Alberto,
On 9 February 2016 at 14:43, Thomas Koster
I have an STM transaction that needs some private, temporary state. The most obvious way is to simply pass pure state as arguments, but for efficiency, I would like this state to be some kind of mutable array, like STArray.
On 9 February 2016 at 22:29, Alberto G. Corona
Why not use the state monad transformer?
http://hackage.haskell.org/package/transformers-0.5.1.0/docs/Control-Monad-T...
atomically $ evalStateT todo initialState
Seems Ok to me
evalStateT :: Monad m => StateT s m a -> s -> m a
in this case:
evalStateT :: StateT MyState STM a -> MyState -> STM a
Thank you for your suggestion. Certainly, if the state was an immutable value, small or otherwise cheaply modified, like a finger tree spine, StateT would be a much cleaner alternative. But unfortunately it is the mutability of the STArray that makes it valuable to my application. "StateT (Vector Value) STM" would not help me much as the state would need to be copied every time it was modified. In fact, I used a variation of this in a very early version of my program, except that the Vector was passed even more simply: as an argument, the way ReaderT does, with no actual transformer in sight. Switching over to ST and passing an STArray instead has improved the throughput of my program greatly (far more than I expect to see from any extra parallelism I might gain from using STM). -- Thomas Koster