from simpleparse.parser import Parser
from simpleparse.common import numbers
from simpleparse.error import ParserSyntaxError

grammar = '''
command     := !, assign/expr
assign      := varname, ts, '=', !, ts, expr
expr        := (lpar?, 
                ts, operand, ts, (op, !, ts, operand, ts)*, ts, 
               rpar?)+
lpar        := '('
rpar        := ')'
operand     := lpar?, ts, number/varname, ts, rpar?
varname     := [a-z]
op          := [+-*/]
<ts>        := [ \t]*
'''

class Calculator(object):
    def __init__(self):
        self.scanner = Parser(grammar, 'command')
    
    def parse(self, command):
        try:
            success, subtags, nextchar = self.scanner.parse(command)
        except ParserSyntaxError, e:
            return str(e)
        else:
            if subtags[0][0] == 'expr':
                try:
                    return eval(command)
                except Exception, e:
                    print "Error:", e.args[0]
            elif subtags[0][0] == 'assign':
                exec command in globals()

if __name__ == '__main__':
    calc = Calculator()
    def prompt():
        return raw_input('> ')

    command = prompt()
    while command != "quit":
        result = calc.parse(command)
        if result:
            print result
        command = prompt()