Hi, I¹ve been reading the Monads aren¹t evil posts with interest I¹m a 2nd week Haskell newbie and I¹m doing my best to use them where (I hope) it is appropriate. Typically I¹m writing my code out without using Monads (normally using list recursion), and then when I get them working, I delve into the Monad world.... This has been going well so far with a bit of help from you guys, but I¹ve hit a snag. In the code below I¹m using a state Monad (getEvolution), but unlike simpler cases I¹m passing around two items of state, and one of these states is also ultimately a result although I don¹t care about the result until I reach an end state. My implementation is a bit ugly to say the least and clearly I¹m forcing round pegs into square holes here reading a bit online I get the impression that I can solve the two-state issue using Monad Transformers, by wrapping a StateT around a regular State object (or even two StateT Monads around an Identity Monad??). I think I understand the theory here, but any attempt to implement it leads to a horrible mess that typically doesn¹t compile. The other problem of having a state that is also a result, I¹m sure what to do about this. Was wondering if anyone could give me a push in the right direction how can I rework my state monad so that it looks less wildly. Many thanks, Phil. mcSimulate :: Double -> Double -> Word64 -> [Double] mcSimulate startStock endTime seedForSeed = expiryStock : mcSimulate startStock endTime newSeedForSeed where expiryStock = evalState ( do replicateM_ (truncate(endTime/timeStep)-1) getEvolution; getEvolution ) $ (startStock,ranq1Init seedForSeed) newSeedForSeed = seedForSeed + 246524 discount :: Double -> Double -> Double -> Double discount stock r t = stock * exp (-r)*t payOff :: Double -> Double -> Double payOff strike stock | (stock - strike) > 0 = stock - strike | otherwise = 0 -- Monad Implementation -- Yuk! evolveUnderlying :: (Double, Word64) -> ( Double, (Double, Word64) ) evolveUnderlying (stock, state) = ( newStock, ( newStock, newState ) ) where newState = ranq1Increment state newStock = stock * exp ( ( ir - (0.5*(vol*vol)) )*timeStep + ( vol*sqrt(timeStep)*normalFromRngState(state) ) ) getEvolution :: State (Double, Word64) Double getEvolution = State evolveUnderlying