Hello Geoffrey, when you really want to make the first function parameter (`a -> (r,a)`) a monad, you can use `r` as the state instead of `a` (and `[a]`). Then both the first parameter and the result are in the same monad `State r` and you can write: update2 :: State r a -> Int -> [a] -> State r [a] update2 sm 0 (a:as) = sm >>= return . (:as) update2 sm i (a:as) = update2 sm (i-1) as >>= return . (a:) Nevertheless in both cases (update' and update2) you need to pass some bogus initial state (or a value in the case of update') to run the computation. In this case I would use the `Writer r` monad anyway. Finally just note that `State a` and `State [a]` are different monads and they can not be easily used in the same computation (although it's indeed possible). Sincerely, Jan. On Thu, Jul 09, 2009 at 10:34:29PM -0600, Geoffrey Marchant wrote:
Which leads to me writing this:
update' s 0 = do (a:as) <- get let (r, a') = runState s a put (a':as) return r update' s i = do (a:as) <- get put as r <- update' s (i-1) as' <- get put (a:as') return r
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