Swift 4 Linked List Generic Types: Cannot compare two generic values

Question

I am trying to implement a linked list in Swift playgrounds using generic types. My remove function is giving me errors about comparing to generic types, even though they are equivalent.

I have already tried conforming to the Equatable and Comparable protocols in my function declaration, but the errors still persist.

class Node<T> {

    var value:T
    var next:Node?

    init(value:T) {
        self.value = value
        self.next = nil
    }

}

func remove<T: Equatable>(value:T) -> Node<T>? {
    if isEmpty {
        return nil

    }
    else {
        var current = head!
        var prev:Node<T>? = nil

        while (current.value != value && current.next != nil) {
            prev = current
            current = current.next!
        }

        if (current.value == value) {
            //Found node. Remove by updating links, return node
            if let prev = prev {
                prev.next = current.next
                current.next = nil
            }
            else {
                self.head = current.next
                current.next = nil
            }

            size -= 1
            return current
        }
        else {
            return nil
        }
    }
}

On this line of my remove function:

while (current.value != value && current.next != nil) {

I am receiving the error:

Binary operator '!=' cannot be applied to operands of type 'T' and 'T'

Also when I conform to Equatable, I am receiving this error on the following line when trying to update the previous node:

Cannot assign value of type 'Node<T>' to type 'Node<T>?'

This error will disappear when I remove the Equatable protocol.

Any ideas? It sounds like I may be missing a simple step when conforming to the protocols, but I'm not sure what is missing... Thanks in advance!

Answer 1

The Equatable conformance should be added to the generic class itself also, not only for the function, like this:

class Node<T: Equatable > {
    // Your code here.
}

By saying current.value != value , you are trying to compare current.value with value . At this point the compiler is sure that value conforms to Equatable , but it is not sure wether current.value does.