This is variant of https://gist.github.com/Thorium/df46659348b0e2146cf6 that is having a time-out. You may want to use this if you e.g. cache a database query result or other mutable data source.

MemoizeWithTimeout.fs

open System
open System.Collections.Concurrent  
let cache = ConcurrentDictionary<(string * obj),Lazy<obj>>()
let cacheTimes = ConcurrentDictionary<string,DateTime>()
let cacheTimeSeconds = 30.
let memoizeConcurrent (caller:string) (f: ('a -> 'b)) = fun x ->
    match cacheTimes.TryGetValue caller with
    | true, time when time < DateTime.UtcNow.AddSeconds(-cacheTimeSeconds)
        -> cache.TryRemove((caller, x|>box)) |> ignore
    | _ -> ()
    (cache.GetOrAdd((caller, x|>box), 
        lazy (( cacheTimes.AddOrUpdate(caller, DateTime.UtcNow, fun _ _ ->DateTime.UtcNow) |> ignore
                f x)|>box)).Force() |> unbox) : 'b

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