type StateMonad() =
member this.ReturnFrom x = x
member this.Return x s = (x, s)
member this.Bind (m, f) s = let (x, s1) = m s
in (f x) s1
let state = new StateMonad()
type IEnvironment =
abstract member UserName : string with get
type ILogger =
abstract member Log : string -> unit
type Container = { Environment : IEnvironment
Logger : ILogger }
let getContainer = fun f -> (f(), f)
let run ctx m = m (fun () -> ctx) |> fst
let rec fac n acc = state { let! container = getContainer
container.Logger.Log (sprintf "%d" acc)
if (n = 0) then return acc
else return! fac (n - 1) (n * acc) }
let compute n = state { let! container = getContainer
container.Logger.Log (sprintf "Begin fac %d" n)
let! result = fac n 1
container.Logger.Log (sprintf "End fac %d" n)
let userName = container.Environment.UserName
container.Logger.Log (sprintf "Computed by %s" userName)
return result }
let prodContainer =
{ Environment = { new IEnvironment with
member this.UserName
with get () =
Environment.UserName }
Logger = { new ILogger with
member this.Log s =
Console.WriteLine ("{0} : {1}", (DateTime.Now), s) } }
let testContainer =
{ Environment = { new IEnvironment with
member this.UserName
with get () =
"John Doe" }
Logger = { new ILogger with
member this.Log s = Console.WriteLine (" {0} ", s) } }
printfn "Result %A" (run prodContainer (compute 10))
printfn "Result %A" (run testContainer (compute 10))
