SplitMonad.hs

newtype RandomT m a = RandomT# { unRandomT# :: (SplitMix64 ->  m (a , SplitMix64) ) }

instance Functor m => Functor (RandomT m) where
  fmap = \ f (RandomT# mf) ->
              RandomT# $  \ seed ->
                              fmap (\(a,s) -> (f a, s)  )   $ mf   seed

instance Applicative m => Applicative (RandomT m) where
  pure = \ x ->  RandomT# $  \ s  ->  pure  ( x , s  )
  (<*>)  = \ (RandomT# frmb) (RandomT# rma) ->  RandomT# $ \ s ->
                    let (# !fseed, !maseed #) = splitGeneratorSplitMix s
                        mfOldSeed= frmb fseed
                        mArgNewSeed = rma  maseed
                        in (fmap (\(f,_s)-> \(x,newSeed)-> (f x, newSeed) ) mfOldSeed)
                            <*> mArgNewSeed

instance Monad m => Monad (RandomT m) where

  (>>=) = \ (RandomT#  ma) f -> RandomT# $  \ s ->
      let (# fseed, nextSeed #) = splitGeneratorSplitMix s
       in
          do
            (a,_boring) <- ma fseed
            unRandomT# (f a) nextSeed