Recursion schemes first steps

rs.rs

// struct FixSexpr(Sexpr<alloc::boxed::Box<FixSexpr>>);
enum SExpr<A=()> {
  Ground(Arc<str>),
  Var(i64),
  App(App<A>)
}
struct App<A> { f : A, a : A}

trait Functor {
  type F<T>;
}
trait Indirect {
  type I<T>;
}
struct FixPoint<F : Functor, I : Indirect>(F::F<I::I<FixPoint<F,I>>>);

enum SExpresion {}
enum Boxed {} 
impl Functor for SExpresion {
  type F<T> = SExpr<T>;
}
impl Indirect for Boxed {
  type I<T> = Box<T>;
}

type SexprFix = FixPoint<SExpresion, Boxed>;