Let me grab this opportunity to advertise the multistate package [1]. The underlying assumption is that the types of the states in your stack are distinct. In that case, type inference can give you something like "automatic zooming" for a MultiState that contains arbitrary heterogenous lists (tuples). Generous usage of the Monad(Multi)State type class would thereby eliminate the need for do1st and do2nd. See the example in the package. The package does not (yet!) work well with existing transformers, for example one might think of running a StateT on one of the states in a MultiStateT. This is not a problem when using MultiStateT exclusively, but of course bad for interoperability. I am open for specific requests in that direction. In the last few days I have been working on adding a MultiRWST, but this is not completely finished yet (and I am not sure yet how to make the interface consistent; the whole run/eval/exec distinction seems unintuitive, especially when you have multiple states..) Lennart [1] https://hackage.haskell.org/package/multistate On 26/03/15 09:28, Ki Yung Ahn wrote:
Consider you have developed library routines that act on (State s1 a). For some reason, you need more than one state simultaneously. Let's say two side by side for simple example, that is (State (s1,s2) a). To use library functions on one state monad in a two state monad, we need to wrapper that lifts actions of (State s1 a) to (State (s1,s2) a).
It is not difficult to write a lifter for such purposes as below. It is kind of like doing liftA in Applicative libarary, but instead of the last argument 'a' but on the fist argument 's' of (State s a). This seems like an idiom that can often come up. So, I tried some searching in some applicative related libraries and monad transformer related libraries but haven't found this idiom yet.
If you had a need for idioms like below, what do you call it? Or, is there a way more general way I've missed that makes this a very special case of it.
import Control.Monad.State import Control.Monad.Error import Control.Applicative
-- lift an action over a state into a pair of states -- do1st :: State s1 a -> State (s1,s2) a do1st m1 = do (s1, s2) <- get let (a, s1') = runState m1 s1 put (s1',s2) return a
do2nd :: State s2 a -> State (s1,s2) a do2nd m2 = do (s1, s2) <- get let (a, s2') = runState m2 s2 put (s1,s2') return a
-- lift an action over a state with error -- into a pair of states with error -- do1 :: Error e => ErrorT e (State s1) a -> ErrorT e (State (s1,s2)) a do1 m1 = do (s1, s2) <- lift get let (ma,s1') = (runState . runErrorT) m1 s1 case ma of Left e -> throwError e Right a -> do lift $ put (s1',s2) return a
do2 :: Error e => ErrorT e (State s2) a -> ErrorT e (State (s1,s2)) a do2 m2 = do (s1, s2) <- lift get let (ma,s2') = (runState . runErrorT) m2 s2 case ma of Left e -> throwError e Right a -> do lift $ put (s1,s2') return a
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