build.sbt

sealed trait List[+T]
case object Nil extends List[Nothing]
case class Cons[+T](head: T, tail: List[T]) extends List[T]

object List {

 def head[T](list: List[T]): T = list match {
   case Nil        => throw new RuntimeException("Empty List")
   case Cons(x, _) => x
 }

 def tail[T](list: List[T]): List[T] = list match {
   case Nil         => Nil
   case Cons(_, xs) => xs
 }

 def map[T, U](list: List[T])(f: T => U): List[U] = list match {
   case Nil         => Nil
   case Cons(x, xs) => Cons(f(x), map(xs)(f))
 }

 def fold[T, U](list: List[T])(default: U)(f: (T, U) => U): U =
   list match {
     case Nil         => default
     case Cons(x, xs) => f(x, fold(xs)(default)(f))
   }

}

List.head(Cons(1, Cons(2, Nil)))
List.map(Cons(1, Cons(2, Nil)))(x => x * x)
List.fold(Cons(1, Cons(2, Nil)))(0)((x,y) => x + y)