Writing a MonadRandom instance for MWC (Haskell)
I want to use an MWC random number generator with a random fu and therefore an instance needs to be implemented.My MonadRandom
attempt to consider a Reader (Gen (PrimState m)) v
a MonadRandom v
. However, the following fails:
$(monadRandom [d|
instance (PrimMonad m) => MonadRandom (Reader (MWC.Gen (PrimState m))) where
getRandomWord16 = ask >>= MWC.uniform
getRandomWord32 = ask >>= MWC.uniform
getRandomWord64 = ask >>= MWC.uniform
|])
with error:
Illegal type synonym family application in instance:
Reader (MWC.Gen (PrimState m_ahwD))
In the instance declaration for
‘MonadRandom (Reader (MWC.Gen (PrimState m_ahwD)))’
What am I doing wrong?
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2 answers
Based on what other members have already said.
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleInstances #-}
import Data.Random
import Data.Random.Source
import qualified System.Random.MWC as MWC
import Control.Monad.Reader
import Control.Monad.Primitive
$(monadRandom [d|
instance (PrimMonad m, s ~ PrimState m) => MonadRandom (ReaderT (MWC.Gen s) m) where
getRandomWord16 = ask >>= lift . MWC.uniform
getRandomWord32 = ask >>= lift . MWC.uniform
getRandomWord64 = ask >>= lift . MWC.uniform
|])
testUniform :: MonadRandom m => Int -> m [Double]
testUniform n = replicateM (fromIntegral n) (sample stdUniform)
n :: Int
n = 10^7
main :: IO ()
main = do
seed <- MWC.create
xs <- runReaderT (testUniform n) seed
print (sum xs / fromIntegral n)
but it doesn't work very well
./RandomFuMWC +RTS -s
0.5000432391067587
3,286,220,896 bytes allocated in the heap
2,427,475,880 bytes copied during GC
600,186,048 bytes maximum residency (12 sample(s))
100,510,656 bytes maximum slop
1249 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max pause
Gen 0 5942 colls, 0 par 1.06s 1.13s 0.0002s 0.0013s
Gen 1 12 colls, 0 par 0.82s 1.23s 0.1024s 0.5787s
INIT time 0.00s ( 0.00s elapsed)
MUT time 1.47s ( 1.39s elapsed)
GC time 1.87s ( 2.36s elapsed)
EXIT time 0.01s ( 0.09s elapsed)
Total time 3.35s ( 3.84s elapsed)
%GC time 56.0% (61.3% elapsed)
Alloc rate 2,242,365,923 bytes per MUT second
Productivity 44.0% of total user, 38.3% of total elapsed
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