Fsharp memoize

memoize.fs

let rec y f x = f (y f) x

let memoize (d:System.Collections.Generic.Dictionary<_,_>) f n = 
   if d.ContainsKey n then d.[n] 
   else d.Add(n, f n);d.[n]

let rec factorialucps factorial' n cont = 
    if n = 0I then cont(1I) else factorial' (n-1I) (fun k -> cont (n*k))

let factorialdpcps  = 
    let d =  System.Collections.Generic.Dictionary<_, _>()
    fun n ->  y (factorialucps >> fun f n -> memoize d f n ) n id


factorialdpcps 15I //1307674368000