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

[Rohit Garg]

On Fri, Jul 22, 2011 at 8:35 PM, aditya siram <aditya.siram at gmail.com> 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 <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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>



-- 
Rohit Garg

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