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
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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On Thursday 03 February 2011 20:18:43, 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?
You need a monad-transformer to use both, Reader and State. You can use either ReaderT Double (State Double) or StateT Double (Reader Double) (they're isomorphic). Then you can query the modifiable state with get (from the MonadState class) and the immutable with ask (from the MonadReader class) type Heron = StateT Double (Reader Double) sqrtH :: Heron Double sqrtH = do a <- ask b <- get let c = 0.5*(b + a/b) if (good enough) then return c else put c >> sqrtH mySqrt a = runReader (evalStateT sqrtH (a*0.5)) a
Hi Daniel,
Ok, but what I was looking for was ReaderT on top, State on the bottom. This is very confusing material, with no apparent conceptual commonality (ad hoc comes to mind) among the many examples I've looked at. Sometimes lift is used, other times a lift helper function, and in this case no use of lift at all.
Michael
--- On Thu, 2/3/11, Daniel Fischer
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?
You need a monad-transformer to use both, Reader and State. You can use either ReaderT Double (State Double) or StateT Double (Reader Double) (they're isomorphic). Then you can query the modifiable state with get (from the MonadState class) and the immutable with ask (from the MonadReader class) type Heron = StateT Double (Reader Double) sqrtH :: Heron Double sqrtH = do a <- ask b <- get let c = 0.5*(b + a/b) if (good enough) then return c else put c >> sqrtH mySqrt a = runReader (evalStateT sqrtH (a*0.5)) a
On Thursday 03 February 2011 21:40:13, michael rice wrote:
Hi Daniel,
Ok, but what I was looking for was ReaderT on top, State on the bottom.
No problem, just change the definition of the Heron type synonym and swap the applcations of runReader[T] and evalState[T] in mySqrt, the monadic sqrtH can remain unchanged :)
This is very confusing material, with no apparent conceptual commonality (ad hoc comes to mind) among the many examples I've looked at. Sometimes lift is used, other times a lift helper function, and in this case no use of lift at all.
That's because only methods of the MonadState and the MonadReader class are used and instances of MonadState are propagated/lifted through ReaderT, instance of MonadReader are propagated/lifted through StateT. ( instance MonadReader r m => MonadReader r (StateT s m) where ask = lift ask local = ... instance MonadState s m => MonadState (ReaderT r m) where get = lift get put = ... ) If you use a function on the inner monad which is not propagated to the entire transformer stack via class instances, you have to use lift (if you have a MonadTrans instance) or something similar.
And swap the arguments.
Thanks for going the extra mile.
Michael
--- On Thu, 2/3/11, Daniel Fischer
Hi Daniel,
Ok, but what I was looking for was ReaderT on top, State on the bottom.
No problem, just change the definition of the Heron type synonym and swap the applcations of runReader[T] and evalState[T] in mySqrt, the monadic sqrtH can remain unchanged :)
This is very confusing material, with no apparent conceptual commonality (ad hoc comes to mind) among the many examples I've looked at. Sometimes lift is used, other times a lift helper function, and in this case no use of lift at all.
That's because only methods of the MonadState and the MonadReader class are used and instances of MonadState are propagated/lifted through ReaderT, instance of MonadReader are propagated/lifted through StateT. ( instance MonadReader r m => MonadReader r (StateT s m) where ask = lift ask local = ... instance MonadState s m => MonadState (ReaderT r m) where get = lift get put = ... ) If you use a function on the inner monad which is not propagated to the entire transformer stack via class instances, you have to use lift (if you have a MonadTrans instance) or something similar.
On 3 February 2011 19:18, michael rice
but how do I get the constant a from the Reader monad?
http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Con... You also need to change the type to use ReaderT. -- Ozgur Akgun
participants (4)
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Daniel Fischer -
michael rice -
Ozgur Akgun -
Thomas Davie