ErrorStateMonad.elm

type alias ErrorM e a =
    Result e a

type alias StateM s a =
    s -> ( a, s )

type alias MyMonad e s a =
    ErrorM e (StateM s a)

andThen : (a -> MyMonad e s b) -> MyMonad e s a -> MyMonad e s b
andThen userFn m =
    case m of
        Err err ->
            Err err

        Ok stateFn ->
            Ok <|
                \state ->
                    let
                        ( a, stateA ) =
                            stateFn state
                    in
                    userFn a
                    -- this is wrong:
                    -- returns:    MyMonad e s (MyMonad e s b)
                    -- instead of: MyMonad e s b
                    -- looks like job for `join : MyMonad e s (MyMonad e s a) -> MyMonad e s a`
                    -- but my brain is crapping out on that one also