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.