[Haskell-cafe] Convert IO Int to Int

[Krzysztof Skrzętnicki]

On Tue, Jun 9, 2009 at 16:14, Daniel Fischer<daniel.is.fischer at web.de> wrote:
> Am Dienstag 09 Juni 2009 15:57:24 schrieb Magnus Therning:
>> On Tue, Jun 9, 2009 at 2:52 PM, ptrash<ptrash at web.de> wrote:
>> > Hmm...it am not getting through it. I just want to generate a random
>> > number and then compare it with other numbers. Something like
>> >
>> > r = randomRIO (1, 10)
>> > if (r > 5) then... else ...
>>
>> You have to do it inside the IO monad, something like
>>
>>     myFunc  = do
>>         r <- randomRIO (1, 10
>>         if r > 5
>>             then ...
>>             else ...
>>
>> /M
>
> Or make the source of the pseudo-random numbers explicit:
>
> import System.Random
>
> function :: (RandomGen g, Random a) => g -> other args -> result
> function gen whatever
>    | r > 5     = blah newgen something
>    | r < 3     = blub newgen somethingElse
>    | otherwise = bling
>      where
>        (r,newgen) = randomR (lo,hi) gen
>
> and finally, when the programme is run:
>
> main = do
>    args <- getArgs
>    sg <- getStdGen
>    foo <- thisNThat
>    print $ function sg foo
>
> If you're doing much with random generators, wrap it in a State monad.

To avoid reinventing the wheel one can use excellent package available
on Hackage:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MonadRandom

> The die function simulates the roll of a die, picking a number between 1 and 6, inclusive, and returning it in the Rand monad.
> Notice that this code will work with any source of random numbers g.
>
> die :: (RandomGen g) => Rand g Int
> die = getRandomR (1,6)
>
> The dice function uses replicate and sequence to simulate the roll of n dice.
>
> dice :: (RandomGen g) => Int -> Rand g [Int]
> dice n = sequence (replicate n die)
>
> To extract a value from the Rand monad, we can can use evalRandIO.
>
> main = do
>   values <- evalRandIO (dice 2)
>   putStrLn (show values)

Best regards

Krzysztof Skrzętnicki