Functional Programming in Scala Book Group Chapter 2

FPInScalaCh2.md

Chapter 2

Core Concepts

• Higher order functions
• Tail call optimization
• Currying

Exercises

2.1) Three Fibonacci implementations

def fibs(x: Int): Long = {
  if (x <= 0) 0
  else if (x == 1) 1
  else fibs(x-1) + fibs(x-2)
}

def fibsTailRec(x: Int): Long = {
  @annotation.tailrec
  def loop(n: Int, last: Long, current: Long): Long = {
    if( n == x ) last
    else loop(n + 1, current, current + last)
  }
  loop(0, 0, 1)
}

def lazyFibs: Stream[Long] = 
  0L #:: 1L #:: lazyFibs.zip(lazyFibs.tail).map { case (first, second) => first + second }

2.2) Polymorphic Sorted

def isSorted[A](as: Array[A], ordered: (A,A) => Boolean): Boolean = {
  as.zip(as.tail).forall{case(a,b)=> ordered(a,b)}
}

def orderedStrings(a: String, b: String) = a < b
isSorted(Array("a"), orderedStrings)
isSorted(Array("a","b","c"), orderedStrings)
isSorted(Array("a","b","a"), orderedStrings)
isSorted(Array("d","b","c","a"), orderedStrings)

2.3) Curry

def curry[A,B,C](f: (A, B) => C): A => B => C = {
  (a: A) => (b: B => f(a, b))
}

2.4) Uncurry

def uncurry[A,B,C](f: A => B => C): (A, B) => C = {
  (a: A, b: B) => f(a)(b)
}

2.5 Compose

def compose[A,B,C](f: B => C, g: A => B): A => C ={
  a: A => f(g(a))
}