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#[derive(Clone)]
pub struct Maybe<T> {
    r: Option<T>,
}

impl<T> Maybe<T> {
    pub fn just(r: Option<T>) -> Maybe<T> {
        return Maybe { r: r };
    }
    pub fn of(r: Option<T>) -> Maybe<T> {
        return Maybe::just(r);
    }
    pub fn val(r: T) -> Maybe<T> {
        return Maybe::just(Some(r));
    }

    pub fn present(&self) -> bool {
        match &self.r {
            Some(_x) => return true,
            None => return false,
        }
    }
    pub fn null(&self) -> bool {
        match &self.r {
            Some(_x) => return false,
            None => return true,
        }
    }
    pub fn let_do<F>(&self, func: F)
    where
        F: FnOnce(&T),
    {
        match &self.r {
            Some(_x) => func(&_x),
            None => (),
        }
    }

    pub fn fmap<F, G>(self, func: F) -> Maybe<G>
    where
        F: FnOnce(Option<T>) -> Maybe<G>,
    {
        return func(self.r);
    }
    pub fn map<F, G>(self, func: F) -> Maybe<G>
    where
        F: FnOnce(Option<T>) -> Option<G>,
    {
        return Maybe::just(func(self.r));
    }
    pub fn bind<F, G>(self, func: F) -> Maybe<G>
    where
        F: FnOnce(Option<T>) -> Option<G>,
    {
        return self.map(func);
    }
    pub fn then<F, G>(self, func: F) -> Maybe<G>
    where
        F: FnOnce(Option<T>) -> Option<G>,
    {
        return self.map(func);
    }
    pub fn chain<F, G>(self, func: F) -> Maybe<G>
    where
        F: FnOnce(Option<T>) -> Maybe<G>,
    {
        return self.fmap(func);
    }
    pub fn ap<F, G>(self, maybe_func: Maybe<F>) -> Maybe<G>
    where
        F: FnOnce(Option<T>) -> Option<G>,
    {
        return maybe_func.chain(|f| self.map(f.unwrap()));
    }

    pub fn option(self) -> Option<T> {
        return self.r;
    }
    pub fn unwrap(self) -> T {
        return self.r.unwrap();
    }
    pub fn or(self, val: T) -> T {
        return self.r.unwrap_or(val);
    }
}

#[test]
fn test_maybe_present() {
    assert_eq!(false, Maybe::just(None::<bool>).present());
    assert_eq!(true, Maybe::val(true).present());

    assert_eq!(true, Maybe::just(None::<bool>).null());
    assert_eq!(false, Maybe::val(true).null());

    let mut val;

    val = false;
    Maybe::just(None::<bool>).let_do(|x| val = *x);
    assert_eq!(false, val);

    val = false;
    Maybe::val(true).let_do(|x| val = *x);
    assert_eq!(true, val);
}
#[test]
fn test_maybe_flatmap() {
    assert_eq!(
        false,
        Maybe::val(true)
            .fmap(|x| return Maybe::val(!x.unwrap()))
            .unwrap()
    );
    assert_eq!(
        true,
        Maybe::val(false)
            .fmap(|x| return Maybe::val(!x.unwrap()))
            .unwrap()
    );

    assert_eq!(
        false,
        Maybe::val(true).map(|x| return Some(!x.unwrap())).unwrap()
    );
    assert_eq!(
        true,
        Maybe::val(false).map(|x| return Some(!x.unwrap())).unwrap()
    );

    assert_eq!(
        true,
        Maybe::val(1)
            .ap(Maybe::val(|x: Option<i16>| if x.unwrap() > 0 {
                return Some(true);
            } else {
                return Some(false);
            }))
            .unwrap()
    );
}
#[test]
fn test_maybe_unwrap() {
    assert_eq!(false, Maybe::just(None::<bool>).or(false));
    assert_eq!(true, Maybe::val(true).or(false));
    use std::panic;

    let none_unwrap = panic::catch_unwind(|| {
        Maybe::just(None::<bool>).unwrap();
    });
    assert_eq!(true, none_unwrap.is_err());
    assert_eq!(true, Maybe::val(true).unwrap());

    assert_eq!(
        true,
        match Maybe::val(true).option() {
            None => false,
            Some(_x) => true,
        }
    );
    assert_eq!(
        false,
        match Maybe::just(None::<bool>).option() {
            None => false,
            Some(_x) => true,
        }
    );
}