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

22 Jul 2011

      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.

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/