A Haskell-like state monad in Standard ML

state_monad.sml

datatype ('a, 'b) state = State of ('a -> 'b * 'a)
infix >>= >>
fun runState (State f) s =
    f s
fun op>>= (act1, fact2) =
    State (fn s =>
    let
        val (iv, is) = runState act1 s
        val act2 = fact2 iv
    in
        runState act2 is
    end)
fun (State f) >> (State g) =
    State (fn s => g (#2 (f s)))
fun return v = State (fn s => (v, s))
val get = State (fn s => (s, s))
fun put s = State (fn _ => ((), s))
fun modify f = State (fn s => ((), f s))
fun gets f = State (fn s => (f s, s))
fun evalState s = #1 o runState s
fun execState s = #2 o runState s;

(* Represents the function
time state;
time deltaTime() {
  time old = state;
  state = Time.now();
  time new = state;
  return new - old;
} *)
val deltaTime = (get >>= (fn old =>
                 put (Time.now ()) >>
                 get >>= (fn new =>
                 return (Time.- (new, old)))))