We can implement a computed size property to calculate the size of a linked list as follows:

var size: Int { 
    switch self { 
    case .node(_, let next): 
        return 1 + next.size 
    case .end: 
        return 0 
    } 
} 

This method recursively goes through LinkedList and counts the number of nodes:

print(functionalLinkedList.size) 

The result is going to be 3 in this example.