scala-foldLeft-vs-foldRight.md

Definition of foldLeft and foldRight

foldLeft

please notice the definition of op : (op: (B, A) => B)

here, A is the element of the caller, B is the intermedia result of function op

foldRight

Example

foldLeft

class Foo(val name: String, val age: Int, val sex: Symbol)

object Foo {
  def apply(name: String, age: Int, sex: Symbol) = new Foo(name, age, sex)
}

val fooList = Foo("Hugh Jass", 25, 'male) ::
  Foo("Biggus Dickus", 43, 'male) ::
  Foo("Incontinentia Buttocks", 37, 'female) ::
  Nil


val stringList = fooList.foldLeft[List[String]](List[String]()) { (z, f) =>
  val title = f.sex match {
    case 'male => "Mr."
    case 'female => "Ms."
  }
  z :+ s"$title ${f.name}, ${f.age}"
}

the result is :

stringList: List[String] = List(Mr. Hugh Jass, 25, Mr. Biggus Dickus, 43, Ms. Incontinentia Buttocks, 37)

foldRight

val stringList2 = fooList.foldRight[List[String]](List[String]()) { (z, f) =>
  val title = f.sex match {
    case 'male => "Mr."
    case 'female => "Ms."
  }
  z :+ s"$title ${f.name}, ${f.age}"
}

you would get error:

Error:(34, 23) value sex is not a member of List[String]
val title = f.sex match {

why ?

notice this code snippet : { (z, f) => }

for foldRight, f is the intermedia result of the function, of course he would have the sex symbol.

so we need to switch them like this:

val stringList2 = fooList.foldRight[List[String]](List[String]()) { (f, z) =>
  val title = f.sex match {
    case 'male => "Mr."
    case 'female => "Ms."
  }
  z :+ s"$title ${f.name}, ${f.age}"
}

result:

stringList2: List[String] = List(Ms. Incontinentia Buttocks, 37, Mr. Biggus Dickus, 43, Mr. Hugh Jass, 25)