Given the first program, it seems that the unchanging first element of the tuple could be handled by a Reader monad, leading to the second program, where b becomes the state, but how do I get the constant a from the Reader monad? Michael ================== import Control.Monad.State type GeneratorState = State (Double,Double) sqrtST :: GeneratorState Double sqrtST = do (a,b0) <- get let b1 = (b0**2.0+a)/(2.0*b0) (if (abs (a-b1**2.0)) < 0.000001 then return b1 else do put (a,b1) sqrtST) mySqrt a = let b = a/2.0 in fst ( runState sqrtST (a,b) ) {- *Main> mySqrt 2.0 1.4142135623746899 -} ================== import Control.Monad.Reader import Control.Monad.State type GeneratorState = State Double sqrtST :: GeneratorState Double sqrtST = do b0 <- get let a = ? b1 = (b0**2.0+a)/(2.0*b0) (if (abs (a-b1**2.0)) < 0.000001 then return b1 else do put b1 sqrtST) mySqrt a = let b = a/2.0 in runReaderT (runState sqrtST b) a