Functional Programming from OO perspective (Sayeret Lambda lecture)
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The document discusses functional programming concepts from an object-oriented perspective. It begins by defining functional programming and explaining why it is important to consider. It then covers concepts like immutable data, referential transparency, and avoiding side effects. The document also discusses how functional programming approaches simplify concepts like inversion of control, factories, security, and abstraction when compared to object-oriented approaches. It introduces functors, applicative functors, and monads as ways to describe working with values in a context. Finally, it discusses how type classes can be used to define common behaviors across different types.
 {
init
if (thereIsData)
f(getData)
close
}
}
service.witData(println)
9 WWW.TIKALK.COM](https://image.slidesharecdn.com/sayeretlambda-fplecture-111029050922-phpapp01/85/Functional-Programming-from-OO-perspective-Sayeret-Lambda-lecture-9-320.jpg)
![In OOP:
trait DataConsumer[T] {
def withData(data: Data): T
{
In FP:
Data => T
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![Option[X]
List[X]
Future[X]
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![trait Future[A] {
def get: A
{
def findUser(name: String): Future[User] = …
…
val future = findUser(userName)
val user = future.get
println(user)
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![trait Future[A] {
def onReady(f: A => Unit)
}
val future = findUser(userName)
future.onReady{user => println(user)}
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![trait Future[A] {
def map[B](f: A => B): Future[B]
{
val user = findUser(userName)
val age: Future[Int] = user.map{user => user.age}
26 WWW.TIKALK.COM](https://image.slidesharecdn.com/sayeretlambda-fplecture-111029050922-phpapp01/85/Functional-Programming-from-OO-perspective-Sayeret-Lambda-lecture-26-320.jpg)
![trait Future[A] {
def get: A
def map[B](f: A => B) = new Future[B] {
def get = f(Future.this.get)
{
{
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![Future[Family]
User => Family
joe.map{joe => jane.map{jane => marry(joe, jane)}}
User => Future[Family]
Future[Future[Family]]
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![Attempt I
trait Future[A] {
def apply[B, C](other: Future[B], f: (A, B) =>
C): Future[C]
}
joe.apply(jane, marry)
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![Attempt II
trait Future[A] {
...
def apply[B](f: Future[A => B]): Future[B]
{
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![Future[User => Family]
User => User => Family
joe.apply(jane.map((marry _).curried))
User => Future[Family]
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![val marry: Future[User => User => Family] = Future(marry)
val partial: Future[User => Family] = futureMarry.apply(joe)
val family: Future[Family] = partial.apply(jane)
Future(marry).apply(joe).apply(jane)
Future(marry)(joe)(jane)
Using apply compiles iff A is a function
38 WWW.TIKALK.COM](https://image.slidesharecdn.com/sayeretlambda-fplecture-111029050922-phpapp01/85/Functional-Programming-from-OO-perspective-Sayeret-Lambda-lecture-38-320.jpg)
![trait Future[A] {
def apply[B, C](b: Future[B])
(implicit ev: A <:< (B => C))
: Future[C]
{
object Future {
def apply[A](a: A): Future[A]
}
Future((marry _).curried)(joe)(jane)
41 WWW.TIKALK.COM](https://image.slidesharecdn.com/sayeretlambda-fplecture-111029050922-phpapp01/85/Functional-Programming-from-OO-perspective-Sayeret-Lambda-lecture-41-320.jpg)
![trait Future[A] {
def apply[B, C](b: Future[B])
(implicit ev: A <:< (B => C)) =
new Future[C] {
def get = Future.this.get(b.get)
{
{
object Future {
def apply[A](a: A) = new Future[A] {
def get = a
}
}
}
42 WWW.TIKALK.COM](https://image.slidesharecdn.com/sayeretlambda-fplecture-111029050922-phpapp01/85/Functional-Programming-from-OO-perspective-Sayeret-Lambda-lecture-42-320.jpg)
![Composing Special
Functions
def findUser(name: String): Future[User]
def findProfile(user: User): Future[Profile]
findProfile(findUser("joe"))
44 WWW.TIKALK.COM](https://image.slidesharecdn.com/sayeretlambda-fplecture-111029050922-phpapp01/85/Functional-Programming-from-OO-perspective-Sayeret-Lambda-lecture-44-320.jpg)
![trait Future[A] {
...
def flatMap[B](f: A => Future[B]): Future[B]
}
findUser("joe").flatMap(findProfile)
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![trait Future[A] {
...
def flatMap[B](f: A => Future[B]): Future[B] =
new Future[B] {
def get = f(Future.this.get).get
}
}
}
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![All Together
trait Context[A] {
def map[B](f: A => B): Context[B]
def apply[B, C](b: Context[B])
(implicit ev: A <:< (B => C)): Context[C]
def flatMap[B](f: A => Context[B]): Context[B]
}
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![• Future[A] is a monoid, if A is a monoid
• Binary operation: Future(A#•)
• (Future as applicative)
• Identity element: Future(A#identity)
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![trait Functor[F[_]] {
def map[A, B](fa: F[A], f: A => B): F[B]
{
implicit object FutureHasFunctor
extends Functor[Future] {
def map[A, B](t: Future[A], f: A => B) =
new Future[B] {
def get = f(t.get)
}
}
{
def age(name: String) = {
val functor = implicitly[Functor[Future]]
functor.map(findUser(name), {u: User => u.age})
}
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![def age(name: String)
(implicit functor: Functor[Future]) {
functor.map(findUser(name), {u: User => u.age})
}
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![def age[F[_] : Functor](name: String) {
functor.map(findUser(name), {u: User => u.age})
}
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![def sequence[A, AP[_] : Applicative]
(apas: List[AP[A]]): AP[List[A]]
val futureOfList = sequence(listOfFutures)
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![trait Applicative[AP[_]] {
def pure[A](a: A) : AP[A]
def apply[A, B](ap: AP[A => B], a: AP[A]): AP[B]
{
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![def sequence[A, AP[_] : Applicative](apas: List[AP[A]]):
AP[List[A]] = {
val applicative = implicitly[Applicative[AP]]
import applicative._
val cons = pure{x: A => xs: List[A] => x :: xs}
apas match {
case Nil => pure(Nil)
case apa :: apas =>
val recursion = sequence(apas) // AP[List[A]]
val partial = apply(cons, apa) // AP[List[A]=>List[A]]
apply(partial, recursion)
}
}
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![val m = Map[Int, Int]()
m.map{case (k, v) => k + v}
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(implicit bf: CanBuildFrom[Repr, B, That])
: That
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Functional Programming from OO perspective (Sayeret Lambda lecture)
- 1. Ittay Dror [email protected] @ittayd http://www.tikalk.com/blogs/ittayd Functional Programming From OOP perspective 26-Oct-11
- 2. What is FP? 2 WWW.TIKALK.COM
- 3. “a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data” 3 WWW.TIKALK.COM
- 4. • Functions as values • Immutable data structures • Mathematical models • Referential transparency 4 WWW.TIKALK.COM
- 5. Why Should We Care? 5 WWW.TIKALK.COM
- 6. Not everything is an object 6 WWW.TIKALK.COM
- 7. May Forget Nothing to service.init return? val data = service.get service.close May forget / call twice println(data) 7 WWW.TIKALK.COM
- 8. IOC 8 WWW.TIKALK.COM
- 9. class Service { def withData[T](f: Data => T) { init if (thereIsData) f(getData) close } } service.witData(println) 9 WWW.TIKALK.COM
- 10. Simplification 10 WWW.TIKALK.COM
- 11. In OOP: trait DataConsumer[T] { def withData(data: Data): T { In FP: Data => T 11 WWW.TIKALK.COM
- 12. Factories are partially applied functions 12 WWW.TIKALK.COM
- 13. val factory_: File => String => User = file => name => readFromFile(name) val factory: String => User = factory_(usersFile) Look ma, no interfaces 13 WWW.TIKALK.COM
- 14. Security 14 WWW.TIKALK.COM
- 15. Immutable data is sharable 15 WWW.TIKALK.COM
- 16. Instead of modifying, create new 16 WWW.TIKALK.COM
- 17. Abstraction 17 WWW.TIKALK.COM
- 18. Functors, Applicatives, Monads 18 WWW.TIKALK.COM
- 19. Usual way of describing: WTF?? 19 WWW.TIKALK.COM
- 20. OO way: Design Patterns 20 WWW.TIKALK.COM
- 21. Values in a context 21 WWW.TIKALK.COM
- 22. Option[X] List[X] Future[X] 22 WWW.TIKALK.COM
- 23. trait Future[A] { def get: A { def findUser(name: String): Future[User] = … … val future = findUser(userName) val user = future.get println(user) 23 WWW.TIKALK.COM
- 24. trait Future[A] { def onReady(f: A => Unit) } val future = findUser(userName) future.onReady{user => println(user)} 24 WWW.TIKALK.COM
- 25. Functor 25 WWW.TIKALK.COM
- 26. trait Future[A] { def map[B](f: A => B): Future[B] { val user = findUser(userName) val age: Future[Int] = user.map{user => user.age} 26 WWW.TIKALK.COM
- 27. val result = new ArrayList(list.size) for (i <- 0 to (list.size - 1)) { result += compute(list(i)) } result Vs. list.map(compute) 27 WWW.TIKALK.COM
- 28. class ComputeTask extends RecursiveTask { def compute = // split and call invokeAll... { val pool = new ForkJoinPool() val task = new ComputeTask() pool.invoke(task) Vs. list.par.map(compute) 28 WWW.TIKALK.COM
- 29. trait Future[A] { def get: A def map[B](f: A => B) = new Future[B] { def get = f(Future.this.get) { { 29 WWW.TIKALK.COM
- 30. def marry(man: User, woman: User): Family = ⠄⠄⠄ val joe = findUser("joe") val jane = findUser("jane") Get Joe and Jane married 30 WWW.TIKALK.COM
- 31. Future[Family] User => Family joe.map{joe => jane.map{jane => marry(joe, jane)}} User => Future[Family] Future[Future[Family]] 31 WWW.TIKALK.COM
- 32. Attempt I trait Future[A] { def apply[B, C](other: Future[B], f: (A, B) => C): Future[C] } joe.apply(jane, marry) 32 WWW.TIKALK.COM
- 33. It is easier to work with single argument functions 33 WWW.TIKALK.COM
- 34. Curried Functions val func: Int => Int => Int = a => b => a + b (marry _).curried 34 WWW.TIKALK.COM
- 35. Attempt II trait Future[A] { ... def apply[B](f: Future[A => B]): Future[B] { 35 WWW.TIKALK.COM
- 36. Future[User => Family] User => User => Family joe.apply(jane.map((marry _).curried)) User => Future[Family] 36 WWW.TIKALK.COM
- 37. We can do better 37 WWW.TIKALK.COM
- 38. val marry: Future[User => User => Family] = Future(marry) val partial: Future[User => Family] = futureMarry.apply(joe) val family: Future[Family] = partial.apply(jane) Future(marry).apply(joe).apply(jane) Future(marry)(joe)(jane) Using apply compiles iff A is a function 38 WWW.TIKALK.COM
- 39. Applicative Functors 39 WWW.TIKALK.COM
- 40. • Put value in context • Apply function in a context to value in a context 40 WWW.TIKALK.COM
- 41. trait Future[A] { def apply[B, C](b: Future[B]) (implicit ev: A <:< (B => C)) : Future[C] { object Future { def apply[A](a: A): Future[A] } Future((marry _).curried)(joe)(jane) 41 WWW.TIKALK.COM
- 42. trait Future[A] { def apply[B, C](b: Future[B]) (implicit ev: A <:< (B => C)) = new Future[C] { def get = Future.this.get(b.get) { { object Future { def apply[A](a: A) = new Future[A] { def get = a } } } 42 WWW.TIKALK.COM
- 43. Composing Functions that Return Value in a Context 43 WWW.TIKALK.COM
- 44. Composing Special Functions def findUser(name: String): Future[User] def findProfile(user: User): Future[Profile] findProfile(findUser("joe")) 44 WWW.TIKALK.COM
- 45. Monads 45 WWW.TIKALK.COM
- 46. trait Future[A] { ... def flatMap[B](f: A => Future[B]): Future[B] } findUser("joe").flatMap(findProfile) 46 WWW.TIKALK.COM
- 47. trait Future[A] { ... def flatMap[B](f: A => Future[B]): Future[B] = new Future[B] { def get = f(Future.this.get).get } } } 47 WWW.TIKALK.COM
- 48. All Together trait Context[A] { def map[B](f: A => B): Context[B] def apply[B, C](b: Context[B]) (implicit ev: A <:< (B => C)): Context[C] def flatMap[B](f: A => Context[B]): Context[B] } 48 WWW.TIKALK.COM
- 49. Laws 49 WWW.TIKALK.COM
- 50. Idiomatic “interface” for context classes 50 WWW.TIKALK.COM
- 51. Monoid • For type A to be (have) a Monoid: • Binary operation „•„: (A, A) => A • Identity element „∅‟: A 51 WWW.TIKALK.COM
- 52. • String is a Monoid (2 ways): • Binary operation: append or prepend • Identity element: “” • Int is a Monoid (2 ways): • Binary operation: + or * • Identity element: 0 or 1 52 WWW.TIKALK.COM
- 53. • Future[A] is a monoid, if A is a monoid • Binary operation: Future(A#•) • (Future as applicative) • Identity element: Future(A#identity) 53 WWW.TIKALK.COM
- 54. Monoids generalize folds 54 WWW.TIKALK.COM
- 55. Folds: reduce values to one • List(x,y,z) => ∅ • x • y • z • Option(x) => if Some ∅ • x else ∅ 55 WWW.TIKALK.COM
- 56. Unfolds: Create values from initial value and function • 1, if (a < 3) Some(a, a+1) else None • Some(1, 2), Some(2, 3), None • ∅•1•2•3 • If our monoid is a List: • Nil ++ List(1) ++ List(2) ++ List(3) • List(1,2,3) 56 WWW.TIKALK.COM
- 57. Since many things are monoids, we can have many generic algorithms 57 WWW.TIKALK.COM
- 58. Problems with subtyping: • Namespace pollution • Need to control the class • Describes behavior, not „is-a‟ • Sometimes ability is conditional: • Future is a monoid if A is a monoid • Sometimes there are different definitions • E.g., 2 monoids for integers • Maybe a class has several facets • E.g. context of 2 values 58 WWW.TIKALK.COM
- 59. Typeclass • Define an API • Create instance for each class that can conform to the API 59 WWW.TIKALK.COM
- 60. Java example: Comparator 60 WWW.TIKALK.COM
- 61. trait Functor[F[_]] { def map[A, B](fa: F[A], f: A => B): F[B] { implicit object FutureHasFunctor extends Functor[Future] { def map[A, B](t: Future[A], f: A => B) = new Future[B] { def get = f(t.get) } } { def age(name: String) = { val functor = implicitly[Functor[Future]] functor.map(findUser(name), {u: User => u.age}) } 61 WWW.TIKALK.COM
- 62. def age(name: String) (implicit functor: Functor[Future]) { functor.map(findUser(name), {u: User => u.age}) } 62 WWW.TIKALK.COM
- 63. def age[F[_] : Functor](name: String) { functor.map(findUser(name), {u: User => u.age}) } 63 WWW.TIKALK.COM
- 64. Generic Programming 64 WWW.TIKALK.COM
- 65. def sequence[A, AP[_] : Applicative] (apas: List[AP[A]]): AP[List[A]] val futureOfList = sequence(listOfFutures) 65 WWW.TIKALK.COM
- 66. trait Applicative[AP[_]] { def pure[A](a: A) : AP[A] def apply[A, B](ap: AP[A => B], a: AP[A]): AP[B] { 66 WWW.TIKALK.COM
- 67. It is NOT important if you don’t understand the next slide 67 WWW.TIKALK.COM
- 68. def sequence[A, AP[_] : Applicative](apas: List[AP[A]]): AP[List[A]] = { val applicative = implicitly[Applicative[AP]] import applicative._ val cons = pure{x: A => xs: List[A] => x :: xs} apas match { case Nil => pure(Nil) case apa :: apas => val recursion = sequence(apas) // AP[List[A]] val partial = apply(cons, apa) // AP[List[A]=>List[A]] apply(partial, recursion) } } 68 WWW.TIKALK.COM
- 69. Beyond 69 WWW.TIKALK.COM
- 70. val m = Map[Int, Int]() m.map{case (k, v) => k + v} 70 WWW.TIKALK.COM
- 71. def map[B, That](f: A => B) (implicit bf: CanBuildFrom[Repr, B, That]) : That 71 WWW.TIKALK.COM
- 72. ? 72 WWW.TIKALK.COM