function expression for the list interpreter

lisp_parser_grammer

function expression:
    token = next_token()
    if token is one of (-, +, 0-9, .):
        putback the token
        val = number()
    elif token is (:
        op = operator()
        exp1 = expression()
        exp2 = expression()
        c = next_token()
        c must be )
        val = calc(op, exp1, exp2)
    else
        SyntaxError
    return val

function number:
    n = get_number()
    return n

function operator:
    op = get_char()
    if op is one of (+, -, *, /)
        return op
    else
        Unknow Operator