Thanks, yes, it should of course been in Lens!! *Main Control.Lens> :t zoom _1 :: State s1 a -> State (s1,s2) a zoom _1 :: State s1 a -> State (s1,s2) a :: State s1 a -> State (s1, s2) a *Main Control.Lens> :t zoom _2 :: State s2 a -> State (s1,s2) a zoom _2 :: State s2 a -> State (s1,s2) a :: State s2 a -> State (s1, s2) a It works beautifully :) 2015년 03월 26일 01:32에 Roman Cheplyaka 이(가) 쓴 글:
It's called 'zoom' in various lens libraries.
On 26/03/15 10:28, 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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