You can simply use a state monad and then extract the data later when you don't want the extra state, viz. | sum :: (Num a) => [a] -> a sum xs = flip execState 0 $ mapM_ (\x -> do { s <- get ; put $ x + s }) xs |
For instance I often keep a pseudorandom generator as the state, and use exception handling.
I'd use a monad transformer. Either `ExceptT` (which would add exceptions to the state monad) or `StateT` (which would add state to an exception monad). On 05/15/2018 12:27 AM, Dennis Raddle wrote:
I find myself using the state and exception monads at large scale in my programs. For instance I often keep a pseudorandom generator as the state, and use exception handling.
In local computations, I may want to use some additional state. Is there a way to add a little extra state temporarily?
Let's say I have
data State1 = State1 StdGen
type M1 = State State1
data State2 = State2 State1 Int
type M2 = State State2
runM2 :: M2 a -> Int -> M1 a
someFunc :: M1 Double someFunc = do r <- <compute something pseudorandomly> r2 <- runM2 (deeperFunc r) 3 <..etc..>
deeperFunc :: Double -> M2 Double
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