type M 's 'a  = <| runState : 's -> ('a * 's) |>

val return : forall 's 'a. 'a -> M 's 'a
let return a = <| runState = (fun s -> (a,s)) |>

val bind : forall 's 'a 'b. M 's 'a -> ('a -> M 's 'b) -> M 's 'b
let bind m f = <| runState = (fun s -> let (a,s) = m.runState s in (f a).runState s) |>

let (>>=) = bind
let (>>) m n = m >>= fun _ -> n