Generics.swift

protocol Node {
    associatedtype Value: Numeric
    
    func execute() -> Value
}

protocol Operator: Node {
    associatedtype LeftNode: Node
    associatedtype RightNode: Node
    
    var left: LeftNode { get }
    var right: RightNode { get }
}

class PlusOper<Left: Node, Right: Node, ValueT>: Operator where
    Left.Value == ValueT,
    Right.Value == ValueT
{
    var left: Left
    var right: Right
    
    init(left: Left, right: Right) {
        self.left = left
        self.right = right
    }
    
    func execute() -> ValueT {
        return self.left.execute() + self.right.execute()
    }
}


class ValueNode<T: Numeric>: Node {    
    let value: T
    
    init(val: T) {
        self.value = val
    }
    
    func execute() -> T {
        return self.value
    }
}

let tree = PlusOper(left: ValueNode(val: 10), right: ValueNode(val: 20))

print(tree.execute())