F# memoize module

memo.fsx

module memo

open System.Collections.Generic

let memoize fn =
  //let cache = new System.Collections.Generic.Dictionary<_,_>()
  let cache = new 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