Limitations of let rec in OCaml

Question

I\'m studying OCaml these days and came across this:

OCaml has limits on what it can put on the righthand side of a let rec. Like this one

let memo_rec f         


        

Answer 1

You can use tying-the-knot techniques to define memoizing fixpoints. See for example those two equivalent definitions:

let fix_memo f =
  let rec g = {contents = fixpoint}
  and fixpoint x = f !g x in
  g := memoize !g;
  !g

let fix_memo f =
  let g = ref (fun _ -> assert false) in
  g := memoize (fun x -> f !g x);
  !g

Or using lazy as reminded by Alain:

let fix_memo f =
  let rec fix = lazy (memoize (fun x -> f (Lazy.force fix) x)) in
  Lazy.force fix