Functional Programming For All

Functional Programming For All talk given at Scala Matsuri 2016

Transcript

1. Functional Programming for All Zachary McCoy ΈΜͳͷؔ਺ܕϓϩάϥϛϯά
   
2. About Me • Scala • Iowa, USA • twitter - @ZachAMcCoy • email - zach.mccoy@banno.com
3. What is Functional Programming? • Programming… with functions • Functions as the main abstraction • Functions as first class values ʮؔ਺Λ༻͍ͨϓϩάϥϛϯάʯ
   ʮؔ਺Λओͳந৅Խͷಓ۩ͱ͢ΔʯʮୈҰڃ஋ͱͯ͠ͷؔ਺ʯ
4. What is Functional Programming? • Controlled side effects • Restricts how we write programs, but not what we can express ʮ੍ޚ͞Εͨ෭࡞༻ʯ
   ʮදݱͷ෯͸ڱΊͣʹɺϓϩάϥϜͷॻ͖ํΛ੍໿͢Δʯ
5. Functional Programming • Pure functional core with a layer of side effects on the outside • Side effect - an action in addition to return values • FP - Evaluating expressions • Imperative - programs are composed of statements த֩͸७ਮؔ਺Ͱͦͷ֎ଆͷ૚Ͱ෭࡞༻͕࣮ߦ͞ΕΔ
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6. What is a function? • An expression involving one or more variables • Domain and Co-Domain • Unique mapping from D -> CD • Immutability, produces something new ؔ਺ͱ͸ͭ΋͘͠͸ෳ਺ͷ஋ʹؔ͢Δࣜ
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7. Pure Functions • No observable side-effects • Anything that isn’t returning a result • Mutation • I/O • Depends only on arguments or subset of ७ਮؔ਺͸෭࡞༻Λ࣋ͨͣɺ݁Ռ͸Ҿ਺ʹͷΈґଘ͢Δ
   
8. Pure Functions • Examples • Hashing • Arithmetic ७ਮؔ਺ྫɿϋογϡԽɺࢉज़ԋࢉ
   
9. Why does purity matter? • Side effects cause order of evaluation to matter • Only have to use local reasoning • Composition and reusability ෭࡞༻͸࣮ߦʗධՁॱͷߟྀ͕ඞཁʹͳΔɺہॴԽɺ
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10. Why does purity matter? • Separate the computation over the input from how to obtain it • Guarantees Referential Transparency ԋࢉͱೖྗΛ༩͑Δํ๏Λ෼཭ɺࢀরಁաੑΛอূ
    
11. Referential Transparency • An expression can be replaced by its value, provided the expression is pure • A function can only be RT if the inputs are also RT • Referential Transparency enables equational reasoning ࢀরಁաੑ
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12. Substitution Model def greaterThan5(i: Int): Option[Int] = if(i > 5) Some(i) else None def createMessage(): String = greaterThan5(3).map(x => "Was greater than 5") getOrElse "Was less than or equal to 5" def createMessage2(): String = (if(3 > 5) Some(3) else None).map(x => "Was greater than 5") getOrElse "Was less than or equal to 5" def createMessage3(): String = None.map(x => "Was greater than 5") getOrElse "Was less than or equal to 5" ஔ͖׵͑Ϟσϧ
    
13. Formalize Referential Transparency • An expression, E, is said to be referentially transparent if E can be replaced with its value without changing the behavior of a program • Same effect and output in the end ࢀরಁաੑͷܗࣜԽɿࣜΛͦͷ஋ͱஔ͖׵͑Δ͜ͱ͕Ͱ͖Δ
    ϓϩάϥϜͷৼΔ෣͍͸࡞༻ΛؚΊมΘͬͯ͸͍͚ͳ͍
14. Referential Transparency • Mathematics! • (2 * 2 = 4) • Returning errors as values, rather than side effecting ࢀরಁաੑ͸਺ֶ
    Τϥʔ͸ɺ෭࡞༻Ͱ͸ͳ͘ɺ஋Ͱฦ͢
15. How does this tie together? • Pure functions enable Referential Transparency • RT enables the Substitution model and Equational Reasoning • Pure functions are a huge gain! ७ਮؔ਺
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16. Scala and FP • Scala doesn't enforce Referential Transparency • We have to work for it • Limit your set of tools: no vars, pulling from out of scope, exceptions 4DBMB
    ͸ࢀরಁաੑΛڧ੍͠ͳ͍ͨΊɺࣗલͰͷରԠ͕ඞཁ
    
17. Scala and FP • Given an impure function of type A => C we can split it into two functions • Pure function of A => B, where B is the description of the result • Impure function of type B => C which is the interpreter of the description ७ਮͰͳ͍ؔ਺
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18. Calculate the oldest ࠷೥௕ऀͷܭࢉ
    case class Person(name: String, age: Int) val p1 = Person("John", 30) val p2 = Person("Jack", 100)
19. Calculate the oldest ͜Ε͸ςετ͢Δͷ͕೉͍͠
    શ෦෭࡞༻Ͱग़ྗ͞Ε͍ͯΔͷͰࢀরಁաͰ͸ͳ͍ def calculateOldest(): Unit = { if(p1.age > p2.age) println(s"${p1.name} is oldest") else if(p2.age > p1.age) println(s"${p2.name} is oldest") else println("They are the same age") }
20. Separation of concerns ؔ৺ࣄͷ෼཭
    def calculateOldest(p1:Person, p2:Person):Unit = { if(p1.age > p2.age) println(s"${p1.name} is oldest") else if(p2.age > p1.age) println(s"${p2.name} is oldest") else println(s"They are the same age") }
21. Return values ໭Γ஋Λ࢖͏͜ͱͰςετ͠΍͘͢ͳͬͨ
    def calculateOldest(p1: Person, p2: Person): Option[Person] = if(p1.age > p2.age) Some(p1) else if(p2.age > p1.age) Some(p2) else None
22. We can still split more ͞Βʹࡉ͔͘෼͚Δ
    def result(maybePerson:Option[Person]): Unit = maybePerson match { case Some(Person(name, age)) => println(s"${p.name} is oldest") case None => println("They are the same age") }
23. A pure function core ͜ΕͰίΞ͕७ਮؔ਺ʹͳͬͨ
    def calculateOldest(p1: Person, p2: Person): Option[Person] def result(maybePerson: Option[Person]): String = maybePerson.map { case Person(name, age) => s"${name} is the oldest" } getOrElse "They are the same age" def combine(p1: Person, p2: Person): Unit = println(result(calculateOldest(p1,p2)))
24. We’re Hiring! zach.mccoy@banno.com Ұॹʹಇ͖·ͤΜ͔