El sáb, 21-08-2010 a las 01:21 +0100, Amy de Buitléir escribió:
I'm dipping a toe in the shallow end of the monad pool. I would appreciate any feedback on the first function here, "mutateList". It seems to be working fine, but I have this feeling that I'm doing some unnecessary mucking about with state and I don't know how to fix it. Since both randomListSelection and mutateGene update the state, it seems unnecessary for mutateList to get and put the state as well. What do ye think?
Indeed. The whole point of using the State monad is that you do not have to keep track of the state manually.
I have a list of 16-bit unsigned words. I want to randomly select an element in the list. In the selected element, I want to flip a bit at random.
---------- The code ----------
{-# LANGUAGE PackageImports #-}
import "mtl" Control.Monad.State import System.Random import Data.Word import Data.Bits
-- | ***** LOOK HERE ***** -- | Flip a random bit in a random element in the list. mutateList :: [Word16] -> State StdGen [Word16] mutateList xs = do g <- get let ((i, x), g') = runState (randomListSelection xs) g let (x', g'') = runState (mutateGene x) g' put g'' return (replaceElement xs i x') -- | ***** LOOK HERE *****
This whole getting the state, passing it on, updating it, etc. is exactly what the State monad allows you to avoid. Think about it this way: A value of type State g a is conceptually sth. like a value of type a that can read and write a state of type g. The do-notation for the State monad then does the following: x <- m (where m::State g a) translates to "x gets bound to the value of m in the current state, and in the next line, the current state is updated according to what m does." With that, your code can be simplified to: mutateList xs = do (i, x) <- randomListSelection xs x' <- mutateGene x return (replaceElement xs i x') Implementationwise, State g a is basically g -> (a,g). The propagation of updated states that you have done above is practically exactly what (>>=) is defined to do in the State monad.
-- | Choose an element at random from a list and return the element and its index randomListSelection :: [a] -> State StdGen (Int, a) randomListSelection xs = do g <- get let s = length xs let (i, g') = randomR (0,s-1) g put g' return (i, xs !! i)
This can also be simplified. The only slight problem is that randomR hasn't the precise type we want, but it is very close. Let's look at the type of randomR: randomR :: (Random a, RandomGen g) => (a, a) -> g -> (a, g) In your case, a=Int, and g=StdGen, so 'randomR (a,b)' has type StdGen -> (Int,StdGen) which is practically the same as State StdGen Int. The only thing missing is some newtype wrapping: randomListSelection xs = do i <- State $ randomR (0,length xs - 1) return (i, xs !! i) Same thing holds for mutateGene. I leave that simplification to you, though. Btw., If you want, you can get rid of this newtype wrapping by using the MonadRandom package. But I guess it is instructive to see it working this way first. Jürgen