[Caml-list] recursive records

[Brigitte Pientka <bpientka@cs.mcgill.ca>]

I am playing around with using recursive records to define observations about
streams simulating essentially the ideas described in "Copatterns:Programming infinite structures by observations" presented at POPL'13.

type 'a susp = Susp of (unit -> 'a)

type 'a str = {hd: 'a  ; tl : ('a str) susp}

let rec ones = {hd = 1 ; tl = Susp (fun () -> ones)}

This works fine and many examples can be elegantly written this way.  However,
when I define the stream ones via the function delay, OCaml fails.

  let delay f = Susp f

  let rec ones = {hd = 1 ; tl = delay (fun () -> ones)};;
Characters 15-53:
   let rec ones = {hd = 1 ; tl = delay (fun () -> ones)};;
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This kind of expression is not allowed as right-hand side of `let rec'

Could someone explain why this fails?

Thanks, Brigitte