清单上的python高阶函数FOLD

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.