# Understanding State Classic List Threaded 4 messages Open this post in threaded view
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## Understanding State

 I'm trying to understand how to use State in a function. I've avoided the topic for several years because State just never seemed very useful, but I figure it's time for me to figure it out: I have the following function > update :: (a -> (r,a)) -> Int -> [a] -> (r, [a]) > update s 0 (a:as) = let (r,a') = s a in (r,a':as) > update s i (a:as) = let (r,as') = update s (i-1) as in (r, a:as') which updates a particular element of a list. Looking at it, I see two parts of the type signature that look like State types, which leads me to think of this: > update' :: State a r -> Int -> State [a] r 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 Now, this just looks awful. The first half, the base condition, is actually "running" a State calculation. And the second half sets the state within the monad twice! I like the idea of using State because it simplifies the type. When I see (a -> (b,a)) I say "Wait a second, that's a State calculation, isn't it?" and then, hopefully, generalize. But I can't write that calculation nearly as concisely. How do I do this properly? -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.haskell.org/pipermail/beginners/attachments/20090710/b855502e/attachment-0001.html
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## Understanding State

 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 > -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.