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