[Haskell-cafe] Reader monad wrapping State monad

[Thomas Davie]

Is the idea here merely an exercise in using the state monad?  This can be easily performed using pure code.

Bob

On 3 Feb 2011, at 19:18, michael rice wrote:

> 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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