What is the appropriate return value when the pattern matched is Nil and we want to return Nil?

Question

I am new to Scala and functional programming in general. So here's my doubt.

In a function with pattern matching, when case Nil is matched, and we want to return Nil, should we return Nil or the data type itself? For example,

def drop[A](l: List[A], n: Int): List[A] = {
    if (n <= 0) l
    else l match {
        case Nil => Nil
        case Cons(_, t) => drop(t, n - 1)
    }
}

This is a function which drops the first n head elements from a Singly Linked List. Here, for the first case, should I return Nil (maybe as a good practice) or should I return l (because it then we won't have to construct the Nil object)?

Answer 1

There is only one singleton instance of the Nil object. When you write Nil you don't create a new one every time, you just use the only one that exists.

It's typically best to write Nil because it's more readable. At least that's what I've always read and written.

Answer 2

Since there is only one instance of Nil so it does not matter since it is the actual same object.

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Now the real question is: what is more readable?

• If you want to make it clear that when you get Nil, then Nil is returned, then write Nil => Nil
• If it seems more logical to return l, even if it is actually Nil, then write Nil => l

IMHO, in your case, Nil => Nil is more clear to me.