Stateモナド

type State<'s, 'a> = State of ('s ->'a * 's)

type State<'s, 'a> = State of ('s ->'a * 's)

(*
//Runを使わないバージョン
let (>>=) (State g) f =
 State <| fun s ->
  let v,s' = g s
  match f v with
        | State h -> h s'
*)
let Run state (State s) = s state
let (>>=) m f =
  State <| fun s ->
   let v,s' = Run s m
   Run s' <| f v
type StateBuilder () = 
  member this.Bind(m, f) = m >>= f
  member this.Return x = State <| fun s -> x, s
let state = StateBuilder () 
//stateモナドを操作する関数群
let Get = State <| fun s -> (s, s)
let Getf = fun _ -> State <| fun s -> (s,s)
let Put s = State <| fun _ -> ((), s)
let Modify f = state {
                let! s = Get
                do! Put (f s)
                }
let Test1 = state{
    let! a = Get             // a=10 (10,10)
    do! Put (a+5)            // ((),15)
    do! Modify ((+)5)        // ((),20)
    let! b = Get             // b=20 (20,20)
    let! d = Modify ((+)10)  // d=() ((),30)
    return b                 // (20,30)
    }
Test1 |> Run 10 |> printfn "%A" 


let Test2 = state{
    let! a = Getf ()      //a=[1;2] ([1;2],[1;2])
    let! b = Put (3::a)   //b=() ([(),[3;1;2])
    let! c = Getf ()      //c=[3;1;2] ([3;1;2],[3;1;2])
    return (List.sum c)   //(6,[3;1;2])
    }
Test2 |> Run [1;2] |> printfn "%A"