Hand-written LR parser

lr.py

def state_0(ch):
    if ch == None:
        return None
    if ch == "1":
        return state_4
    if ch == 'int':
        return state_1
    if ch == 'plus':
        return state_1
    assert False

def state_1(ch):
    if ch == None:
        return None
    if ch == "+":
        return state_2
    assert False

def state_2(ch):
    if ch == "1":
        return state_4
    if ch == 'int':
        return state_3
    assert False

def state_3(ch):
    if ch == None:
        return reduce(3, 'plus', ch)
    if ch == '+':
        return reduce(3, 'plus', ch)
    assert False

def state_4(ch):
    if ch == None:
        return reduce(1, 'int', ch)
    if ch == '+':
        return reduce(1, 'int', ch)
    assert False

def reduce(count, goto, ch):
    print 'reduce', count, goto
    for i in range(count):
        stack.pop(-1)
    stack.append(stack[-1](goto))
    return stack[-1](ch)

stack = [state_0]
for ch in ["1", "+", "1", "+", "1", None]:
    state = stack[-1](ch)
    if state is None:
        print 'accept'
        break
    print 'shift', state, repr(ch)
    stack.append(state)