module ListRev2;


data Closure<A> = Lam1(Closure<A>, Closure<A>) | Lam2 | Lam3(A);

// let rec apply c a = match c with
//   Lam1(f,g) -> apply f (apply g a) | Lam2 -> a | Lam3(x) -> x :: a ;;

def List<A> apply<A>(Closure<A> c, List<A> a) =
    case c {
    Lam1(f,g) => apply(f, apply(g,a));
    Lam2 => a;
    Lam3(x) => Cons(x,a);
};

def Closure<A> comp<A>(Closure<A> f, Closure<A> g) = Lam1(f,g);

// let rev2 l =
//   let rec walk = function
//     | [] -> Lam2
//     | x :: xs -> comp (walk xs) (Lam3 x)
// in
//   apply (walk l) [];;

def Closure<A> walk<A>(List<A> l) =
    case l {
    Nil => Lam2;
    Cons(x,xs) => comp(walk(xs),Lam3(x));
};


def List<A> rev2<A>(List<A> l) = apply(walk(l), Nil);

def List<A> start<A>(List<A> l) = rev2(l);

{

}