use std::marker::PhantomData;

struct StateM<F, S, A>(F, PhantomData<S>, PhantomData<A>);

impl StateM<(), (), ()>
{
    fn unit<S, A>(a: A) -> StateM<impl FnOnce(S) -> (A, S), S, A> {
        StateM(move |s| (a, s), PhantomData, PhantomData)
    }
}