The only performance benefits (that I know of) to writing code in the functional style come in the form of concurrency support. Support for multi-threading literally comes for free in functional languages. You don’t need to worry about locking or anything messy like that since nothing is mutable. Don’t get me wrong, abstractions like actors really help, but they are merely an embodiment of the functional style: shared-nothing continuations passing. Code written imperatively can still perform well asynchronously, but it is much harder to do.

Is that the reason scala.List looks the way it does, performance? There’s very little (or maybe nothing) that’s more important than performance at this foundational level so I understand that one makes compromises, but it’s not exactly a vote of confidence in the practicality of functional code.

object MyList {
// syntactically easy way to create a list
def apply[A](elems : A *) =
elems.foldRight(MyNil:MyList[A]) {case (head, accum) => MyCons(head, accum)}
}

sealed abstract class MyList[+A] {
def head : A
def tail : MyList[A]

def foldLeft[B](init:B)(f:(A,B) => B) : B = {
def loop(lst:MyList[A], accum:B) : B = lst match {
case MyCons(head, tail) => loop(tail, f(head, accum))
case _ => accum
}

loop(this, init)
}

def reverse : MyList[A] =
foldLeft(MyList[A]()){(head, accum) => MyCons(head, accum)}

def foldRight[B](init:B)(f:(A,B) => B) : B =
reverse.foldLeft(init)(f)

def length : Int =
foldLeft(0) {case (_, accum) => accum + 1}

def indices : MyList[Int] =
foldRight((MyList[Int](), 0)) {case (_, (accum, count)) => (MyCons(count, accum), count +1 )} _1

def last : A =
reverse.head

def foreach(f:A => Unit) : Unit =
foldLeft(()) {case (head, _) => f(head)}

def map[B](f:A => B) : MyList[B] =
foldRight(MyList[B]()) {case (head, accum) => MyCons(f(head), accum)}

}

case class MyCons[+A](head:A, tail:MyList[A]) extends MyList[A]

case object MyNil extends MyList[Nothing] {
override def head = error (“head of an empty list”)
override def tail = error (“tail of an empty list”)
}