Functional IoC Container Example

open System

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))