Nil checks demistifaction

1

Hello, new guy here. I’m playing with the language a little bit and I’ve decided to implement some data structures. While implementing Linked lists I’ve stomped upon strange behavior. How is that compiler can’t recognize @tail as not nil?

class LinkedList(V)
  getter head : Node(V)?
  getter tail : Node(V)?
  
  ...
  
  # Pushes new node value on the end of the list
  def push(value : V)
    if @head.nil?
      @head = Node.new(value)
      @tail = @head
    elsif !@tail.nil?
      @tail.next = Node.new(value)
      @tail = @tail.next
    end
  end
end

function doesn’t compile and returns

In linked_list.cr:63:13

 63 | @tail.next = Node.new(value)
            ^---
Error: undefined method 'next=' for Nil (compile-time type is (LinkedList::Node(Int32) | Nil))

error.

I’ve eventually fixed it with .not_nil! method but I was wondering how the mechanism behind null reference checks actually works and what are the best practices?

Thanks!

2

See if var - Crystal.

2 Likes
3

Oh, that was fast, thank you. So if I understood correctly it’s because of possible mutation of the state from some other thread(fiber)? That means that not_nil! method is the go to way when I don’t expect nil to be present?

4

Correct, another fiber could mutate that value after the check but before it’s used.

No. As mentioned in the examples you’d want to do like

if h = @head
  # h is not nil
end

or

@head.try do |h|
  # h is not nil 
end

EDIT: not_nil! is best used when you can be 100% sure it won’t be nil, but the compiler isn’t able to prove it, such as some application specific state etc.

2 Likes
5

Thank you!

6

It’s also nkot necessary to assign the instance variable inside the if. It’s usually done for brevity but you can do:

head = @head
if head
 ... 
end
2 Likes