#!/bin/python

OPERATORS = ['+', '-', '*', '/']

def maybe(func):
    """Turns Exceptions into return values."""
    def inner(*args):
        for a in args:
            if isinstance(a, Exception):
                return a
        try:
            return func(*args)
        except Exception as e:
            return e
    return inner

def repeat(func, until):
    """Repeats a function until its return value meets the stop criterion."""
    def inner(*args):
        while True:
            result = func(*args)
            if until(result):
                return result
    return inner

is_int = lambda i: isinstance(i, int)
get_number = lambda: int(input('Enter an integer: '))
safe_get_number = repeat(maybe(get_number), until=is_int)

is_operator = lambda o: o in OPERATORS
get_operator = lambda: input('Enter an operator (+, -, *, /): ')
safe_get_operator = repeat(get_operator, until=is_operator)

calculate = lambda number1, operator, number2: \
        number1 + number2 if operator == '+' \
        else number1 - number2 if operator == '-' \
        else number1 * number2 if operator == '*' \
        else number1 / number2 if operator == '/' \
        else None

main = lambda: calculate(
        safe_get_number(),
        safe_get_operator(),
        safe_get_number(),
        )

forever = lambda _: False
main_loop = repeat(lambda: print(main()), until=forever)

main_loop()