DSL - The Kotlin Way

DSLs IN A KOTLIN WAY Ubiratan Soares November / 2019

SELECT * FROM accounts WHERE id=12345

SELECT * FROM accounts WHERE id=12345 " "

String sql = SQLiteQueryBuilder .select("*") .from("accounts") .where("id=12345") .toString(); https://github.com/alexfu/SQLiteQueryBuilder

<!DOCTYPE html> <html> <body> <h1>Hey!!"h1> <div>Ho!<span>Lets go!!"span> !"div> !"body> !"html>

val tree = createHTMLDocument().html { body { h1 { +"Hey"
} div { +"Ho!" span { +"Lets go!" } } } } https://github.com/Kotlin/kotlinx.html

TRAILLING NOTATION

Trailling Lamba, at your service!

“A small step for the IDE, but a giant leap
for DSLs” - Me, 2019

INFIX NOTATION

class Extractor { fun ﬁrstLetter(target: String) = target[0].toString() } val
k = Extractor().ﬁrstLetter("Kotlin")

class Extractor { infix fun firstLetter(target: String) = target[0].toString() }
val k = Extractor() firstLetter "Kotlin" NO PUNCTUATION!

class Extractor { infix fun firstLetter(target: String) = target[0].toString() }
val k = Extractor() firstLetter "Kotlin" NO PARENTHESIS

object Extract { infix fun firstLetterOf(target: String) = target[0].toString() }
val first = Extract firstLetterOf “Awesome"

object extract { infix fun firstLetterOf(target: String) = target[0].toString() }
val first = extract firstLetterOf “Awesome"

“PLAIN ENGLISH” STATEMENTS

initials from “Kotlin" and "Rocks" gluedWith dots !" K.R

object initials { inﬁx fun from(target: String) = Tinker( collected
= mutableListOf(), incoming = target ) }

class Tinker( private val collected: MutableList<String>, incoming: String ) {
}

init { collected += incoming.ﬁrst().toString() } }

init { collected += incoming.ﬁrst().toString() } inﬁx fun and(another: String) = Tinker(collected, another) }

class Tinker(#$%) { inﬁx fun gluedWith(option: Joiner) = TODO() }

class Tinker(#$%) { inﬁx fun gluedWith(option: Joiner) = collected.reduce {
previous, letter &' } }

previous, letter &' val next = when (option) { is nothing &' letter is dots &' ".$letter" } } }

previous, letter &' val next = when (option) { is nothing &' letter is dots &' ".$letter" } "$previous$next" } }

class InitialsTinker( private val collected: MutableList<String>, incoming: String ) {
init { collected += incoming.first().toString() } infix fun and(another: String) = InitialsTinker(collected, another) infix fun gluedWith(option: Joiner) = collected.reduce { previous, letter &' val next = when (option) { is nothing &' letter is dots &' ".$letter" } "$previous$next" } }

sealed class Joiner object nothing : Joiner() object dots :
Joiner()

val dsl = initials from "Domain" and "Speciﬁc" and "Language"
gluedWith nothing //DSL

val dsl = initials from "Domain" and "Speciﬁc" and "Language"
gluedWith dots //D.S.L

IN THE WILD https://github.com/kotlintest KotlinTest custom matchers class KotlinTestDemo :
StringSpec({ "assert string length" { "DSL".length shouldBe 3 } })

OPERATOR OVERLOADS

val list = mutableListOf(1, 2, 3) list += 4 /()
* Adds the speciﬁed [element] to this mutable collection. *+ @kotlin.internal.InlineOnly public inline operator fun <T> MutableCollection<in T>.plusAssign(element: T) { this.add(element) }

OPERATING WITH java.util.Date

fun addMonths(target: Date, months: Int): Date { // TODO }

fun addMonths(target: Date, months: Int): Date { val calendar =
Calendar.getInstance() }

Calendar.getInstance() calendar.time = target calendar.add(Calendar.MONTH, months) }

Calendar.getInstance() calendar.time = target calendar.add(Calendar.MONTH, months) return calendar.time }

Calendar.getInstance() calendar.time = target calendar.add(Calendar.MONTH, months) return calendar.time } val twoMonthsLater = addMonths(Date(), 2)

fun subtractDays(target: Date, days: Int): Date { val calendar =
Calendar.getInstance() calendar.time = target calendar.add(Calendar.DAY_OF_YEAR, -days) return calendar.time } val oneWeekAgo = subtractDays(Date(), 7)

fun evaluateDates() { val atPast = Date() - 2.days val
atFuture = Date() + 2.years }

data class DateIncrement( val dateField : Int, val amount: Int
) val Int.days: DateIncrement get() = DateIncrement(Calendar.DAY_OF_YEAR, this) val Int.months: DateIncrement get() = DateIncrement(Calendar.MONTH, this) val Int.years: DateIncrement get() = DateIncrement(Calendar.YEAR, this)

fun Date.applyIncrement( increment: DateIncrement, operation: IncrementOperation): Date = with(Calendar.getInstance()) {
// TODO }

val (ﬁeld, amount) = increment }

val (ﬁeld, amount) = increment val translatedAmount = when(operation) { DECREASE &' -amount INCREASE &' amount } }

val (ﬁeld, amount) = increment val translatedAmount = when(operation) { DECREASE &' -amount INCREASE &' amount } time = this@applyIncrement }

val (ﬁeld, amount) = increment val translatedAmount = when(operation) { DECREASE &' -amount INCREASE &' amount } time = this@applyIncrement add(ﬁeld, translatedAmount) }

val (ﬁeld, amount) = increment val translatedAmount = when(operation) { DECREASE &' -amount INCREASE &' amount } time = this@applyIncrement add(ﬁeld, translatedAmount) time }

operator fun Date.plus(increment: DateIncrement): Date = applyIncrement(increment, INCREASE) operator fun
Date.minus(increment: DateIncrement): Date = applyIncrement(increment, DECREASE) val atPast = Date() - 2.days val atFuture = Date() + 2.years enum class IncrementOperation { INCREASE, DECREASE }

A PROBLEM LIKE ANDROID SPANNABLES

val spannable = SpannableString("Pain is not enough!") spannable.setSpan( ForegroundColorSpan(Color.RED), 0,
3, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE ) spannable.setSpan( StyleSpan(BOLD), 12, spannable.length, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE ) // plain old SpannableString.java

val spanned = buildSpannedString { bold { append("This") } append("
is ") italic { append("ﬁne") } } // Shiny new SpannableStringBuilder.kt

,- SomeActivity.kt val dsl = bold("DSLs") + " are "
+ italic("awesome") labelHello.text = dsl

fun bold(given: CharSequence) = TODO() span(given, StyleSpan(Typeface.BOLD)) fun italic(given: CharSequence)
= TODO() span(given, StyleSpan(Typeface.ITALIC)) val dsl = bold("DSLs") + " are " + italic("awesome")

fun bold(given: CharSequence) = span(given, StyleSpan(Typeface.BOLD)) fun italic(given: CharSequence) =
span(given, StyleSpan(Typeface.ITALIC)) val dsl = bold("DSLs") + " are " + italic("awesome")

fun span(target: CharSequence, style: Any) : SpannableString =

fun span(target: CharSequence, style: Any) : SpannableString = when (target)
{ }

{ is String &' SpannableString(target) }

{ is String &' SpannableString(target) is SpannableString &' target }

{ is String &' SpannableString(target) is SpannableString &' target else &' throw CannotBeSpanned }

{ is String &' SpannableString(target) is SpannableString &' target else &' throw CannotBeSpanned }.let { spannable &' }

{ is String &' SpannableString(target) is SpannableString &' target else &' throw CannotBeSpanned }.let { spannable &' spannable.apply { setSpan(style, 0, length, SPAN_EXCLUSIVE_EXCLUSIVE) } }

{ is String &' SpannableString(target) is SpannableString &' target else &' throw CannotBeSpanned }.let { spannable &' spannable.apply { setSpan(style, 0, length, SPAN_EXCLUSIVE_EXCLUSIVE) } } object CannotBeSpanned : IllegalArgumentException()

operator fun SpannableString.plus(s: SpannableString) = SpannableString(TextUtils.concat(this, s)) operator fun SpannableString.plus(s:
String) = SpannableString(TextUtils.concat(this, s)) val spanned = bold("DSLs") + " are " + italic("awesome")

IN THE WILD https://github.com/ReactiveX/RxKotlin RxKotlin CompositeDisposable enhancement private val composite
= CompositeDisposable() private val stream = Observable.just(1, 2, 3) composite += stream.subscribe()

LAMBDAS + RECEIVERS

THE END OF A HAPPY HISTORY

data class Person( var name: String, var married: Boolean )

fun Person.freeAgain() { this.married = false }

fun Person.freeAgain() { this.married = false } val changeToSingle = Person./freeAgain

fun theEndOfHistory() { val unhappy = Person("Alice Rodriguez", true) val
optimistic = Person("Bob Rodriguez", true) }

optimistic = Person("Bob Rodriguez", true) unhappy.freeAgain() println(unhappy) }

optimistic = Person("Bob Rodriguez", true) unhappy.freeAgain() println(unhappy) changeToSingle(optimistic) println(optimistic) }

val changeToSingle = Person./freeAgain val unhappy = Person("Alice Rodriguez", true)
val optimistic = Person("Bob Rodriguez", true) marriedNoMore(unhappy) changeToSingle(optimistic) val marriedNoMore : Person.() &' Unit = { this.married = false } Called in the same way !!!

fun theTrueEndForAlice(block : Person.() &' Unit) { val newPerson =
Person("Alice", false) newPerson.block() println(newPerson) }

fun theTrueEndForAlice(block : Person.() &' Unit) { val newPerson =
Person("Alice", false) newPerson.block() println(newPerson) } fun main() { theTrueEndForAlice { name = "Alice Springs" } } Trailling Notation !!!!

fun theTrueEndForAlice(block : Person.() &' Unit) { println(Person("Alice", false).apply(block)) }
fun main() { theTrueEndForAlice { name = "Alice Springs" } }

The secret sauce A Lambda extension used as the last
argument of a high order function (plus trailling notation !)

TYPE-SAFE BUILDERS

fun shareMyCatalog() { val myCatalog: Catalog = TODO() println(myCatalog) }
data class Movie( val title: String, val rating: Int ) data class Catalog( val movies: List<Movie>, val updatedAt: Date )

data class Catalog( val movies: List<Movie>, val updatedAt: Date )
fun shareMyCatalog() { val myCatalog: Catalog = TODO() println(myCatalog) }

class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun build() = Catalog(movies, updatedAt) } fun shareMyCatalog() { val myCatalog: Catalog = TODO() println(myCatalog) }

class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun build() = Catalog(movies, updatedAt) } fun catalog(block: CatalogBuilder.() &' Unit) = CatalogBuilder().apply(block).build() fun shareMyCatalog() { val myCatalog = catalog { } println(myCatalog) }

class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun build() = Catalog(movies, updatedAt) } fun catalog(block: CatalogBuilder.() &' Unit) = CatalogBuilder().apply(block).build() fun shareMyCatalog() { val myCatalog = catalog { updatedAt = Date() - 1.days } println(myCatalog) }

data class Movie( val title: String, val rating: Int )
fun shareMyCatalog() { val myCatalog = catalog { updatedAt = Date() - 1.days } println(myCatalog) }

class MovieBuilder { var title: String = "" var rating: Int = 0 fun build() = Movie(title, rating) } fun shareMyCatalog() { val myCatalog = catalog { updatedAt = Date() - 1.days } println(myCatalog) }

class MovieBuilder { var title: String = "" var rating: Int = 0 fun build() = Movie(title, rating) } class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun build() = Catalog(movies, updatedAt) } fun shareMyCatalog() { val myCatalog = catalog { updatedAt = Date() - 1.days } println(myCatalog) }

class MovieBuilder { var title: String = "" var rating: Int = 0 fun build() = Movie(title, rating) } class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun movie(block: MovieBuilder.() &' Unit) { val movie = MovieBuilder().apply(block).build() movies += movie } fun build() = Catalog(movies, updatedAt) } fun shareMyCatalog() { val myCatalog = catalog { updatedAt = Date() - 1.days } println(myCatalog) }

fun shareMyCatalog() { val myCatalog = catalog { updatedAt =
Date() - 1.days } println(myCatalog) } data class Catalog( val movies: List<Movie>, val updatedAt: Date ) data class Movie( val title: String, val rating: Int ) class MovieBuilder { var title: String = "" var rating: Int = 0 fun build() = Movie(title, rating) } class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun movie(block: MovieBuilder.() &' Unit) { movies += MovieBuilder().apply(block).build() } fun build() = Catalog(movies, updatedAt) }

Date() - 1.days movie { title = “TaxiDriver" rating = 9 } } println(myCatalog) } data class Catalog( val movies: List<Movie>, val updatedAt: Date ) data class Movie( val title: String, val rating: Int ) class MovieBuilder { var title: String = "" var rating: Int = 0 fun build() = Movie(title, rating) } class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun movie(block: MovieBuilder.() &' Unit) { movies += MovieBuilder().apply(block).build() } fun build() = Catalog(movies, updatedAt) }

Date() - 1.days movie { title = “Taxi Driver" rating = 9 } movie { title = "The GoodFellas" rating = 10 } } println(myCatalog) } data class Catalog( val movies: List<Movie>, val updatedAt: Date ) data class Movie( val title: String, val rating: Int ) class MovieBuilder { var title: String = "" var rating: Int = 0 fun build() = Movie(title, rating) } class CatalogBuilder { var updatedAt: Date = Date() var movies: List<Movie> = mutableListOf() fun movie(block: MovieBuilder.() &' Unit) { movies += MovieBuilder().apply(block).build() } fun build() = Catalog(movies, updatedAt) }

I ﬁnally understand !!

IN THE WILD (I) Kotlinx.HTML val tree = createHTMLDocument().html {
body { h1 { +"Hey" } div { +"Ho!" span { +"Lets go!" } } } } https://github.com/Kotlin/kotlinx.html

IN THE WILD (II) Ktor val server = embeddedServer(Netty, port
= 8080) { routing { get("/") { call.respondText("Hello World!", ContentType.Text.Plain) } } } server.start(wait = true) https://ktor.io/

IN THE WILD (III) Spek object SetFeature: Spek({ Feature("Set") {
Scenario("adding items") { When("adding foo") { set.add(“foo") } Then("it should have a size of 1") { assertEquals(1, set.size) } } } https://spekframework.org

FINAL  REMARKS

Kotlin + DSLs

CALL TO ACTION ! • Learn more about invoking instances
in Kotlin • Learn about scoping with @DSLMarker • Design your own DSLs for fun and proﬁt !

UBIRATAN SOARES Brazilian Computer Scientist Senior Software Engineer @ N26
GDE for Android and Kotlin @ubiratanfsoares ubiratansoares.dev

https://speakerdeck.com/ubiratansoares

THANK YOU Do you want to write awesome Kotlin? N26
is hiring! https://n26.com/en/careers

DSL - The Kotlin Way

DSL - The Kotlin Way

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