Improving Random

[Alexey Khudyakov]

On 27.05.2020 05:13, Carter Schonwald wrote:
> Zemyla!
> 
> Yeah, absolutely agree that their pr has the wrong shape for monad 
> random, which is why I’m confused by dominics pronouncement :), though 
> its a great contrib.
> 
> It should be (ignoring m being applicative vs monad)
> (For context, their design is roughly  ... MonadRandom m s g | m,g -> s 
> ?  I’m typing on a phone so i might be restating it wrong ... :) )
> 
> 
> The *better* choice is
> Class PRNG g =>  MonadRandom m g | m-> g where ...
> 
> And then their example instance  should be something like
> 
> Newtype MWCT  m a = ... newtype wrapper around StateT m a threading an 
> MWC indexed by the state token of the underlying PrimMonad
> (Or newtype MWCT s m a = .... stateT thing)
> 
> Instance (PrimMonad m) => MonadRandom  (MWCT m) (MCWGen (PrimState m)) 
> where ...
> 
> Or something along those lines.  It absolutely shouldn’t be in the style 
> of being on any PrimMonad, but part of a stack that provides that instance.
> 
One couldn't use StateT to thread state token for two reasont. First 
StateT from transformer since it allows to thread values of kind * and 
IO's state token is #.

State transformer threading state token on top of some other monad 
_will_ cause problems if it's placed on top of list monad causing
unwanted buffer sharing and possibly loss of referential transparency


> This is ignoring the freeze/thaw stuff which really is just “record and 
> restore RNG state”, and i think is  an artifact of ther design choice in 
> their PR. Which has a lot of great stuff I’m reviewing and poking at.
> 
Frozen/thaw (or immutable/mutable) distinction is inevitable once one 
starts working with in place mutation. In order to get snapshot of PRNG 
state at some particular point it have to be copied.