 1) => "00000001"
	(encode 8) => "00001000"
	(encode 127) => "01111111")
	(facts
	"encoding numbers greater or equal than 128"
	(encode 300) => "1010110000000010")
	(facts
	"decoding varints"
	(decode "1010110000000010") => 300))
	(defspec coding-and-decoding
	1000
	(prop/for-all [num (gen/large-integer* {:min 0})]
	(= (-> num encode decode) num)))

view raw varint.core-test.clj hosted with ❤ by GitHub

and this is the resulting code:

	(ns varint.core)
	(defn- pad-left [length element bin-num]
	(concat (repeat (- length (count bin-num)) element) bin-num))
	(defn- add-most-significat-bits [bytes]
	(flatten (concat (map #(cons "1" %) (butlast bytes))
	(cons "0" (last bytes)))))
	(defn- partition-in-blocks-of [block-size coll]
	(partition-all block-size block-size coll))
	(defn- bin-str->bytes [bin-str]
	(->> bin-str
	reverse
	(partition-in-blocks-of 7)
	(map reverse)
	(map (partial pad-left 7 "0"))))
	(defn- int->bin-str [num]
	(Long/toBinaryString num))
	(def ^:private drop-most-significat-bits (partial map rest))
	(defn- varint->bytes [varint]
	(->> varint
	(partition-in-blocks-of 8)
	drop-most-significat-bits))
	(defn- bytes->bin-str [bytes]
	(-> bytes
	reverse
	flatten))
	(defn- int-pow [b exp]
	(reduce * (repeat exp b)))
	(defn- char->int [ch]
	(Integer/parseInt (str ch)))
	(def ^:private bin-str->bits (partial map char->int))
	(defn- bits->int [bits]
	(->> bits
	reverse
	(map-indexed #(* %2 (int-pow 2 %1)))
	(reduce +)))
	(defn- bin-str->int [bin-str]
	(-> bin-str
	bin-str->bits
	bits->int))
	(defn- bytes->varint [bytes]
	(->> bytes
	add-most-significat-bits
	(apply str)))
	(defn encode [num]
	(-> num
	int->bin-str
	bin-str->bytes
	bytes->varint))
	(defn decode [varint]
	(-> varint
	varint->bytes
	bytes->bin-str
	bin-str->int))

view raw varint.core.clj hosted with ❤ by GitHub

I used a mix of a bit of TDD, a lot of REPL-driven development (RDD) and some property-based testing.

Basically, the cycle I followed was like this:

1. Write a failing test (using more complicated examples than the typical ones you use when doing only TDD).
2. Explore and triangulate on the REPL until I made the test pass with some ugly complete solution.
3. Refactor the code to make it more readable.

I've noticed that when I mix TDD and RDD, I end up doing bigger steps and keeping less tests than when doing only TDD. I think this is because most of the baby steps and "triangulation tests" are done on the REPL which gives you a much faster feedback cycle.

This way I wrote both encode and decode functions.

Once I had then working with a few examples, I added property-based tests using Clojure test.check library (a QuickCheck like library for Clojure) to automatically explore more examples by checking that the property of "decoding and encoded number returns the number" held for 1000 randomly generated examples.

I think that these three techniques are complementary and its combination makes me more productive.

To document the development process I commit after each TDD green step and after each refactoring. I also committed the REPL history.

See all the commits here if you want to follow the process.

You can find all the code on GitHub.

Books I read (January - August 2016)

January
- El Eternauta, Héctor Germán Oesterheld and Francisco Solano López
- Barcelona. Los vagabundos de la chatarra, Jorge Carrión and Sagar Fornies
- Rip Van Winkle, Washington Irving
- La guerra interminable (The Forever War), Joe Haldeman
- Maintanable JavaScript, Nicholas C. Zakas
- Ve y pon un centinela (Go Set a Watchman), Harper Lee
- El nombre del viento (The Name of the Wind), Patrick Rothfuss
- You Don't Know JS: Async & Performance, Kyle Simpson
- Sapiens: A Brief History of Humankind, Yuval Noah Harari
- The Principles of Object Oriented JavaScript, Nicholas C. Zakas

February
- The Leprechauns of Software Engineering, Laurent Bossavit
- The Wise Man's Fear, Patrick Rothfuss
- Fundamentals of Object-Oriented Design in UML, Meilir Page-Jones
- Old Man's War, John Scalzi
- Cevdet Bey e hijos (Cevdet Bey ve Oğulları), Orhan Pamuk
- Ringworld, Larry Niven
- Why Nations Fail: The Origins of Power, Prosperity, and Poverty, Daron Acemoglu and James A. Robinson

March
- The Grumpy Programmer's Guide To Building Testable PHP Applications, Chris Hartjes
- The Grapes of Wrath, John Steinbeck
- The Coding Dojo Handbook, Emily Bache
- Fahrenheit 451, Ray Bradbury

April
- Implementation Patterns, Kent Beck (3rd time)
- The Power of Habit: Why We Do What We Do in Life and Business, Charles Duhigg
- How to Win Friends and Influence People, Dale Carnegie
- Understanding the Four Rules of Simple Design, Corey Haines (2nd time)
- La llamada de Cthulhu (The Call of Cthulhu), El horror de Dunwich (The Dunwich Horror) and Las ratas en las paredes (The Rats in the Walls), H. P. Lovecraft
- The Tales of Beedle the Bard, J. K. Rowling

May
- The Slow Regard of Silent Things, Patrick Rothfuss
- Building Microservices, Designing Fine-Grained Systems, Sam Newman
- Man's Search for Meaning, Viktor Frankl
- Manifiesto del Partido Comunista (Das Kommunistiche Manifest), K. Marx and F. Engels
- Web Development with Clojure, Build Bulletproof Web Apps with Less Code, Dimitri Sotnikov
- Patterns of Enterprise Application Architecture, Martin Fowler
- The Checklist Manifesto: How to Get Things Right, Atul Gawande
- How to Fail at Almost Everything and Still Win Big: Kind of the Story of My Life, Scott Adams

June
- Pragmatic Thinking and Learning, Refactor Your Wetware, Andy Hunt
- Testable JavaScript, Mark Ethan Trostler
- The Secrets of Consulting: A Guide to Giving & Getting Advice Successfully, Gerald M. Weinberg
- La desfachatez intelectual. Escritores e intelectuales ante la política, Ignacio Sánchez-Cuenca
- REST in Practice, Jim Webber, Savas Parastatidis and Ian Robinson
- Mindset, Carol Dweck

July
- The Call of the Wild, Jack London
- Weinberg on Writing: The Fieldstone Method, Gerald M. Weinberg
- Dinner at the Homesick Restaurant, Anne Tyler
- How to Solve It: A New Aspect of Mathematical Method, G. Polya
- Object Oriented Software Engineering: A Use Case Driven Approach, Ivar Jacobson
- A Far Cry from Kensington, Muriel Spark

August
- The Difference Engine, William Gibson and Bruce Sterling
- Clojure Applied, From Practice to Practitioner, Ben Vandgrift and Alex Miller
- Vampir, Joann Sfar
- Los mares del Sur, Manuel Vázquez Montalbán
- El océano al final del camino (The Ocean at the End of the Lane), Neil Gaiman
- La Guerra Civil Española, Paul Preston and José Pablo García
- Wiser: Getting Beyond Groupthink to Make Groups Smarter, Cass R. Sunstein and Reid Hastie

MOOCs: Machine Learning on Coursera

I recently finished this wonderful Coursera course by Andrew Ng of Stanford University:

• Machine Learning

I really enjoyed the course. It was very interesting. Andrew Ng did a great job introducing machine learning, data mining, and statistical pattern recognition.

I would like to thank Coursera and Andrew Ng for this great course!

Kata: Variation on Lights Out, flipping behavior on the backend

Lately, in Clojure Developers Barcelona's events, we've been practicing by doing several variations on the Lights Out Kata.

First, we did the kata in ClojureScript using Reagent and Figwheel (described in this previous post).

In the last two meetups (this one and this other one), we redid the kata.

This time instead of doing everything on the client, we used the Compojure library to develop a back end service that reset and flipped the lights, and a simple client that talked to it using cljs-http and core.async libraries.

As we've been doing lately, we did mob programming and REPL-driven development.

First, let's see the code we did for the server:

These are the tests:

	(ns lights-out-server.handler-test
	(:require [clojure.test :refer :all]
	[ring.mock.request :as mock]
	[lights-out-server.handler :refer :all]))
	(deftest test-lights-out
	(testing "resetting lights"
	(let [response (app (mock/request :post "/reset-lights" {:m 3 :n 3}))]
	(is (= (:status response) 200))
	(is (= (:body response) "{\"lights\":[[1,1,1],[1,1,1],[1,1,1]]}"))))
	(testing "flipping lights"
	(app (mock/request :post "/reset-lights" {:m 3 :n 3}))
	(let [response (app (mock/request :post "/flip-light" {:x 0 :y 0}))]
	(is (= (:status response) 200))
	(is (= (:body response) "{\"lights\":[[0,0,1],[0,1,1],[1,1,1]]}"))))
	(testing "flipping lights twice"
	(app (mock/request :post "/reset-lights" {:m 3 :n 3}))
	(app (mock/request :post "/flip-light" {:x 1 :y 1}))
	(let [response (app (mock/request :post "/flip-light" {:x 0 :y 0}))]
	(is (= (:status response) 200))
	(is (= (:body response) "{\"lights\":[[0,1,1],[1,0,0],[1,0,1]]}")))))

view raw lights-out-server-handler-test.clj hosted with ❤ by GitHub

and this is the definition of the routes and their handlers using Compojure:

	(ns lights-out-server.handler
	(:require
	[compojure.core :refer :all]
	[ring.middleware.cors :as cors-middleware]
	[ring.middleware.json :as json-middleware]
	[ring.middleware.defaults :refer [wrap-defaults api-defaults]]
	[lights-out-server.lights :as lights]))
	(defroutes app-routes
	(POST "/reset-lights" [m n]
	(let [m (Integer/parseInt m)
	n (Integer/parseInt n)]
	(lights/reset-lights! m n)
	{:status 200 :body {:lights @lights/lights}}))
	(POST "/flip-light" [x y]
	(let [x (Integer/parseInt x)
	y (Integer/parseInt y)]
	(lights/flip-light! [x y])
	{:status 200 :body {:lights @lights/lights}})))
	(def app
	(-> app-routes
	(wrap-defaults api-defaults)
	(json-middleware/wrap-json-response {:keywords? true :bigdecimals? true})
	(cors-middleware/wrap-cors
	:access-control-allow-origin #"http://0.0.0.0:3449"
	:access-control-allow-methods [:post])))

view raw ligths-outh-server-handler.clj hosted with ❤ by GitHub

We had some problems with the Same-origin policy until we discovered and learned how to use the Ring middleware for Cross-Origin Resource Sharing.

Finally, this is the code that flips and resets the lights (it's more or less the same code we wrote for the client-only version of the kata we did previously):

	(ns lights-out-server.lights)
	(def lights (atom nil))
	(def ^:private light-on 1)
	(def ^:private light-off 0)
	(defn- neighbors? [[i0 j0] [i j]]
	(or (and (= j0 j) (= 1 (Math/abs (- i0 i))))
	(and (= i0 i) (= 1 (Math/abs (- j0 j))))))
	(defn- neighbors [m n pos]
	(for [i (range m)
	j (range n)
	:when (neighbors? pos [i j])]
	[i j]))
	(defn light-off? [light]
	(= light light-off))
	(defn- flip-light [light]
	(if (light-off? light)
	light-on
	light-off))
	(defn- flip [lights pos]
	(update-in lights pos flip-light))
	(defn- flip-neighbors [m n pos lights]
	(->> pos
	(neighbors m n)
	(cons pos)
	(reduce flip lights)))
	(defn- all-lights-on [m n]
	(vec (repeat m (vec (repeat n light-on)))))
	(defn- num-rows []
	(count @lights))
	(defn- num-colums []
	(count (first @lights)))
	(defn reset-lights! [m n]
	(reset! lights (all-lights-on m n)))
	(defn flip-light! [pos]
	(swap! lights (partial flip-neighbors (num-rows) (num-colums) pos)))

view raw lights-out-server-lights.clj hosted with ❤ by GitHub

Now, let's see the code we wrote for the client:

First, the core namespace where everything is initialized (notice the channel creation on line 12):

	(ns kata-lights-out.core
	(:require
	[kata-lights-out.lights-view :as light-view]
	[cljs.core.async :as async]))
	(enable-console-print!)
	;; -------------------------
	;; Initialize app
	(def m 4)
	(def n 4)
	(def lights-channel (async/chan))
	(defn init! []
	(light-view/mount lights-channel m n))
	(init!)

view raw lights-out-client-core.cljs hosted with ❤ by GitHub

Then the lights-view namespace which contains the code to render all the components:

	(ns kata-lights-out.lights-view
	(:require
	[reagent.core :as r]
	[kata-lights-out.lights :as l]))
	(def ^:private light-on "1")
	(def ^:private light-off "0")
	(defn- all-lights-off-message-content [lights]
	(if (l/all-lights-off? lights)
	"Lights out, Yay!"
	{:style {:display :none}}))
	(defn- all-lights-off-message-component [lights]
	[:div#all-off-msg (all-lights-off-message-content lights)])
	(defn- on-light-click [lights-channel pos]
	(l/flip-light! lights-channel pos))
	(defn- render-light [light]
	(if (l/light-off? light)
	light-off
	light-on))
	(defn- light-component [lights-channel i j light]
	^{:key (+ i j)}
	[:button
	{:on-click #(on-light-click lights-channel [i j])}
	(render-light light)])
	(defn- row-lights-component [lights-channel i row-lights]
	^{:key i}
	[:div (map-indexed
	(partial light-component lights-channel i)
	row-lights)])
	(defn- home-page [lights-channel lights]
	(fn []
	[:div [:h2 "Kata Lights Out"]
	(map-indexed
	(partial row-lights-component lights-channel)
	@lights)
	[all-lights-off-message-component @lights]]))
	(defn mount [lights-channel m n]
	(l/listen-lights-updates! lights-channel)
	(l/reset-lights! lights-channel m n)
	(r/render
	[home-page lights-channel l/lights]
	(.getElementById js/document "app")))

view raw lights-out-client-lights-view.cljs hosted with ❤ by GitHub

The lights-view's code is also very similar to the one we did for the client-only version of the kata.

And finally the lights namespace which is in charge of talking to the back end and updating the lights atom:

	(ns kata-lights-out.lights
	(:require
	[reagent.core :as r]
	[cljs-http.client :as http]
	[cljs.core.async :as async])
	(:require-macros
	[cljs.core.async.macros :refer [go go-loop]]))
	(def ^:private lights (r/atom []))
	(def ^:private light-off 0)
	(defn light-off? [light]
	(= light light-off))
	(defn- extract-lights [response]
	(->> response
	:body
	(.parse js/JSON)
	.-lights
	js->clj))
	(defn listen-to-lights-updates! [lights-channel]
	(go-loop []
	(when-let [response (async/<! lights-channel)]
	(reset! lights (extract-lights response))
	(recur))))
	(defn flip-light! [lights-channel [x y]]
	(async/pipe
	(http/post "http://localhost:3000/flip-light"
	{:with-credentials? false
	:form-params {:x x :y y}})
	lights-channel
	false))
	(defn reset-lights! [lights-channel m n]
	(async/pipe
	(http/post "http://localhost:3000/reset-lights"
	{:with-credentials? false
	:form-params {:m m :n n}})
	lights-channel
	false))
	(defn all-lights-off? [lights]
	(every? light-off? (flatten lights)))

view raw lights-out-client-lights.cljs hosted with ❤ by GitHub

Notice how we used the pipe function to take the values that were coming from the channel returned by the call to cljs-httphttps://github.com/r0man/cljs-http's post function and pass them to the channel from which the code that updates the lights state is taking values.

You can find the code we produced in these two GitHub repositories: the server and the client (see the flip-lights-in-backend branch).

As usual it was a great pleasure to do mob programming and learn with the members of Clojure Developers Barcelona.

MOOCs: Networking for Web Developers on Udacity

I recently finished this Udacity course:

• Networking for Web Developers

I found it very interesting. I did it, mainly, because I'd like to start filling some gaps in my knowledge about the stack of networking protocols under the hood of HTTP.

After the course, I've started to read Ilya Grigorik's High Performance Browser Networking book bit by bit.

Thanks a million Udacity!

Interesting Talk: "Taming Asynchronous Workflows with Functional Reactive Programming"

I've just watched this great talk by Leonardo Borges

• Taming Asynchronous Workflows with Functional Reactive Programming

You can find the slides here.

Kata: Lights Out in ClojureScript using Reagent and Figwheel

In a recent Clojure Developers Barcelona event we did the Lights Out Kata in ClojureScript using Reagent (a minimalistic ClojureScript interface to React.js) and Figwheel (which builds your ClojureScript code and hot loads it into the browser as you are coding).

We did mob programming and REPL-driven development.

Once at home I went on working on the kata.

This is the resulting code:

First, the core namespace where everything is initialized:

	(ns kata-lights-out.core
	(:require
	[reagent.core :as r]
	[kata-lights-out.lights :as l]
	[kata-lights-out.lights-view :as light-view]))
	(enable-console-print!)
	;; -------------------------
	;; Initialize app
	(def m 3)
	(def n 3)
	(def lights (r/atom []))
	(defn init! []
	(l/reset-lights! lights m n)
	(light-view/mount lights))
	(init!)

view raw lights_out_kata_core.cljs hosted with ❤ by GitHub

The lights-view namespace which contains the code to render all the components:

	(ns kata-lights-out.lights-view
	(:require
	[reagent.core :as r]
	[kata-lights-out.lights :as l]))
	(def ^:private light-on "1")
	(def ^:private light-off "0")
	(defn- all-lights-off-message-content [lights]
	(if (l/all-lights-off? lights)
	"Lights out, Yay!"
	{:style {:display :none}}))
	(defn- all-lights-off-message-component [lights]
	[:div#all-off-msg
	(all-lights-off-message-content lights)])
	(defn- on-light-click [pos lights]
	(swap! lights (partial l/flip-lights! lights pos)))
	(defn- render-light [light]
	(if (l/light-off? light)
	light-off
	light-on))
	(defn- light-component [lights i j light]
	^{:key (+ i j)}
	[:button
	{:on-click #(on-light-click [i j] lights)}
	(render-light light)])
	(defn- row-lights-component [lights i row-lights]
	^{:key i}
	[:div (map-indexed (partial light-component lights i) row-lights)])
	(defn- home-page [lights]
	[:div [:h2 "Kata Lights Out"]
	(map-indexed (partial row-lights-component lights) @lights)
	[all-lights-off-message-component @lights]])
	(defn mount [lights]
	(r/render
	[home-page lights]
	(.getElementById js/document "app")))

view raw lights_out_kata_view.cljs hosted with ❤ by GitHub

Finally, the lights namespace which contains all the business logic:

	(ns kata-lights-out.lights)
	(def ^:private light-on 1)
	(def ^:private light-off 0)
	(defn- neighbors? [[i0 j0] [i j]]
	(or (and (= j0 j) (= 1 (Math/abs (- i0 i))))
	(and (= i0 i) (= 1 (Math/abs (- j0 j))))))
	(defn- neighbors [m n pos]
	(for [i (range m)
	j (range n)
	:when (neighbors? pos [i j])]
	[i j]))
	(defn light-off? [light]
	(= light light-off))
	(defn- flip-light [light]
	(if (light-off? light)
	light-on
	light-off))
	(defn- flip [lights pos]
	(update-in lights pos flip-light))
	(defn- num-rows [lights]
	(count lights))
	(defn- num-colums [lights]
	(count (first lights)))
	(defn- flip-neighbors [pos lights]
	(->> pos
	(neighbors (num-rows lights) (num-colums lights))
	(cons pos)
	(reduce flip lights)))
	(defn- all-lights-on [m n]
	(vec (repeat m (vec (repeat n light-on)))))
	(defn reset-lights! [lights m n]
	(reset! lights (all-lights-on m n)))
	(defn flip-lights! [lights pos]
	(swap! lights (partial flip-neighbors pos)))
	(defn all-lights-off? [lights]
	(every? zero? (flatten lights)))

view raw lights_out_kata_lights.cljs hosted with ❤ by GitHub

You can check all the code in the everything-in-client branch of this GitHub repository.

As usual it was a great pleasure to do mob programming with the members of Clojure Developers Barcelona.

Thanks!

Kata: Bank Account in Clojure using outside-in TDD with Component and Midje

I did the Printing Account Statement subset of the Bank Account kata in Clojure using Component and Midje.

I started the kata in a Barcelona Clojure Developers event.

The truth is that, since I was learning how to use the Component library, I didn't use TDD.

Instead I worked on the REPL to get everything in place and make it work.

Then I wrote the tests I would have liked to write if I had used outside-in TDD with Midje.

I find that, when I'm learning something new, it works better for me what Brian Marick describes in these tweets:

(6) the immediately-following writing of tests to capture what I've learned counts as Schön-style reflection on just-prior practice.

— Brian Marick (@marick) July 26, 2016

Now I'll show you the tests I actually wrote afterwards, in the order I would have written them doing outside-in TDD.

This is the acceptance test I would have started with:

	(ns bank-account.acceptance-test
	(:require
	[midje.sweet :refer :all]
	[bank-account.test-helpers :refer [output-lines make-dates]]
	[com.stuartsierra.component :as component]
	[bank-account.factories :as factories]
	[bank-account.account :as account]))
	(unfinished date-fn)
	(facts
	"printing an account statement"
	(let [config {:format {:date-format "dd/MM/yyyy"
	:separator "||"
	:num-decimals 2
	:header "date || credit || debit || balance"}}
	dates (partial make-dates "dd/MM/yyyy")
	account-system (assoc (factories/make-system config)
	:transactions
	(factories/in-memory-transactions #(date-fn)))
	account (-> account-system component/start :account)]
	(do
	(account/deposit! account 1000)
	(account/deposit! account 2000)
	(account/withdraw! account 500)
	(output-lines
	account/print-statement account))
	=> ["date || credit || debit || balance"
	"14/01/2012 || || 500.00 || 2500.00"
	"13/01/2012 || 2000.00 || || 3000.00"
	"10/01/2012 || 1000.00 || || 1000.00"]
	(provided
	(date-fn) =streams=> (dates ["10/01/2012" "13/01/2012" "14/01/2012"]))))

view raw bank_kata_acceptance_test.clj hosted with ❤ by GitHub

Then I would have written this unit test for Account:

	(ns bank-account.account-test
	(:require
	[midje.sweet :refer :all]
	[midje.open-protocols :refer [defrecord-openly]]
	[com.stuartsierra.component :as component]
	[bank-account.account :as account]
	[bank-account.factories :as factories]
	[bank-account.transactions-operations.transactions-operations :as transactions-operations]
	[bank-account.statement-printing.statement-printer :as statement-printer]))
	(unfinished register!)
	(unfinished balanced-transactions)
	(unfinished print-statement)
	(defrecord-openly FakeTransactions []
	transactions-operations/TransactionsOperations
	(register! [this amount])
	(balanced-transactions [this]))
	(defrecord-openly FakePrinter []
	statement-printer/StatementPrinter
	(print-statement [this balanced-transactions]))
	(defn new-account [transactions printer]
	(-> (factories/account)
	(merge {:transactions transactions
	:printer printer})
	component/start))
	(facts
	"about account operations"
	(fact
	"it registers deposit transactions"
	(let [fake-transactions (->FakeTransactions)
	an-account (new-account fake-transactions :not-used)]
	(account/deposit! an-account 50) => irrelevant
	(provided
	(register! fake-transactions 50) => irrelevant :times 1)))
	(fact
	"it registers withdrawals transactions"
	(let [fake-transactions (->FakeTransactions)
	an-account (new-account fake-transactions :not-used)]
	(account/withdraw! an-account 100) => irrelevant
	(provided
	(register! fake-transactions -100) => irrelevant :times 1)))
	(fact
	"it prints the transactions in the statement"
	(let [fake-transactions (->FakeTransactions)
	fake-printer (->FakePrinter)
	an-account (new-account fake-transactions fake-printer)]
	(account/print-statement an-account) => irrelevant
	(provided
	(balanced-transactions
	fake-transactions) => ...some-balanced-transactions... :times 1
	(print-statement
	fake-printer
	...some-balanced-transactions...) => irrelevant :times 1))))

view raw bank_kata_account_test.clj hosted with ❤ by GitHub

And these are the ones for InMemoryTransactions, ConsoleStatementPrinter and NiceReverseStatementFormat:

	(ns bank-account.in-memory-transactions-test
	(:require
	[midje.sweet :refer :all]
	[midje.open-protocols :refer [defrecord-openly]]
	[com.stuartsierra.component :as component]
	[bank-account.factories :as factories]
	[bank-account.transactions-operations.transactions-operations :as transactions]
	[bank-account.test-helpers :refer [make-date]]))
	(unfinished date-fn)
	(defn new-in-memory-transactions [date-fn]
	(component/start (factories/in-memory-transactions date-fn)))
	(fact
	"about transactions"
	(facts
	"in memory"
	(fact
	"returns balanced transactions lines for all registered transactions"
	(let [date (partial make-date "dd/MM/yyyy")
	first-transaction {:amount 1000 :date (date "10/02/2016")}
	second-transaction {:amount 1500 :date (date "13/05/2016")}
	third-transaction {:amount -500 :date (date "14/08/2016")}
	in-memory-transactions (new-in-memory-transactions #(date-fn))]
	(do
	(transactions/register! in-memory-transactions (:amount first-transaction))
	(transactions/register! in-memory-transactions (:amount second-transaction))
	(transactions/register! in-memory-transactions (:amount third-transaction))
	(transactions/balanced-transactions in-memory-transactions))
	=> [(assoc first-transaction :balance 1000)
	(assoc second-transaction :balance 2500)
	(assoc third-transaction :balance 2000)]
	(provided (date-fn) =streams=> [(:date first-transaction)
	(:date second-transaction)
	(:date third-transaction)])))))

view raw bank_kata_in_memory_transactions_test.clj hosted with ❤ by GitHub

	(ns bank-account.console-statement-printer-test
	(:require
	[midje.sweet :refer :all]
	[midje.open-protocols :refer [defrecord-openly]]
	[com.stuartsierra.component :as component]
	[bank-account.factories :as factories]
	[bank-account.statement-formatting.statement-format :refer [StatementFormat]]
	[bank-account.statement-printing.statement-printer :as printer]
	[bank-account.test-helpers :refer [output-lines]]))
	(unfinished format-statement-lines)
	(unfinished header)
	(defrecord-openly FakeFormat []
	StatementFormat
	(header [this])
	(format-statement-lines [this statement-lines]))
	(defn new-console-printer
	([format print-fn]
	(component/start (merge (factories/console-printer)
	{:format format
	:print-fn print-fn})))
	([format]
	(new-console-printer format identity)))
	(fact
	"about printing statements"
	(fact
	"it asks the format for the header"
	(let [fake-format (->FakeFormat)
	a-printer (new-console-printer fake-format)]
	(printer/print-statement
	a-printer :not-used-in-this-test) => irrelevant
	(provided
	(header fake-format) => irrelevant :times 1)))
	(fact
	"it asks the format to format all balanced transactions"
	(let [fake-format (->FakeFormat)
	a-printer (new-console-printer fake-format)]
	(printer/print-statement
	a-printer
	...some-balanced-transactions...) => irrelevant
	(provided
	(format-statement-lines
	fake-format
	...some-balanced-transactions...)
	=> ...some-formatted-statement-lines... :times 1)))
	(fact
	"it prints the header and formatted lines"
	(let [some-header "some-header"
	some-formatted-statement-lines ["statement-line-1" "statement-line-2"]
	expected-output-lines (cons some-header some-formatted-statement-lines)
	fake-format (->FakeFormat)
	a-printer (new-console-printer fake-format println)]
	(output-lines
	printer/print-statement
	a-printer
	...some-balanced-transactions...) => expected-output-lines
	(provided
	(header fake-format) => some-header
	(format-statement-lines
	fake-format
	...some-balanced-transactions...) => some-formatted-statement-lines))))

view raw bank_kata_console_statement_printer_test.clj hosted with ❤ by GitHub

	(ns bank-account.nice-reverse-statement-format-test
	(:require
	[midje.sweet :refer :all]
	[midje.open-protocols :refer [defrecord-openly]]
	[com.stuartsierra.component :as component]
	[bank-account.factories :as factories]
	[bank-account.statement-formatting.statement-format :as statement-format]
	[bank-account.test-helpers :refer [make-date]]))
	(defn- new-nice-reverse-format [config]
	(component/start (factories/nice-reverse-statement-format config)))
	(fact
	"about formatting statements"
	(facts
	"using NiceReverseStatementFormat"
	(let [config {:date-format "dd/MM/yyyy"
	:separator "||"
	:num-decimals 2
	:header "date || credit || debit || balance"}
	date (partial make-date (:date-format config))
	nice-reverse-format (new-nice-reverse-format config)]
	(fact
	"it returns the configured header"
	(statement-format/header nice-reverse-format) => (:header config))
	(fact
	"it formats the statement lines"
	(let [balanced-transactions [{:balance 1000 :amount 1000 :date (date "10/02/2016")}
	{:balance 2500 :amount 1500 :date (date "13/05/2016")}
	{:balance 2000 :amount -500 :date (date "14/08/2016")}]]
	(statement-format/format-statement-lines
	nice-reverse-format
	balanced-transactions)
	=> ["14/08/2016 || || 500.00 || 2000.00"
	"13/05/2016 || 1500.00 || || 2500.00"
	"10/02/2016 || 1000.00 || || 1000.00"])))))

view raw bank_kata_statement_format_test.clj hosted with ❤ by GitHub

You can check the rest of the tests and code in this GitHub repository.

Doing this kata I learned and practiced how to use Component.

I also learned how to use Midje's defrecord-openly and provided macros to mock protocols which helped me correct something I did wrong in another kata.