Chapter 6 Linked Lists: Conformance to collection, subscript method seems problematic with a force unwrap

chaugen55 · June 26, 2024, 10:26pm

In chapter 6, the authors propose a great solution to make LinkedList conform to the Collection protocol. This makes perfect sense to use a linked list with a familiar set of APIs.

I have a concern though regarding the subscript method declaration, which can be found here:

github.com

kodecocodes/alg-materials/blob/editions/4.0/06-linked-list/projects/finished/LinkedList.playground/Sources/LinkedList.swift

// Copyright (c) 2021 Razeware LLC
// For full license & permission details, see LICENSE.markdown.

public struct LinkedList<Value> {
  
  public var head: Node<Value>?
  public var tail: Node<Value>?
  
  public init() {}
  
  public var isEmpty: Bool {
    head == nil
  }
  
  public mutating func push(_ value: Value) {
    copyNodes()
    head = Node(value: value, next: head)
    if tail == nil {
      tail = head
    }

This file has been truncated. show original

public subscript(position: Index) -> Value {
  position.node!.value
}

This seems problematic because the Index.node has a type of Node<Value>?. If the node value is nil, then this force unwrapping will crash. Take this example:

let emptyList = LinkedList<Int>()
print(emptyList[0]) // crash, since no nodes exist yet

Should we just be extra careful when using linked list subscripts, and ensure that they aren’t empty before trying to subscript? This would mirror the behavior of array subscripting out of range, but seems like a non-optimal solution for users.

robertomachorro · June 27, 2024, 12:41am

This is a good point, in general it’s good to avoid ! unless it’s with a constant.
Hopefully the author can add to the rationale here.