Functional Kotlin with Arrow (jLove 2021)

What is a monad? Do I need a PhD to write in functional style? How do I do that in Kotlin? And other questions that you were afraid to ask.

In this talk, we'll walk through Arrow Core, the functional companion to the Kotlin standard library. We'll cover both the concepts that every software engineer may add to their project right now and more advanced structures, starting from non-empty list, going over Validated and Either followed by monad comprehension.

Transcript

1. Functional Kotlin with Arrow Karin-Aleksandra Monoid jLove, 2021
2. About Senior Software Engineer Background in Kotlin & Scala (both front and backend) Living in Berlin with two cats
3. Agenda oWhat is Arrow? oNonEmptyList oEither oMonad Comprehensions
4. What is Arrow?
5. What is Arrow? Core (more functional programming) FX (functional effects) Optics (for traversing and transforming immutable data structures) Meta (writing compiler plugins) FX Optics Meta Core
6. What is Arrow? Core (more functional programming) FX (functional effects) Optics (for traversing and transforming immutable data structures) Meta (writing compiler plugins) FX Optics Meta Core
7. NonEmptyList
8. Non-empty list Payment fields ​Type Is Required? Phone​ String(100)​ - Reference​ String(100)​ Required​ Amount​ Decimal​ Required​
9. Non-empty list Is JSON {“payments”: []} ok? 🤔 Payment fields ​Type Is Required? Phone​ String(100)​ - Reference​ String(100)​ Required​ Amount​ Decimal​ Required​
10. Non-empty list NonEmptyList.of() // does not compile NonEmptyList.of(1, 2, 3, 4, 5) // NonEmptyList<Int>
11. Non-empty list NonEmptyList.of() // does not compile NonEmptyList.of(1, 2, 3, 4, 5) // NonEmptyList<Int>
12. Either
13. Java Exceptions if (somethingWrong) throw new SomeException(...)
14. Java Exceptions - “Handling” in signature void testFun() throws SomeException
15. Java Exceptions - Handling with try try { testFun() } catch (SomeException e) { ... }
16. Java Exceptions IntStream.range(0, 10) .forEach(i -> testFun()) void testFun() throws SomeException
17. Java Exceptions IntStream.range(0, 10) .forEach(i -> testFun()) void testFun() throws SomeException
18. Java Exceptions IntStream.range(0, 10) .forEach(hideException(i -> testFun()));
19. Java Exceptions unhideSomeException(() -> IntStream.range(0, 10) .forEach( hideException(i -> testFun() )));
20. Kotlin Exceptions No more checked exceptions (like Scala, Groovy and everyone else)
21. Kotlin Exceptions (0 until 10).forEach { testFun() }
22. Null fun String.toIntOrNull(): Int? o Not expressive o ????? o null-checks
23. Result Type inline class Result<out T> : Serializable
24. Result Type fun deserialize(): Something { return try { doSomething() } catch (_: Exception) { try { doSomethingElse() } catch(_: Exception) { doSomethingElseAgain() }}... fun deserialize(): Something { return runCatching { doSomething() }.recoverCatching { doSomethingElse() }.getOrElse { doSomethingElseAgain() } }
25. Sealed Type sealed class Result class Success : Result() class FullMoonFailure : Result() class ElonMuskTweetedFailure : Result()
26. Either sealed class Either<out A, out B> Which can be: o Either.Left<A> o Either.Right<B>
27. Validated sealed class Validated<out A, out B> Which can be: o Validated.Invalid<A> o Validated.Valid<B>
28. Validated vs Either o Either stops at first fail, validated collects all: User(phone, name, age) User(“test”, XÆA-12 Musk, -10) o Either: invalid phone o Validated: invalid phone, name*, age * https://www.kalzumeus.com/2010/06/17/falsehoods-programmers-believe-about-names/
29. Ior sealed class Ior<out A, out B> Which can be: o Ior.Left<A> o Ior.Right<B> o Ior.Both<A,B>
30. Monad comprehensions
31. Arrow Monads https://chrisdone.com/posts/monads-are-burritos/
32. Arrow Monads inline fun <T, R> Iterable<T>.flatMap( transform: (T) -> Iterable<R> ): List<R>
33. Chaining - flatMap val data = getFromDB(...) .flatMap { user -> getFromDB(...).flatMap { stockInfo-> getFromDB(...).flatMap { historicalData-> PageInfo(...) }}}}}}}}} private fun <T> getFromDB(param: String): Either<ErrorResponse, T>
34. Chaining - monad comprehensions val data = either { val user = getFromDB(request.userId).bind() val stockInfo = getFromDB(request.stockId).bind() val historicalData = getFromDB(request.stockId).bind() // ... PageInfo(user, stockInfo, historicalData, ...) } return data private fun <T> getFromDB(param: String): Either<ErrorResponse, T>
35. Next steps o Core: Option, Eval, extentions o FX: suspend, parZip, parTraverse, raceN... o Optics