[Haskell-beginners] Random computation with fixed seed value
(Control.Monad.Random)
Daniel Fischer
daniel.is.fischer at web.de
Fri Nov 26 06:16:14 EST 2010
On Friday 26 November 2010 08:20:38, Jan Snajder wrote:
> Hi,
>
> I'd like to evaluate a random computation using Control.Monad.Random,
> but I'd like to be able to fix the seed value. I know I can use mkStdGen
> from System.Random to get an initial generator. I also know I can use
>
> evalRand :: RandomGen g => Rand g a -> g -> a
>
> from Control.Monad.Random to evaluate a random computation. But the
> problem is I don't see how I can turn a StdGen value that I get from
> mkStdGen into a Rand type.
What would you need that for?
Rand is basically the State monad (newtype wrapped), which is basically
s -> (a,s)
(newtype wrapped).
You can write your computation so that it works with whatever pseudo-random
generator it is given and then let g be determined by the value passed to
evalRand. That is the default use case.
For example
die :: (RandomGen g) => Rand g Int
die = getRandomR (1,6)
dice :: (RandomGen g) => Int -> Rand g [Int]
dice n = sequence (replicate n die)
main :: IO ()
main = do
print $ evalRand (dice 10) (mkStdGen 123)
print $ evalRand (dice 10) (mkBlumBlumShub 2379009 1234567890983)
(for a hypothetical
data BlumBlumShub = BBS !Integer !Integer
instance RandomGen BlumBlumShub where ...
mkBlumBlumShub :: Integer -> Integer -> BlumBlumShub
)
> What am I missing here?
>
> Best,
> Jan
>
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