[Haskell-cafe] Explicit approach to lazy effects For probability monad?

Olaf Klinke olf at aatal-apotheke.de
Sun Oct 20 20:45:38 UTC 2019

> It kind of seems like this can't work if >>= has type 'm a -> (a -> m b) 
> -> mb', assuming m a = (a,state).  The second argument needs to have 
> type (m a -> m b) instead of (a -> m b), since the computation needs to 
> have access to the state produced by the first "m a" computation, in 
> order to (optionally) depend on the state thread for first computation.

My earlier suggestion of using the pure reader monad is not useful for two reasons: 
(1) A single random number is shared among all sub-computations, which destroys statistical independence. 
(2) Multi-dimensional distributions arguably require more than a single floating point number to parametrize. 

Maybe the following two monads hint at a better way, although none of them exhibits the two desirable properties I suggested in my first reply. It seems that both exhibit the sequential nature that you want to avoid, if the underlying monad m is sequential (i.e. stateful). 

(a) The free monad over the reader functor.

type Distribution a 
   = Free ((->) Double) a 
   = Pure a | Impure (r -> Free ((->) Double) a)
This is a reader monad of functions 
(Double,Double,...,Double) -> a
where the tuple length may be any finite number. 
-- @choose p@ chooses the first distribution with probability @p at . 
choose :: Double -> Distribution a -> Distribution a -> Distribution a
choose p x y = Impure (\r -> if r <= p then x else y)
-- Feed a random number generator into a distribution. 
-- You get to decide which monad that is. 
sample :: Monad m => m r -> Free ((->) r) a -> m a
sample _ (Pure a) = pure a
sample gen (Impure f) = (sample gen) =<< (fmap f gen)

(b) A reader monad which passes copies of the random number generator around, not the generator state. Again, you get to choose which monad the random number generator lives in. 

type DistrT m a
   = ReaderT (m Double) m a
   ~ m Double -> m a
-- 'chooseT' queries the random number generator once 
-- and then passes it down to either of the two choices. 
chooseT :: Monad m => Double -> DistrT m a -> DistrT m a -> DistrT m a
chooseT p x y = ReaderT (\gen -> gen >>= (\r -> if r <= p then runReaderT x gen else runReaderT y gen))
sampleT = runReaderT :: DistrT m a -> m Double -> m a

Apologies that I can not be any more helpful at the moment. But the topic really interests me, please do post any progress on haskell-cafe. 


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