Kotlin Sequences Deep Dive Phil Shadlyn | @physphil November 16, 2020 #dcAmericas

What is a Sequence?

What are Kotlin Sequences? “A sequence is an iterable type that can be operated upon without creating unnecessary intermediate collections, by running all applicable operations on each element before moving onto the next.” https://typealias.com/guides/kotlin-sequences-illustrated-guide/

Great... What does that mean?

An Example ● TV Show reference app ● Display 10 favourites and their rating ● Consume API of 30,000 TV Shows

data class NetworkTvShow( val id: String val title: String, val year: String?, val ratings: List?, val favourite: Boolean, val characters: List? ) data class TvShow( val title: String, val year: Int, val rating: Int, val favourite: Boolean ) Server model App model

Order of operations: ● Get list of shows from API ● Filter by favourite ● Convert network model to domain model ● Take first 10 elements ● Display in list

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList)

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList)

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList) New collection

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList) New collection

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList) New collection

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) view.displayFavouriteList(favouritesList)

val favouritesList = tvShowApi.getShowsList() .filter { it.favourite } .map { it.toTvShow() } .take(10) 30,000 shows 31,941 operations 30,000 operations 1,931 operations 10 operations 3 new collections

val favouritesList = tvShowApi.getShowsList() .asSequence() .filter { it.favourite } .map { it.toTvShow() } .take(10) .toList() 30,000 shows 39 operations 19 operations 10 operations 10 operations

Whoa… what just happened?

What are Kotlin Sequences? “A sequence is an iterable type that can be operated upon without creating unnecessary intermediate collections, by running all applicable operations on each element before moving onto the next.” https://typealias.com/guides/kotlin-sequences-illustrated-guide/

The Wire Favourite Seinfeld Favourite Breaking Bad Game of Thrones Favourite Mad Men filter { it.favourite } The Wire Seinfeld Game of Thrones map { it.toTvShow() } The Wire Seinfeld Game of Thrones take(10) The Wire Seinfeld Game of Thrones … … … …

Collection Processing ● Operations executed eagerly ● Operate on every element ● Intermediate collections

The Wire Favourite Seinfeld Favourite Breaking Bad Game of Thrones Favourite Mad Men filter { it.favourite } The Wire Seinfeld Game of Thrones map { it.toTvShow() } The Wire Seinfeld Game of Thrones take(10) The Wire Seinfeld Game of Thrones … …

Sequence Processing ● Operations executed lazily ● One element at a time ● Chain executed with terminal operator ● No intermediate collections ● Can short circuit

Intermediate Operators ● Return a Sequence ● Build out operation chain ● filter, map, take, etc...

val favouritesList = tvShowApi.getShowsList() .asSequence() .filter { it.favourite } .map { it.toTvShow() } .take(10) .toList()

Terminal Operators ● Return a result ● Trigger execution of operation chain ● toList(), count(), maxOf(), etc...

val favouritesList = tvShowApi.getShowsList() .asSequence() .filter { it.favourite } .map { it.toTvShow() } .take(10) .toList()

Sequence Summary ● Operations executed lazily ● Execute chain on each item ● Chain executed with terminal operator ● No intermediate collections ● Can short circuit

How can I create a Sequence?

Creating Sequences /** * Creates a [Sequence] instance that wraps the original * collection returning its elements when being iterated. */ public fun Iterable.asSequence(): Sequence val showSequence = tvShowApi.getShowsList().asSequence()

Creating Sequences /** * Creates a sequence that returns the specified values. */ public fun sequenceOf(vararg elements: T): Sequence val showSequence = sequenceOf(TheWire, Seinfeld, GameOfThrones)

Creating Sequences /** * Returns a sequence defined by the starting value [seed] * and the function [nextFunction] */ public fun generateSequence( seed: T?, nextFunction: (T) -> T? ): Sequence val incrementalSequence = generateSequence(1) { it + 1 }

Should I use a Collection or Sequence?

Use a Collection if: ● Few items ● Few operators ● Stateful operations ● Missing Sequence operators

Use a Sequence if: ● Many items ● Many operations ● Stateless operations ● Short circuits

Want to Learn More? ● Kotlin Sequences docs https://kotlinlang.org/docs/reference/sequences.html ● Kotlin Sequences: An Illustrated Guide https://typealias.com/guides/kotlin-sequences-illustrated-guide/ ● Kotlin Sequence Tutorial https://winterbe.com/posts/2018/07/23/kotlin-sequence-tutorial/ ● Kotlin Vocabulary: Collections and Sequences https://www.youtube.com/watch?v=77hfjIYwouw

Thank You! Phil Shadlyn | @physphil November 16, 2020 #dcAmericas