"""Correctness parser for S-expression grammar. Note that this program
commits a massive style faux pas: the variables token and tokens are
global variables. In other words, they are accessible from anywhere in
the program. I chose to do this because this any attempt to do this
'right' results in a bunch of parameters getting passed around that
obscures the idea that this demonstration program is trying to
convey. Sometimes 'proper' isn't always 'simplest.'"""

import sys

def tokenize(lexemes):
    global tokens
    # Make a copy of lexemes (don't just point tokens at it)
    tokens = list(lexemes)
    for i in range(len(tokens)):
        if tokens[i] not in ["(" , ")" , "'" , None]:
            tokens[i] = 'atom'

def nextToken():
    """Grab the next token. Set the current token to None if there are
    no more tokens."""
    global token
    #print 'Matched',token
    if len(tokens)>0:
        token = tokens.pop(0)
    else:
        token = None

def match(expected):
    """Match the current token against the expected value. If
    successful, grab the next token. Set the current token to None if
    there are no more tokens."""
    if token==expected:
        nextToken()
    else:
        raise Exception('Parse error',token,expected)

def P():
    if token in ['atom' , "'" , '(']:
        E()
    else:
        raise Exception('Parse error')

def E():
    if token == 'atom':
        match('atom')
    elif token == "'":
        match("'")
        E()
    elif token == "(":
        match("(")
        E()
        Es()
        match(")")
    else:
        raise Exception('Parse error')

def Es():
    if token in ['atom' , "'" , '(']:
        E()
        Es()
    elif token == ")":
        pass

"""Open up the file, grab the program, and parse it."""

file = open(sys.argv[1],'r')
data = file.read()
file.close()
lexemes = data.split()
tokenize(lexemes)
token = tokens.pop(0)
P()
print "Success"