Very slow guards in my monadic random implementation (haskell)
- by danpriduha
Hi! I was tried to write one random number generator implementation, based on number class. I also add there Monad and MonadPlus instance.
What mean "MonadPlus" and why I add this instance? Because of I want to use guards like here:
-- test.hs --
import RandomMonad
import Control.Monad
import System.Random
x = Rand (randomR (1 ::Integer, 3)) ::Rand StdGen Integer
y = do
a <-x
guard (a /=2)
guard (a /=1)
return a
here comes RandomMonad.hs file contents:
-- RandomMonad.hs --
module RandomMonad where
import Control.Monad
import System.Random
import Data.List
data RandomGen g => Rand g a = Rand (g ->(a,g)) | RandZero
instance (Show g, RandomGen g) => Monad (Rand g)
where
return x = Rand (\g ->(x,g))
(RandZero)>>= _ = RandZero
(Rand argTransformer)>>=(parametricRandom) = Rand funTransformer
where
funTransformer g | isZero x = funTransformer g1
| otherwise = (getRandom x g1,getGen x g1)
where
x = parametricRandom val
(val,g1) = argTransformer g
isZero RandZero = True
isZero _ = False
instance (Show g, RandomGen g) => MonadPlus (Rand g)
where
mzero = RandZero
RandZero `mplus` x = x
x `mplus` RandZero = x
x `mplus` y = x
getRandom :: RandomGen g => Rand g a ->g ->a
getRandom (Rand f) g = (fst (f g))
getGen :: RandomGen g => Rand g a ->g -> g
getGen (Rand f) g = snd (f g)
when I run ghci interpreter, and give following command
getRandom y (mkStdGen 2000000000)
I can see memory overflow on my computer (1G). It's not expected, and if I delete one guard, it works very fast. Why in this case it works too slow?
What I do wrong?
