parser.py

class Identifier(object):
	def __init__(self, name):
		if isinstance(name, Identifier):
			name = name.name
		self.name = name
	
	def __repr__(self):
		return self.name

class Symbol(Identifier):
	def __repr__(self):
		return '\'' + self.name

class List(list):
	def __repr__(self):
		return '(' + ' '.join(map(repr, self)) + ')'

class Parser(object):
	def __init__(self, data):
		self.data = data
		self.off = 0
	
	def getch(self):
		return self.data[self.off]
	
	def move(self):
		self.off += 1
	
	def parse(self):
		ch = self.getch()
		while ch in ' \r\n\t':
			self.move()
			ch = self.getch()
		if ch == '(':
			return self.parseList()
		elif ch == '\'':
			return self.parseSymbol()
		elif ch in '-0123456789':
			return self.parseNumber()
		elif ch == '.':
			return self.parseDots()
		else:
			return self.parseIdentifier()
	
	def parseList(self):
		self.move()
		elems = List()
		while self.getch() != ')':
			elems.append(self.parse())
		self.move()
		return elems
	
	def parseIdentifier(self):
		identifier = ''
		while True:
			ch = self.getch()
			if ch == '.' or ch == ')' or ch == ' ':
				break
			identifier += ch
			self.move()
		return Identifier(identifier)
	
	def parseDots(self):
		identifier = ''
		while True:
			ch = self.getch()
			if ch != '.':
				break
			identifier += ch
			self.move()
		return Identifier(identifier)
	
	def parseSymbol(self):
		self.move()
		return Symbol(self.parse())
	
	def parseNumber(self):
		number = ''
		first = self.getch()
		if first in '-+':
			number = first
			self.move()
		seenDot = False
		while True:
			ch = self.getch()
			if ch in '0123456789':
				number += ch
			elif ch == '.':
				assert not seenDot
				seenDot = True
				number += ch
			else:
				break
			self.move()
		
		if seenDot:
			return float(number)
		else:
			return int(number)

def parse(data):
	return Parser(data).parse()