F# fibonacci with memoize

memofib1.fsx

let memoize fn =
  let cache = new System.Collections.Generic.Dictionary<_,_>()
  printfn "counter: %A" cache.Count
  
  fun x ->
    printfn "%A is received" x
    match cache.TryGetValue x with
    | true, v -> v
    | false, _ -> let v = fn (x)
                  cache.Add(x,v)
                  v

let rec fib n =
  match n with
  | 0 | 1 -> n
  | n -> fib (n-1) + fib (n-2)

let memo_fib = memoize fib

let sw = new System.Diagnostics.Stopwatch()
sw.Start()
memo_fib 45 |> printfn "%A"
sw.ToString() |> printfn "%s"