新スレ記念にState Monadを作ってみましたよ。(　ﾟдﾟ)ﾉ

#light

type State<'s, 'a> = ST of ('s -> ('a * 's))
　with
　　static member Run (ST x) = x
　　static member Get () = ST <| fun s -> (s, s)
　　static member Put s = ST <| fun _ -> ((), s)
　　static member Eval x = State.Run x >> fst
　　static member Exec x = State.Run x >> snd

type StateBuilder () =
　member b.Return a = ST <| fun s -> (a, s)
　member b.Bind (ST x, f) = ST <| fun s -> let (a, s') = x s in State.Run (f a) s'
　member b.Let (x, f) = f x

let state = StateBuilder ()

let _ =
　state
　　{ let x = 100
　　　let! s = State.Get ()
　　　do! State.Put (x + s)
　　　return () }
　|> State.Exec <| 1000
　|> (print_any >> print_newline)


(実行結果)
1100