python high order function FOLD on lists

Question

class Node:
    def __init__(self, head, tail):
        self.head = head
        self.tail = tail
        self.isEmpty = False

class Empty:
    def __init__(self):
        self.isEmpty = True

def fold(ls,f,z):
    if ls.isEmpty:
        return z
    else:
        return f(ls.head,fold(ls.tail,f,z))

print(fold(Node(4,Node(3,Node(2,Node(1,e)))),lambda x,y:x+y,0)) ## Sum
print(fold(Node(3,Node(2,Node(1,e))),lambda x,y:x*y,1)) ## Multiply
print(fold(Node(3,Node(2,Node(1,e))),lambda x,y:x-y,0)) ## Minus

Everything works fine,except the minus print with lambda. What am I doing wrong here?

The result should be "0"(3-2-1) in this example list.

Answer 1

A hint to get you started: fold comes in two forms: left and right, corresponding to left and right associativity. If you examine your implementation of fold, you should discover which one it is. Expand the "Minus" expression to see the full affect.