Applying FP Patterns - Scala.io 2016

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Intro Composition Monoids Errors Conclusion Applying FP Patterns Markus Hauck (@markus1189) Scala.IO, 2016 Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 1

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Intro Composition Monoids Errors Conclusion Introduction FP becomes more popular there are many useful patterns Monoids Better Error Handling Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 2

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Intro Composition Monoids Errors Conclusion Lego vs Duplo pictures from shop.lego.com Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 3

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Intro Composition Monoids Errors Conclusion Lego vs Duplo pictures from shop.lego.com Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 4

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Intro Composition Monoids Errors Conclusion Lego vs Duplo Duplo favours large specialized building blocks blocks tend to be too big limited reuse Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 5

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Intro Composition Monoids Errors Conclusion Lego vs Duplo Duplo favours large specialized building blocks blocks tend to be too big limited reuse Lego focuses on small composable building blocks blocks can conveniently be reused for other purposes limited use of specialized building blocks Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 5

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Intro Composition Monoids Errors Conclusion Lego vs Duplo Duplo favours large specialized building blocks blocks tend to be too big limited reuse Lego focuses on small composable building blocks blocks can conveniently be reused for other purposes limited use of specialized building blocks OO tends to be like Duplo, FP tends to be like Lego Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 5

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Intro Composition Monoids Errors Conclusion Monoids intuition: “combine stuﬀ” you can create values from thin air via Monoid.empty combine two values via Monoid.combine / |+| additionally: laws (don’t write buggy implementations) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 6

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Intro Composition Monoids Errors Conclusion Monoid Laws // 1) left identity empty |+| x === x // 2) right identity x |+| empty === x // 3) associative x |+| (y |+| z) === (x |+| y) |+| z Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 7

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Intro Composition Monoids Errors Conclusion Monoid Typeclass trait Monoid[A] { def empty: A def combine(lhs: A, rhs: A): A // infix operator: |+| } implicit val plus: Monoid[Int] = new Monoid[Int] { def empty: Int = 0 def combine(lhs: Int, rhs: Int): Int = lhs + rhs } object Monoid { def apply[A:Monoid]: Monoid[A] = implicitly } Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 8

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Intro Composition Monoids Errors Conclusion Using Our Monoid > Monoid[Int].empty 0 > Monoid[Int].combine(1,2) 3 > 1 |+| 2 3 > 42 |+| Monoid[Int].empty 42 > List(1,2,3).foldLeft(Monoid[Int].empty)(_ |+| _) 6 Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 9

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Intro Composition Monoids Errors Conclusion More Monoids Monoid instance not unique: addition / multiplication / min / max most collections are Monoids: List / Vector / Set let’s see some more examples Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 10

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Intro Composition Monoids Errors Conclusion Monoid Zoo List[A] is a Monoid Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 11

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Intro Composition Monoids Errors Conclusion Monoid Zoo List[A] is a Monoid A => B if B is a Monoid Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 11

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Intro Composition Monoids Errors Conclusion Monoid Zoo List[A] is a Monoid A => B if B is a Monoid (A,B) if A and B are Monoids Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 11

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Intro Composition Monoids Errors Conclusion Monoid Zoo List[A] is a Monoid A => B if B is a Monoid (A,B) if A and B are Monoids Future[A] if A is a Monoid Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 11

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Intro Composition Monoids Errors Conclusion Monoid Zoo List[A] is a Monoid A => B if B is a Monoid (A,B) if A and B are Monoids Future[A] if A is a Monoid Map[A,B] if B is a Monoid Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 11

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Intro Composition Monoids Errors Conclusion Monoid Zoo List[A] is a Monoid A => B if B is a Monoid (A,B) if A and B are Monoids Future[A] if A is a Monoid Map[A,B] if B is a Monoid val m1 = Map("as" -> 21, "bs" -> 4) val m2 = Map("as" -> 21, "cs" -> 2) m1 |+| m2 // Map("as" -> 42, "bs" -> 4, "cs" -> 2) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 11

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Intro Composition Monoids Errors Conclusion Monoids Compose (Lego Principle) Config => A Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 12

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Intro Composition Monoids Errors Conclusion Monoids Compose (Lego Principle) Config => A Config => Future[A] Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 12

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Intro Composition Monoids Errors Conclusion Monoids Compose (Lego Principle) Config => A Config => Future[A] Config => Future[Map[String,A]] Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 12

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Intro Composition Monoids Errors Conclusion Monoids Compose (Lego Principle) Config => A Config => Future[A] Config => Future[Map[String,A]] Config => Future[Map[String,(A,B)]] Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 12

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Intro Composition Monoids Errors Conclusion Monoids Compose (Lego Principle) Config => A Config => Future[A] Config => Future[Map[String,A]] Config => Future[Map[String,(A,B)]] Config => Future[Map[String,(A,Option[B])]] Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 12

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Intro Composition Monoids Errors Conclusion Monoids Compose (Lego Principle) Config => A Config => Future[A] Config => Future[Map[String,A]] Config => Future[Map[String,(A,B)]] Config => Future[Map[String,(A,Option[B])]] Config => Future[Map[String,(Set[A],Option[B])]] Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 12

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Intro Composition Monoids Errors Conclusion I thought this was about applying patterns! Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 13

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Intro Composition Monoids Errors Conclusion analysis of a potentially huge text calculate metrics over text word count char count min/max word length avg word length . . . (be ﬂexible) goal: single traversal ↔ easy composition Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 14

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Intro Composition Monoids Errors Conclusion RDDs and Folds abstract class RDD[T] { /** * Aggregate the elements of each partition, * and then the results for all the partitions, * using a given associative function and a * neutral "zero value". */ def fold(zeroValue: T)(op: (T, T) => T): T } Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 15

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Intro Composition Monoids Errors Conclusion Monoidal RDDs implicit class MonoidRDD[T](val rdd: RDD[T]) { // avoid conflicts with fold/reduce etc def combine(implicit M: Monoid[T]): T = rdd.fold(M.empty)(M.combine(_,_)) } Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 16

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Intro Composition Monoids Errors Conclusion The Program val sc: SparkContext = ??? val file: String = ??? val data = sc.textFile(file). // read the file flatMap(_.split("""\W+""")). // split into words map(expand) // action! def expand(w: String) = (1, w.length, Map(w -> 1)) val (words,chars,wordMap) = data.combine Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 17

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Intro Composition Monoids Errors Conclusion Running this program Scala.io The Scala event in France Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 18

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Intro Composition Monoids Errors Conclusion Running this program Scala.io The Scala event in France Seq("Scala","io","The","Scala","event","in",...) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 18

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Intro Composition Monoids Errors Conclusion Running this program Scala.io The Scala event in France Seq("Scala","io","The","Scala","event","in",...) Seq( // expand(w: String) = (1,w.length,Map(w->1)) (1, 5, Map("Scala" -> 1)), (1, 2, Map("io" -> 1)), (1, 3, Map("The" -> 1)), (1, 5, Map("Scala" -> 1)), // ... ) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 18

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Intro Composition Monoids Errors Conclusion Running this program Scala.io The Scala event in France Seq("Scala","io","The","Scala","event","in",...) Seq( // expand(w: String) = (1,w.length,Map(w->1)) (1, 5, Map("Scala" -> 1)), (1, 2, Map("io" -> 1)), (1, 3, Map("The" -> 1)), (1, 5, Map("Scala" -> 1)), // ... ) (7,28,Map("Scala" -> 2,"io" -> 1,"The" -> 1,...)) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 18

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Intro Composition Monoids Errors Conclusion Easy Extension val data: RDD[String] = ??? // as before def expand(w: String) = ( 1, Max(w.length), // max word length Min(w.length), // min word length Map(w.length -> Set(w)) // words by count ) val (count,max,min,byCount) = data.combine Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 19

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Intro Composition Monoids Errors Conclusion The new program Scala.io The Scala event in France Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 20

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Intro Composition Monoids Errors Conclusion The new program Scala.io The Scala event in France Seq("Scala","io","The","Scala","event","in",...) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 20

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Intro Composition Monoids Errors Conclusion The new program Scala.io The Scala event in France Seq("Scala","io","The","Scala","event","in",...) Seq( (1, Max(5), Min(5), Map(5 -> Set("Scala"))), (1, Max(2), Min(2), Map(2 -> Set("io"))), (1, Max(3), Min(3), Map(3 -> Set("The"))), (1, Max(5), Min(5), Map(5 -> Set("Scala"))), (1, Max(5), Min(5), Map(5 -> Set("event"))), // ... ) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 20

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Intro Composition Monoids Errors Conclusion The new program Scala.io The Scala event in France Seq("Scala","io","The","Scala","event","in",...) Seq( (1, Max(5), Min(5), Map(5 -> Set("Scala"))), (1, Max(2), Min(2), Map(2 -> Set("io"))), (1, Max(3), Min(3), Map(3 -> Set("The"))), (1, Max(5), Min(5), Map(5 -> Set("Scala"))), (1, Max(5), Min(5), Map(5 -> Set("event"))), // ... ) (7,Max(6),Min(2),Map(5->Set("Scala","event"),...)) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 20

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Intro Composition Monoids Errors Conclusion More Monoid Tricks “ﬁlter” values via mempty value map + reduce == two phase computation via monoids ﬁnger trees, choose Monoid, get random access list / queue / . . . Monoids Theme and Variations (Functional Pearl) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 21

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Intro Composition Monoids Errors Conclusion Part Two: Errors Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 22

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Intro Composition Monoids Errors Conclusion The Traditional Way Java style: try/catch/finally checked exceptions compiler help reduce return value checking unchecked exceptions only visible via docs, if documented at all currently the only way to go in Java errors? return null / custom classes Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 23

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Intro Composition Monoids Errors Conclusion The Functional Way Scala: only unchecked exceptions but: throw and catch discouraged in FP anyway FP: type system + ﬁrst class values Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 24

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Intro Composition Monoids Errors Conclusion Out Of The Box Either / Try Either unbiased (before 2.12) Try not a lawful monad. . . get the most by using a FP library Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 25

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Intro Composition Monoids Errors Conclusion Try and Catch def convert(is: String*): List[Int] = is.map(_.toInt).toList convert("1","2","3","Hello World!","5") java.lang.NumberFormatException: For input string: "Hello World" at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) at java.lang.Integer.parseInt(Integer.java:580) at java.lang.Integer.parseInt(Integer.java:615) ... Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 26

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Intro Composition Monoids Errors Conclusion Try and Catch: Not Compositonal try { convert("1","2","3","Hello World!","5") } catch { case e: NumberFormatException => println("Oops") } type says nothing about errors only one option: fail fast what about getting all errors or non-fatal warnings problem is that try/catch does not compose (Duplo) Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 27

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Intro Composition Monoids Errors Conclusion Example Time password validation constraints: length ≥ 8 contains at least one number contains no spaces contains at least one upper char Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 28

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Intro Composition Monoids Errors Conclusion Validating Passwords sealed trait LoginError case object PwTooShort extends LoginError case object PwContainsSpace extends LoginError def checkLength(s: String): Option[LoginError] def checkSpace(s: String): Option[LoginError] def convert[E](o: Option[E]): Xor[E, Unit] def login(s: String): Xor[LoginError,Token] = for { _ <- convert(checkLength(input)) _ <- convert(checkSpace(input)) // ... token = createToken() } yield token Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 29

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Intro Composition Monoids Errors Conclusion Using our Function > cabbage Sorry the password must be more than 8 chars > boiled cabbage Sorry, the password must contain at least one digit > 1 boiled cabbage Sorry, the password cannot have spaces > 50damnedboiledcabbages Sorry, the password must contain at least one upper char Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 30

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Intro Composition Monoids Errors Conclusion Using our Function Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 31

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Intro Composition Monoids Errors Conclusion Validated scenario: multiple unrelated error conditions, result is fail/success Either,Xor,etc. are made for fail-fast, or short circuiting solution: Validated / Validation Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 32

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Intro Composition Monoids Errors Conclusion Passwords with Validated sealed trait LoginError case object PwTooShort extends LoginError case object PwContainsSpace extends LoginError case object PwContainsNoDigit extends LoginError def checkLength(s: String): Option[LoginError] def checkSpace(s: String): Option[LoginError] Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 33

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Intro Composition Monoids Errors Conclusion Passwords with Validated def convert[E](o:Option[E]):ValidatedNel[E, Unit] def login(s: String): ValidatedNel[LoginError,Token] = { (convert(checkLength(input)) |+| convert(checkSpace(input)) |+| ... ).as(createToken()) } Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 34

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Intro Composition Monoids Errors Conclusion Using Validated login("cabbage") Invalid(NonEmptyList(PwTooShort, PwContainsNoDigit, PwContainsNoSpecialChar)) > cabbage Sorry the password is too short (min. 8 chars), contains no digit and no special character Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 35

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Intro Composition Monoids Errors Conclusion Using Validated Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 36

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Intro Composition Monoids Errors Conclusion Ior (cats) and \&/ (scalaz) what about “non-fatal” exceptions instead of short circuiting, continue with warning there might still be fatal situations with short circuiting solution: Ior and \&/ Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 37

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Intro Composition Monoids Errors Conclusion First Class Errors replace try / catch with ﬁrst class values Either / Xor and friends = fail-fast Validated /Validation = multiple independent errors Ior = fail/succeed/succeed with warnings Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 38

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Intro Composition Monoids Errors Conclusion FP has some nice patterns for you Monoids: combine stuﬀ Functional Error Handling Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 39

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Intro Composition Monoids Errors Conclusion Enough Duplo, time for Lego! . THANKS! Markus Hauck (@markus1189) - Applying FP Patterns - codecentric AG 40