Hands-on Arrow

Transcript

1. Karin-Aleksandra Monoid GDG Nuremberg, 2021 Hands-on Arrow (How we filled Kotlin standard library gaps without converting code to Scala)
2. Agenda • What is Arrow? • NonEmptyList • Either • Monad Comprehensions
3. What is Arrow?
4. What is Arrow? 4 4 Core FX Optics Meta
5. What is Arrow? 5 5 Core FX Optics Meta
6. NonEmptyList
7. Non-empty list Is JSON {“payments”: []} ok? 7 7 Payments List of items (Phone, Reference, Amount) Phone ... String(100) Required Reference ... String(100) Required Amount ... Decimal Required
8. Non-empty list NonEmptyList.of() // does not compile NonEmptyList.of(1, 2, 3, 4, 5) // NonEmptyList<Int> 8 8
9. Non-empty list NonEmptyList.of() // does not compile NonEmptyList.of(1, 2, 3, 4, 5) // NonEmptyList<Int> 9 9
10. Either
11. Java Exceptions if (somethingWrong) throw new SomeException(...) 11 11
12. Java Exceptions - “Handling” in signature void testFun() throws SomeException 12 12
13. Java Exceptions - Handling with try try { testFun() } catch (SomeException e) { ... } 13 13
14. Java Exceptions IntStream.range(0, 10) .forEach(i -> testFun()) 14 14
15. Java Exceptions IntStream.range(0, 10) .forEach(i -> testFun()) 15 15
16. Java Exceptions IntStream.range(0, 10) .forEach(hideException(i -> testFun())); 16 16
17. Java Exceptions unhideSomeException(() -> IntStream.range(0, 10) .forEach( hideException(i -> testFun() ))); 17 17
18. Kotlin Exceptions • No more checked exceptions (like Scala, Groovy and everyone else) 18 18
19. Kotlin Exceptions (1 until 10).forEach { testFun() } 19 19
20. Null fun String.toIntOrNull(): Int? • Not expressive • ????? • null-checks 20 20
21. Result Type inline class Result<out T> : Serializable 21 21
22. Result Type 22 22 fun deserialize(): Something { return try { doSomething() } catch (_: Exception) { try { doSomethingElse() } catch(_: Exception) { doSomethingElseAgain() }}... fun deserialize(): Something { return runCatching { doSomething() }.recoverCatching { doSomethingElse() }.getOrElse { doSomethingElseAgain() } }
23. Sealed Type sealed class Result class Success : Result() class FullMoonFailure : Result() class ElonMuskTweetedFailure : Result() 23 23
24. Either sealed class Either<out A, out B> 24 24
25. Validated sealed class Validated<out A, out B> 25 25
26. Validated vs Either • Either stops at first fail, validated collects all: User(phone, name, age) User(“test”, XÆA-12 Musk, -10) • Either: invalid phone • Validated: invalid phone, name*, age 26 26 * https://www.kalzumeus.com/2010/06/17/falsehoods-programmers-believe-about-names/
27. Monad comprehensions
28. Require requireNotNull(request.userId) requireNotNull(request.transaction.id) 28 28
29. Require public inline fun <T : Any> requireNotNull (value: T?): T { contract { returns() implies (value != null) } return requireNotNull(value) { "Required value was null." } } 29 29
30. if-else-if-else-if-else if(request.userId == null) return buildExceptionResponse(...) if(request.transaction.id == null) return buildExceptionResponse(...) ... 30 30
31. Arrow Monads 31 31 https://chrisdone.com/posts/monads-are-burritos/
32. Arrow Monads 32 32 inline fun <T, R> Iterable<T>.flatMap( transform: (T) -> Iterable<R> ): List<R>
33. Chaining - flatMap val response = checkForNull(...) .flatMap { userId -> checkForNull(...).flatMap { paymentId -> checkForNull(...).flatMap { items -> checkForNull(...).flatMap { status -> checkForNull(...).map { ( … )}}}}}}}}} 33 33
34. Chaining - monad comprehensions val response = Either.fx { val userId = checkForNull(request.userId).bind() val paymentId = checkForNull(request.transaction.id).bind() val items = checkForNull(request.transaction.items).bind() val status = client.checkPaymentStatus(userId,...) // ... } return response.getOrHandle { it } private fun <T : Any> checkForNull(param: T?, paramName: String): Either<ErrorResponse, T> 34 34