use std::marker::PhantomData;

trait TransformStep<S, X> { }

struct Start;


impl<S> TransformStep<S, S> for Start { }

struct Transform<S, F, Xin, Xout, P>
where
    P: TransformStep<S, Xin>,
    F: Fn(Xin) -> Xout,
{
    previous: P,
    transform: F,
    _a: PhantomData<S>,
    _b: PhantomData<Xin>,
    _c: PhantomData<Xout>,
}

impl<S, F, Xin, Xout, P> TransformStep<S, Xout> for Transform<S, F, Xin, Xout, P>
where
    P: TransformStep<S, Xin>,
    F: Fn(Xin) -> Xout,
{ }

impl<S, F, Xin, Xout, P> Transform<S, F, Xin, Xout, P>
where
    P: TransformStep<S, Xin>,
    F: Fn(Xin) -> Xout,
{
    pub fn new(f: F) -> Transform<S, F, Xin, Xout, P> {
        Transform {
            previous: Start,
            transform: f,
            _a: PhantomData,
            _b: PhantomData,
            _c: PhantomData,
        }
    }

    pub fn extend<XXout, G, PP>(self, f:G) -> Transform<S, G, Xout, XXout, PP>
    where
        PP: TransformStep<S, Xout>,
        G: Fn(Xout) -> XXout,
    {
        Transform {
            previous: self,
            transform: f,
            _a: PhantomData,
            _b: PhantomData,
            _c: PhantomData,
        }
    }
}


fn main() {
    let t = Transform::new(|x:u64| x + 10)
        .extend(|x| x * 12)
        .extend(|x| x.to_string());
}