[Caml-list] fib with lazy list

["SooHyoung Oh" <shoh@duonix.com>]

I'm trying to implement "fib" function with lazy list.
I met the error with message:
    File "eg1.ml", line 19, characters 19-35:
    This kind of expression is not allowed as right-hand side of `let rec'

Can anyone solve this problem?

------- source begin ---------
type 'a lzlist = LzCons of 'a * 'a lzlist Lazy.t;;
let rec forever n = LzCons (n, lazy (forever n));;
let rec lzlist f n = LzCons (n, lazy (lzlist f (f n)));;
let hd (LzCons (n, l)) = n;;
let tl (LzCons (n, l)) = Lazy.force l;;
let from n =
    let add1 m = m + 1 in
    lzlist add1 n
;;
let rec map f lls =
    LzCons (f (hd lls), lazy (map f (tl lls)))
;;
let rec nth lls n =
    match n with
    | 0 -> hd lls
    | n -> nth (tl lls) (n-1)
;;

let rec fib_list = map fib (from 0)    (* ERROR *)
and fib n =
    match n with
    | 1 -> 1
    | n -> nth fib_list (n-1) + nth fib_list (n-2)
;;

let main = fib 5;;
-------------- end of source ------------------
---
SooHyoung Oh

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