# Mutable Function Using Nonlocal

def make_withdraw(balance):
    """Return a withdraw function with a starting balance."""
    def withdraw(amount):
        nonlocal balance
        if amount > balance:
            return 'Insufficient funds'
        balance = balance - amount
        return balance
    return withdraw

withdraw = make_withdraw(100)
withdraw(25)
# 75
withdraw(25)
# 50
withdraw(60) 
# 'Insufficient Funds'
withdraw(15)
# 35


# Mutable Function Using A List

def make_withdraw_list(balance):
    b = [balance]
    def withdraw(amount):
        if amount > b[0]:
            return 'Insufficient funds'
        b[0] = b[0] - amount
        return b[0]
    return withdraw
    
withdraw_list = make_withdraw_list(100)
withdraw_list(25)
# 75


# Multiple Mutable Functions

jen = make_withdraw(10000000)
mitas = make_withdraw(100)
mitas(10) # Mitas gets lunch
# 90
jen(1000) # Jen treats the 61A staff to dinner
# 9999000
mitas(2000) # Mitas wants to build a PC
# 'Insufficient funds'
jen(2000) # Jen lends Mitas money
# 9997000


# Referential Transparency

mul(add(2, mul(4, 6)), add(3, 5))
mul(add(2,    24    ), add(3, 5))
mul(        26       , add(3, 5))
mul(        26       ,     8    ) # equivalent to the above


account = make_withdraw(100)

add(account(25), account(25))
add(     75    , account(25)) # not equivalent to the above