open System
open System.Collections.Concurrent  

let memoize cacheTimeSeconds (caller:string) (f: ('a -> 'b)) =
    let cacheTimes = ConcurrentDictionary<string,DateTime>()
    let cache = ConcurrentDictionary<'a, 'b>()    
    fun x ->
        match cacheTimes.TryGetValue caller with
        | true, time when time < DateTime.UtcNow.AddSeconds(-cacheTimeSeconds)
            -> cache.TryRemove(x) |> ignore
        | _ -> ()
        cache.GetOrAdd(x, fun x -> 
            cacheTimes.AddOrUpdate(caller, DateTime.UtcNow, fun _ _ ->DateTime.UtcNow)|> ignore
            f(x)
            )

let memoizeAsync cacheTimeSeconds (caller:string) (f: ('a -> Async<'b>)) =
    let cacheTimes = ConcurrentDictionary<string,DateTime>()
    let cache = ConcurrentDictionary<'a, System.Threading.Tasks.Task<'b>>()    
    fun x ->
        match cacheTimes.TryGetValue caller with
        | true, time when time < DateTime.UtcNow.AddSeconds(-cacheTimeSeconds)
            -> cache.TryRemove(x) |> ignore
        | _ -> ()
        cache.GetOrAdd(x, fun x -> 
            cacheTimes.AddOrUpdate(caller, DateTime.UtcNow, fun _ _ ->DateTime.UtcNow)|> ignore
            f(x) |> Async.StartAsTask
            ) |> Async.AwaitTask

(* Example test:
let cacheTimeSeconds = 30.
let myfun = memoize "myTest" cacheTimeSeconds (fun f -> Console.WriteLine("SideEffect"); f + 1)
myfun 3 // "SideEffect"
myfun 5 // "SideEffect"
myfun 3
System.Threading.Thread.Sleep ((cacheTimeSeconds |> int) * 1000 + 100)
myfun 3 // "SideEffect"
myfun 3
*)