def eval(expr, env):
    return match expr:
        case Atom(x): Success(lookup(env, x))
        case List(['quote', x]): Success(x)
        case List(['if', cond, then, else_]):
            eval(cond, env).flatmap(
                lambda v: eval(then if v else else_, env)
            )
        case List([fn, *args]):
            eval(fn, env).flatmap(
                lambda f: traverse(args, lambda a: eval(a, env))
                    .flatmap(lambda vals: apply(f, vals))
            )