use std::io::Error;

pub trait State {
    fn handle(self) -> Result<Box<State>, Error>;
}

struct A {}
impl State for A {
    fn handle(self) -> Result<Box<State>, Error> {
        println!("Running handle of A");
        Ok(Box::new(B {}))
    }
}

struct B {}
impl State for B {
    fn handle(self) -> Result<Box<State>, Error> {
        println!("Running handle of B");
        Ok(Box::new(A {}))
    }
}

#[test]
fn assert_works() {
    macro_rules! assert_state {
        ($machine:ident, $state:ident) => {
            use std::any::Any;
            let any: Box<Any> = Box::new($machine);
            assert!(
                if let Ok(_) = any.downcast::<$state>() {
                    true
                } else {
                    false
                },
                "Failed to get to {} state.",
                stringify!($state)
            );
        };
    }

    let to_b = Box::new(A {}).handle().unwrap();

    assert_state!(to_b, B);
}