Re: [Haskell-beginners] Haskell state monad example - type mismatch error

On Fri, Jul 22, 2011 at 8:35 PM, aditya siram wrote:

Hi, Your main function needs to be: main :: IO () main = putStrLn $ show $ runState addThreeRandoms seed =>(70496,695785320) if you want to preserve the final state

or: main' :: IO () main' = putStrLn $ show $ execState addThreeRandoms seed =>695785320

if you don't.

Thanks. This cleared up the matter.

-deech

On Fri, Jul 22, 2011 at 9:59 AM, Antoine Latter wrote:

...

On Fri, Jul 22, 2011 at 9:50 AM, Rohit Garg wrote:

...

Hi,

I am trying out a simple haskell state monad example. I think I have understood the concept of monads, but I am getting stuck at using State monad. As far as I understand, the code below should compile, but it is throwing a type mismatch error in the argument to show. The rest of the code, however, type checks all right.

You need to include the function 'runState' or 'evalState' somewhere - a value of type 'State x y' is not a function, so trying to apply it to values as if it were a function is not going to work.

http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona... http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona...

Antoine

...

If any one can point out what I am doing wrong, it would be really helpful.

Thanks and regards, Rohit

=============================== import Control.Monad.State import Data.Word

type LCGState = Word32

lcg :: LCGState -> (Integer, LCGState) lcg s0 = (output, s1)    where s1 = 1103515245 * s0 + 12345          output = fromIntegral s1 * 2^16 `div` 2^32

seed :: LCGState seed = 5

getRandom :: State LCGState Integer getRandom = do    s0 <- get    let (x,s1) = lcg s0    put s1    return x

addThreeRandoms :: State LCGState Integer addThreeRandoms = do    a <- getRandom    b <- getRandom    c <- getRandom    return (a+b+c)

main :: IO () main = putStrLn show(addThreeRandoms seed)

-- Rohit Garg

http://rpg-314.blogspot.com/

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-- Rohit Garg http://rpg-314.blogspot.com/