Treasure.map(): Functional programming in JVM-based languages

Transcript

1. 1 Treasure.Map(): Functional Programming in JVM-based languages Karin-Aleksandra Monoid GDSC KPI, 2022
2. 2 • Senior Software Engineer at CircleCI • Background in Kotlin & Scala (both front and backend) • Living in Potsdam with two cats • GDE in Kotlin About
3. 3 Why?
4. 4 “Why can we tell people about internals of JVM and no one bats an eye but should I only add a word “monad” and everyone is too scared because it’s so difficult?” – Conclusions on the speaker dinner/Joker 2021 Motivation
5. 5 Agenda Elements of functional programming Monads Error handling When to use?
6. 6 Languages • Kotlin • Java • Scala • Clojure • Groovy
7. 7 Elements of functional programming
8. 8 What is functional programming? https://blog.knoldus.com/
9. 9 Pure functions • Same input = same output • No “side-effects” • Not altering any global state
10. 10 Immutability • Java: records, final, vavr • Kotlin: val, read-only collection interfaces, Arrow, kotlinx.collections.immutable • Scala: vals, scala.collection.* and scala.collection.immutable.* • Clojure: everything but def • Groovy: @Immutable meta-annotation, asImmutable()
11. 11 True immutability vs reference immutability? Java: final List<Integer> list = Arrays.asList(1, 2, 3); We can’t reassign it like list = Arrays.asList(1, 2, 6); But can alter the list itself e.g. with set(0, 10) We can wrap with unmodifiableList or use List.of to begin with, but we don’t know by type whether provided list is mutable (ArrayList), semi-mutable (Arrays.asList) or unmodifiable. Also deep structures are not covered.
12. 12 Collection immutability: Groovy See: Java (but there is .asImmutable())
13. 13 Collection immutability: Scala scala.collection.* - you cannot modify but somebody else might be able to. scala.collection.immutable.* - nobody can modify collection itself scala.collection.mutable.* - you can modify this
14. 14 Collection immutability: Kotlin Collection, List, etc - you cannot modify but somebody else might be able to. MutableCollection, MutableList, etc - you can modify this ImmutableList (from kotlinx.collections.immutable) - nobody can modify collection/list
15. 15 Collection immutability: Clojure Everything is truly immutable
16. 16 Map (🐸, 🐸, 🐸).map(🐸 -> 🦄) => (🦄, 🦄, 🦄) Java items.stream().map(frog -> unicorn).toList() Groovy items.collect(frog -> unicorn) Kotlin items.map { frog -> unicorn } Scala items.map(frog => unicorn) Clojure (map (constantly unicorn) items)
17. 17 Filter (🐸, 🐸, 🦄, 🦄, 🐸, 🦄).filter(🐸?) => (🐸, 🐸, 🐸) Java items.stream().filter(Animal::isFrog).toList() Groovy items.findAll { it.isFrog() } Kotlin items.filter { it.isFrog() } Scala items.filter(isFrog(_)) Clojure (filter is-frog? items)
18. 18 Fold (👩, 👩, 👧, 👦).fold(+) => 󰔮 Java items.stream().collect(collector) Groovy items.inject(acc, …) Kotlin items.fold(acc) {...} Scala tems.fold(acc)(...) Clojure (r/fold function acc items)
19. 19 FlatMap (🟧, 🟨, 🟩, 🟦).flatMap(⬜ -> (🤍, ⚪)) => (🧡, 🟠, 💛, 🟡, 💚, 🟢, 💙, 🔵) Java items.stream.flatMap(...).toList() Groovy items.collectMany(...) Kotlin items.flatMap {...} Scala items.flatMap(...) Clojure (mapcat transform items)
20. 20 Monads
21. 21 A monad A monad is just a monoid in a category of the endofunctors
22. 22 Monads Do you know monads? Do you use a `List`? That’s a monad!
23. 23 Monads in the simple terms A type: • An instance of which can be created • Which has flatMap() function
24. 24 Monad comprehensions combined = for { a <- listA b <- listB } yield a * b Which is equal to combined = listA.flatMap(a => listB.flatMap(b => a * b))
25. 25 Monad comprehensions combined = for { a <- optionA b <- optionB } yield a * b Which is equal to combined = optionA.flatMap(a => optionB.flatMap(b => a * b))
26. 26 Error handling
27. 27 if (somethingWrong) throw new SomeException(...) Java Exceptions
28. 28 Java Exceptions - “Handling” in signature void testFun() throws SomeException
29. 29 Java Exceptions - Handling with try try { testFun() } catch (SomeException e) { ... }
30. 30 Java Exceptions IntStream.range(0, 10) .forEach(i -> testFun()) void testFun() throws SomeException
31. 31 Java Exceptions IntStream.range(0, 10) .forEach(i -> testFun()) void testFun() throws SomeException
32. 32 Java Exceptions IntStream.range(0, 10) .forEach(hideException(i -> testFun()));
33. 33 Java Exceptions unhideSomeException(() -> IntStream.range(0, 10) .forEach( hideException(i -> testFun() )));
34. 34 All other languages Checked exceptions are gone!
35. 35 Demo K: (0 until 10).forEach { testFun() } S: (0 until 10).foreach { testFun() } C: (run test-fun (range 10)) G: (1..<3).each { testFun() }
36. 36 How to handle problems: Groovy ¯\_(ツ)_/¯
37. 37 How to handle problems: Kotlin, safer null fun String.toIntOrNull(): Int?
38. 38 How to handle problems: Kotlin, Result type runCatching { doSomething() }
39. 39 How to handle problems: Kotlin, sealed types sealed class Result class SuccessfullyInvested : Result() class LostMoney : Result()
40. 40 How to handle problems: Scala, Option Something or none def toIntOption: Option[Int] Some(5) or None
41. 41 How to handle problems: Scala, Either val result: Either[DbError, User] = db.findUser(id) Left(UserNotFound) or Right(User(123))
42. 42 How to handle problems: Scala, Either Either is also a monad val userSummary = for { user <- db.findUser(id) balance <- db.findUserBalance(user.bankId) } yield UserSummary(user, balance)
43. 43 Either Kotlin+Arrow: Either<Failure, Success> Java+vavr: Either<Failure, Success>
44. 44 How to handle problems: Clojure ¯\_(ツ)_/¯
45. 45 How to handle problems: Clojure ¯\_(ツ)_/¯ REPL
46. 46 Frameworks and libraries Java, Groovy: vavr Kotlin: Arrow Scala: Cats, zio Clojure: just continue using REPL
47. 47 • You are using elements of FP already • Consider support of libraries • Consider team knowledge • Immutability may be slow-ish Conclusions