import ballerina/io;

class Stack {
    private any[] s = [];

    function push(any a) {
        self.s.push(a);
    }

    function length() returns int {
        return self.s.length();
    }

    function isEmpty() returns boolean {
        return self.length() == 0;
    }

    function pop() returns any? {
        if !self.isEmpty() {
            return self.s.pop();
        }
        return ();
    }

    function peek() returns any? {
        if !self.isEmpty() {
            return self.s[self.length() - 1];
        }
        return ();
    }

    function clear() {
        self.s.removeAll();
    }

    function toString() returns string {
        return self.s.toString();
    }
}

public function main() {
    Stack s = new;
    s.push(1);
    s.push(2);
    io:println("Stack contains ", s);
    io:println("Number of elements in stack = ", s.length());
    any item = s.pop();
    io:println(item, " popped from the stack");
    io:println("Last element is now ", s.peek());
    s.clear();
    io:println("Stack cleared");
    io:println("Is stack now empty? ", s.isEmpty() ? "yes" : "no");
}