Functional Android

For the most part, programming in Android has meant living in the imperative programming world. Recently, many aspects of functional programming have become standard with the adoption of Kotlin and RxJava. What does it mean to use functional paradigm properties in our Android code, and how can it help us? In this talk you’ll learn some of the fundamentals of functional programming, and what this might look like on Android.

Transcript

1. FUNCTIONAL ANDROID
   Victoria Gonda
   @TTGonda
   Photo by taylor hernandez on Unsplash
   

   View Slide

2. @TTGonda
   WHO AM I?
   ➤ Android Engineer at Buffer
   ➤ Author for Ray Wenderlich
   ➤ Chicago
   ➤ Dancer
   

   View Slide

3. @TTGonda
   THANKS TO ANUP COWKUR
   ➤ His blog posts provided lots of help in the
   research of this topic.
   ➤ Functional Programming for Android
   Developers
   

   View Slide

4. @TTGonda
   WHY FUNCTIONAL PROGRAMMING?
   ➤ We’re seeing an increase of the use of the
   paradigm (Kotlin, RxJava, Λrrow)
   ➤ Features can be used to write cleaner, more
   expressive code
   ➤ Simplifies concurrency
   ➤ Easier to test
   Photo by Jennifer Pallian on Unsplash
   

   View Slide

5. @TTGonda
   WHAT IS FUNCTIONAL PROGRAMMING?
   ➤ Came from lambda calculus
   ➤ Mimics the evaluation of mathematical
   functions
   ➤ Easier to test
   ➤ Some key characteristics of FP
   ➤ Declarative
   ➤ Explicitness
   ➤ Concurrency
   ➤ Higher Order Functions
   ➤ Immutability
   Photo by Patrick Tomasso on Unsplash
   

   View Slide

6. @TTGonda
   WHAT ARE WE GOING TO TALK ABOUT?
   ➤ Pure functions
   ➤ No side effects
   ➤ Ordering
   ➤ Immutability
   ➤ Concurrency
   ➤ Higher order functions
   ➤ Error handling
   

   View Slide

7. @TTGonda
   PURE FUNCTIONS
   ➤ Always gives the same output for a given
   input
   ➤ Independent of the outside world
   ➤ No side effects
   ➤ Easy to test
   Photo by Toa Heftiba on Unsplash
   

   View Slide

8. @TTGonda
   PURE FUNCTIONS
   fun blend(ingredients: List): Milkshake {
   return Milkshake(milk.get(), icecream.get(), ingredients)
   }
   

   View Slide

9. @TTGonda
   PURE FUNCTIONS
   fun blend(milk: Milk, icecream: IceCream,
   ingredients: List): Milkshake {
   return Milkshake(milk, icecream, ingredients)
   }
   

   View Slide

10. @TTGonda
    NO SIDE EFFECTS
    ➤ Don’t change anything outside
    ➤ Lessens the dependence on context
    ➤ Eliminates surprises
    ➤ You guessed it, easier to test
    Photo by Annie Shelmerdine on Unsplash
    

    View Slide

11. @TTGonda
    NO SIDE EFFECTS
    fun blend(milk: Milk, icecream: IceCream,
    ingredients: List): Milkshake {
    return glasses.pop()
    .fill(Milkshake(milk, icecream, ingredients))
    }
    

    View Slide

12. @TTGonda
    NO SIDE EFFECTS
    fun blend(glass: Glass, milk: Milk, icecream: IceCream,
    ingredients: List): Milkshake {
    return glass.fill(Milkshake(milk, icecream, ingredients))
    }
    

    View Slide

13. @TTGonda
    ORDERING
    ➤ Things that can be executed in any order
    ➤ Something we get to do when we have pure
    functions
    ➤ This means they can also be concurrent!
    Photo by Gabriel Gurrola on Unsplash
    

    View Slide

14. @TTGonda
    ORDERING
    makeToast()
    mixEggs()
    fryBacon()
    

    View Slide

15. @TTGonda
    ORDERING
    fun makeToast() {
    cooks.pop()
    .prepare(Toast)
    }
    

    View Slide

16. @TTGonda
    ORDERING
    fun makeToast(cook: Cook) {
    cook.prepare(Toast)
    }
    

    View Slide

17. @TTGonda
    IMMUTABILITY
    ➤ No changing things
    ➤ Want something different, make a new one
    ➤ You know it won’t change on you
    ➤ Watch for mutable child objects
    Photo by Alireza Etemadi on Unsplash
    

    View Slide

18. @TTGonda
    IMMUTABILITY
    class Burger(var type: String)
    

    View Slide

19. @TTGonda
    IMMUTABILITY
    public class Burger {
    private String type;
    public Burger(String type) {
    this.type = type;
    }
    public final String getType() {
    return this.type;
    }
    public final void setType(String type) {
    this.type = type;
    }
    }
    

    View Slide

20. @TTGonda
    IMMUTABILITY
    val burger = Burger("veggie")
    burger.type = "beef"
    

    View Slide

21. @TTGonda
    IMMUTABILITY
    class Burger(val type: String)
    

    View Slide

22. @TTGonda
    IMMUTABILITY
    final public class Burger {
    final private String type;
    public Burger(String type) {
    this.type = type;
    }
    public final String getType() {
    return this.type;
    }
    }
    

    View Slide

23. @TTGonda
    IMMUTABILITY
    val burger = Burger("veggie")
    burger.type = "beef"
    

    View Slide

24. @TTGonda
    IMMUTABILITY
    class Burger(var type: String?)
    if (burger.type != null)
    burger.type.length
    

    View Slide

25. @TTGonda
    IMMUTABILITY
    class Burger(val type: String?)
    if (burger.type != null)
    burger.type.length
    

    View Slide

26. @TTGonda
    IMMUTABILITY
    class Burger(var type: String, val toppings: MutableList)
    val burger = Burger("veggie", mutableListOf())
    burger.toppings.add("onion")
    

    View Slide

27. @TTGonda
    IMMUTABILITY
    class Burger(var type: String, val toppings: List)
    val burger = Burger("veggie", listOf())
    

    View Slide

28. @TTGonda
    IMMUTABILITY
    final public class Burger {
    final private String type;
    final private List toppings;
    public Burger(String type, List toppings) {
    /* ... */
    }
    public final String getType() { /* ... */ }
    public final List getToppings() {
    List toppingsCopy = new ArrayList<>();
    toppingsCopy.addAll(toppings);
    return toppingsCopy;
    }
    }
    

    View Slide

29. @TTGonda
    CONCURRENCY
    ➤ We want concurrency, but it can be hard
    ➤ Using pure functions with no side effects
    and immutability helps
    ➤ We can do things on different threads,
    without worrying about unexpected
    behavior
    ➤ Locks aren’t required, so we don’t have to
    worry about deadlock
    Photo by ål nik on Unsplash
    

    View Slide

30. @TTGonda
    CONCURRENCY
    fun addToppings(burger: Burger, toppings: List)
    : Observable {
    burger.toppings.addAll(toppings)
    return Observable.just(burger)
    .subscribeOn(kitchenThread)
    }
    

    View Slide

31. @TTGonda
    CONCURRENCY
    Observable.merge(
    addToppings(burger, listOf("tomato")),
    addToppings(burger, listOf("lettuce")))
    .observeOn(deliveryThread)
    .subscribe { finishedBurger ->
    deliver(finishedBurger)
    }
    

    View Slide

32. @TTGonda
    CONCURRENCY
    fun addToppings(burger: ImmutableBurger, toppings: List)
    : Observable? {
    val newBurger = burger.copy(toppings = toppings)
    return Observable.just(newBurger)
    .subscribeOn(kitchenThread)
    }
    

    View Slide

33. @TTGonda
    HIGHER ORDER FUNCTIONS
    ➤ Functions as first class citizens
    ➤ Pass functions into a function
    ➤ Share logic the way you do an object
    ➤ Lose single abstract method interfaces
    Photo by Emmy Smith on Unsplash
    

    View Slide

34. @TTGonda
    HIGHER ORDER FUNCTIONS
    fun makePotatoes(potatoes: Potatoes,
    cookPotatoes: (Potatoes) -> CookedPotatoes) {
    val cleanPotatoes = washAndPeelPotatoes(potatoes)
    val cookedPotatoes = cookPotatoes(cleanPotatoes)
    eatPotatoes(cookedPotatoes)
    }
    

    View Slide

35. @TTGonda
    HIGHER ORDER FUNCTIONS
    makePotatoes(Potatoes(), ::fryPotatoes)
    makePotatoes(Potatoes()) {
    fryPotatoes(it)
    }
    makePotatoes(Potatoes(), {
    fryPotatoes(it)
    })
    makePotatoes(Potatoes(), { potatoes ->
    fryPotatoes(potatoes)
    })
    

    View Slide

36. @TTGonda
    HIGHER ORDER FUNCTIONS
    makePotatoes(Potatoes(), ::fryPotatoes)
    makePotatoes(Potatoes(), ::bakePotatoes)
    makePotatoes(Potatoes(), ::boilEm)
    makePotatoes(Potatoes(), ::mashEm)
    makePotatoes(Potatoes(), ::stickEmInAStew)
    

    View Slide

37. @TTGonda
    HIGHER ORDER FUNCTIONS
    val customer = Customer()
    customer.setOnFinishedEatingListener {
    clearDishes()
    bringCheck()
    }
    

    View Slide

38. @TTGonda
    ERROR HANDLING
    ➤ Everything is a value or computation
    ➤ Avoiding unexpected states
    ➤ Errors are treated as values
    Photo by chuttersnap on Unsplash
    

    View Slide

39. @TTGonda
    ERROR HANDLING
    deliver(burger)
    

    View Slide

40. @TTGonda
    ERROR HANDLING
    try {
    deliver(burger)
    } catch (error: DroppedPlateException) {
    makeNew(burger)
    }
    

    View Slide

41. @TTGonda
    ERROR HANDLING
    try {
    deliver(burger)
    deliver(pancakes)
    deliver(fries)
    deliver(hashbrowns)
    } catch (error: DroppedPlateException) {
    // ?????
    }
    

    View Slide

42. @TTGonda
    ERROR HANDLING
    try {
    deliver(burger)
    } catch (error: DroppedPlateException) {
    makeNew(burger)
    }
    try {
    deliver(pancakes)
    } catch (error: DroppedPlateException) {
    makeNew(pancakes)
    }
    try {
    deliver(fries)
    } catch (error: DroppedPlateException) {
    makeNew(fries)
    }
    try {
    deliver(hashbrowns)
    } catch (error: DroppedPlateException) {
    makeNew(hashbrowns)
    }
    

    View Slide

43. @TTGonda
    ERROR HANDLING
    val result = deliver(burger)
    if (!result.success) {
    makeNew(burger)
    }
    

    View Slide

44. @TTGonda
    ERROR HANDLING
    if (!deliver(burger).success) {
    makeNew(burger)
    }
    

    View Slide

45. @TTGonda
    TOOLS
    ➤ Kotlin programming language
    ➤ RxJava library
    ➤ Λrrow library
    Photo by Leti Kugler on Unsplash
    

    View Slide

46. @TTGonda
    { previousState, result ->
    when (result) {
    is FoldersResult.GetLastState -> previousState
    is FoldersResult.LoadFoldersTask -> {
    when {
    result.status == TaskStatus.SUCCESS ->
    FoldersUiModel.FoldersLoaded(result.folders)
    result.status == TaskStatus.FAILURE ->
    FoldersUiModel.Failed
    result.status == TaskStatus.IN_FLIGHT ->
    FoldersUiModel.InProgress
    else -> FoldersUiModel.Idle()
    }
    }
    }
    }
    

    View Slide

47. @TTGonda
    { previousState, result ->
    when (result) {
    is FoldersResult.GetLastState -> previousState
    is FoldersResult.LoadFoldersTask -> {
    when {
    result.status == TaskStatus.SUCCESS ->
    FoldersUiModel.FoldersLoaded(result.folders)
    result.status == TaskStatus.FAILURE ->
    FoldersUiModel.Failed
    result.status == TaskStatus.IN_FLIGHT ->
    FoldersUiModel.InProgress
    else -> FoldersUiModel.Idle()
    }
    }
    }
    }
    

    View Slide

48. @TTGonda
    { previousState, result ->
    when (result) {
    is FoldersResult.GetLastState -> previousState
    is FoldersResult.LoadFoldersTask -> {
    when {
    result.status == TaskStatus.SUCCESS ->
    FoldersUiModel.FoldersLoaded(result.folders)
    result.status == TaskStatus.FAILURE ->
    FoldersUiModel.Failed
    result.status == TaskStatus.IN_FLIGHT ->
    FoldersUiModel.InProgress
    else -> FoldersUiModel.Idle()
    }
    }
    }
    }
    

    View Slide

49. @TTGonda
    { previousState, result ->
    when (result) {
    is FoldersResult.GetLastState -> previousState
    is FoldersResult.LoadFoldersTask -> {
    when {
    result.status == TaskStatus.SUCCESS ->
    FoldersUiModel.FoldersLoaded(result.folders)
    result.status == TaskStatus.FAILURE ->
    FoldersUiModel.Failed
    result.status == TaskStatus.IN_FLIGHT ->
    FoldersUiModel.InProgress
    else -> FoldersUiModel.Idle()
    }
    }
    }
    }
    

    View Slide

50. @TTGonda
    { previousState, result ->
    when (result) {
    is FoldersResult.GetLastState -> previousState
    is FoldersResult.LoadFoldersTask -> {
    when {
    result.status == TaskStatus.SUCCESS ->
    FoldersUiModel.FoldersLoaded(result.folders)
    result.status == TaskStatus.FAILURE ->
    FoldersUiModel.Failed
    result.status == TaskStatus.IN_FLIGHT ->
    FoldersUiModel.InProgress
    else -> FoldersUiModel.Idle()
    }
    }
    }
    }
    

    View Slide

51. @TTGonda
    RESOURCES
    ➤ Functional Programming for Android Developers by Anup Cowkur
    ➤ Functional Reactive Programming with RxJava by Francisco González
    ➤ Functional Programming in Kotlin with Arrow by Jorge Castillo on Caster.io
    

    View Slide

52. THANKS!
    Victoria Gonda
    @TTGonda
    Photo by taylor hernandez on Unsplash
    

    View Slide