FP In scala chapter 3

In [26]:

    

sealed trait List[+A] // `List` data type, parameterized on a type, `A`
case object Nil extends List[Nothing] // A `List` data constructor representing the empty list
/* Another data constructor, representing nonempty lists. Note that `tail` is another `List[A]`,
which may be `Nil` or another `Cons`.
 */
case class Cons[+A](head: A, tail: List[A]) extends List[A]


    

    






defined trait List
defined object Nil
defined class Cons

In [27]:

    

object List{
      def apply[A](as: A*): List[A] = // Variadic function syntax
        if (as.isEmpty) Nil
        else Cons(as.head, apply(as.tail: _*))
    
      def sum(ints: List[Int]): Int = ints match { // A function that uses pattern matching to add up a list of integers
        case Nil => 0 // The sum of the empty list is 0.
        case Cons(x,xs) => x + sum(xs) // The sum of a list starting with `x` is `x` plus the sum of the rest of the list.
      }

    }


    

    






defined object List

In [29]:

    

val x = List(1,2,3,4,5) match {
    case Cons(x, Cons(2, Cons(4, _))) => x
    case Nil => 42
    case Cons(x, Cons(y, Cons(3, Cons(4, _)))) => x + y
    case Cons(h, t) => h + List.sum(t)
    case _ => 101
  }


    

    






x: Int = 3

In [ ]:

    




    

In [7]:

    

val ll = Cons(1, Nil)


    

    






ll: cmd2.INSTANCE.$ref$cmd0.Cons[Int] = Cons(1, Nil)