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

[aditya siram]

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.

-deech


On Fri, Jul 22, 2011 at 9:59 AM, Antoine Latter <aslatter at gmail.com> wrote:
> On Fri, Jul 22, 2011 at 9:50 AM, Rohit Garg <rpg.314 at gmail.com> 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-Monad-State-Lazy.html#v:runState
> http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Monad-State-Lazy.html#v:evalState
>
> 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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>
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