[Haskell-cafe] Reader monad wrapping State monad

[michael rice]

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




      
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