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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Vanessa McHale
Functional Compiler Engineer | Chicago, IL

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