Functional Programming exercise with Scala

Question

I have recently started reading the book Functional Programming in Scala by Paul Chiusano and Rúnar Bjarnason, as a means to learn FP. I want to learn it because it will open my head a bit, twist my way of thinking and also hopefully make me a better programmer overall, or so I hope.

In their book, Chp. 3, they define a basic singly-linked-list type as follows:

package fpinscala.datastructures

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

object List {
    def sum(ints: List[Int]): Int = ints match {
    case Nil => 0
    case Cons(x,xs) => x + sum(xs)
    }
    def product(ds: List[Double]): Double = ds match {
    case Nil => 1.0
    case Cons(0.0, _) => 0.0
    case Cons(x,xs) => x * product(xs)
    }
    def apply[A](as: A*): List[A] =
    if (as.isEmpty) Nil
    else Cons(as.head, apply(as.tail: _*))
}

I'm now working on implementing the tail method, which shall work similarly to the tail method defined in Scala libraries. I guess that the idea here, is to define a tail method inside the List object, what they call a companion method, and then call it normally in another file (like a Main file).

So far, I have this:

def tail[A](ls: List[A]): List[A] = ls match {
    case Nil => Nil
    case Cons(x,xs) => xs
    }

Then I created a Main file in another folder:

package fpinscala.datastructures

object Main {
    def main(args:Array[String]):Unit = {
       println("Hello, Scala !! ")
       val example = Cons(1, Cons(2, Cons(3, Nil)))
    val example2 = List(1,2,3)
       val example3 = Nil
    val total = List.tail(example)
    val total2 = List.tail(example3)
    println(total2)
}
}

This works and gives me:

Hello, Scala !! 
Cons(2,Cons(3,Nil))

My question is:

Is this the correct way to write the tail method, possibly, as the authors intended? And is this package structure correct? Because it feels very wrong to me, although I just followed the authors package.

I also don't know if I should have used a specific type instead of writing a polymorphic method (is this the name?)...

Bear with me, for I am a newbie in the art of FP.

It seems perfectly fine for me. Though : 1. You don't want to show Cons outside of sealed trait, so it's better to make it private 2. Why do you need example & total variables in your main function? — mavarazy
– mavarazy, Commented Aug 21, 2016 at 11:21
Thanks! How can I "encapsulate" the Cons in order to make it private? I understand the example but it still is a bit too much for me to change the actual class, and in the book they ask to add simple methods like tail and such... so I'm at a loss. Regarding the examples, well, I wanted to test it? — Bruno Oliveira
– Bruno Oliveira, Commented Aug 21, 2016 at 12:12

John · Accepted Answer · 2016-08-21 14:02:49Z

2

In the default Scala list implementation, attempting to take the tail of an empty list would throw an UnsupportedOperationException. So you might want something more like

def tail[A](ls: List[A]): List[A] = ls match {
    case Nil => throw new UnsupportedOperationException()
    case Cons(x,xs) => xs
}

Also, qantik's answer where he suggests using the :: operator would work with Scala's default list implementation, but since there isn't a :: method defined on this custom list implementation it won't work.

Finally, you may want to consider defining tail so that instead of doing

val list = List(1, 2, 3)
val restOfList = tail(list).

you could instead do

val list = List(1, 2, 3)
val restOfList = list.tail

That would require defining the method on the List trait, as opposed to in the List object.

answered Aug 21, 2016 at 14:02

John

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Comments

qantik · Accepted Answer · 2016-08-21 11:20:27Z

-1

Looks okay. What about?

def tail[A](xs: List[A]): List[A] = xs match {
    case Nil => Nil
    case head :: xxs => xxs
}

answered Aug 21, 2016 at 11:20

qantik

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