pub trait Mode {
    type Output<T>;

    fn bind<T, F>(f: F) -> Self::Output<T>
    where
        F: FnOnce() -> T;

    fn map<T, U, F>(t: Self::Output<T>, f: F) -> Self::Output<U>
    where
        F: FnOnce(T) -> U;

    fn combine<T, U, V, F>(t: Self::Output<T>, u: Self::Output<U>, f: F) -> Self::Output<V>
    where
        F: FnOnce(T, U) -> V;
}

pub struct Emit;

impl Mode for Emit {
    type Output<T> = T;

    fn bind<T, F>(f: F) -> Self::Output<T>
    where
        F: FnOnce() -> T,
    {
        f()
    }

    fn map<T, U, F>(t: Self::Output<T>, f: F) -> Self::Output<U>
    where
        F: FnOnce(T) -> U,
    {
        f(t)
    }

    fn combine<T, U, V, F>(t: Self::Output<T>, u: Self::Output<U>, f: F) -> Self::Output<V>
    where
        F: FnOnce(T, U) -> V,
    {
        f(t, u)
    }
}

pub struct Check;

impl Mode for Check {
    type Output<T> = ();

    fn bind<T, F>(_: F) -> Self::Output<T>
    where
        F: FnOnce() -> T,
    {
    }

    fn map<T, U, F>(_: Self::Output<T>, _: F) -> Self::Output<U>
    where
        F: FnOnce(T) -> U,
    {
    }

    fn combine<T, U, V, F>(_: Self::Output<T>, _: Self::Output<U>, _: F) -> Self::Output<V>
    where
        F: FnOnce(T, U) -> V,
    {
    }
}