2_Getting_started_with_functional_programming_in_Scala

• 스칼라로 순수함수형 코딩을 해보자.

• 스칼라의 기본 문법?

  • tail recursive function
  • high order function
  • polymorphic

2.1 Introducing Scala the language: an example

object MyModule {
  def abs(n: Int): Int =
    if (n < 0) -n
    else n

  private def formatAbs(x: Int) = {
    val msg = "The absolute value of %d is %d"
    msg.format(x, abs(x))
  }

  def main(args: Array[String]): Unit =
    println(formatAbs(-42))
}

• object object
• private keywork
• return type 표기 방법

2.2 Running our program

scalac MyModule.scala
scala MyModule

• scala interpreter

2.3 Modules, objects, and namespaces

• namespacs? 주소?
• module? 파티션?

2.4 Higher-order functions: passing functions to functions

• writing loops functionally

def factorial(n: Int): Int = {
  def go(n: Int, acc: Int): Int =
    if (n <= 0) acc
    else go(n-1, n*acc)
  go(n, 1)
}

• tail recursive

2.4.2 Writing our first higher-order function

object MyModule {
...
  private def formatAbs(x: Int) = {
    val msg = "The absolute value of %d is %d."
    msg.format(x, abs(x))
  }
  private def formatFactorial(n: Int) = {
    val msg = "The factorial of %d is %d."
    msg.format(n, factorial(n))
  }
  def main(args: Array[String]): Unit = {
    println(formatAbs(-42))
    println(formatFactorial(7))
  }
}

• formatAbs, formatFactorial는 거의 비슷하지 않나?
• 그런데 high order function 같은건 왜 필요한가?
• 함수형 이름짓기란?? n이란 변수는 괜찮은가??

2.5 Polymorphic functions: abstracting over types

def findFirst(ss: Array[String], key: String): Int = {
  @annotation.tailrec
  def loop(n: Int): Int =
    if (n >= ss.length) -1
    else if (ss(n) == key) n
    else loop(n + 1)
 loop(0)
}

def findFirst[A](as: Array[A], p: A => Boolean): Int = {
  @annotation.tailrec
  def loop(n: Int): Int =
    if (n >= as.length) -1
    else if (p(as(n))) n
    else loop(n + 1)
  loop(0)
}

2.5.2 Calling HOFs with anonymous functions

• 익명함수 사용하기?
• findFirst 예시
• 익명함수의 원래 정체는 뭐지? 왜 이상한 람다식만 적어도 잘 돌아가는거지?

2.6 Following types to implementations

• As you might have seen when writing isSorted, the universe of possible implementations is significantly reduced when implementing a polymorphic function
• partial function : 익명함수와 무엇이 다른가?
• composition function : 합성 함수