On 9 February 2012 10:59, Aleksey Khudyakov
So is it possible to use the fast and efficient mersenne generator with the convenient and general random API?
I think design of Random type class basically precludes efficient generators with large periods and consequently large state. Look at next function:
next :: g -> (Int, g)
It means that state has to be copied but for efficiency we want to mutate it in place. I consider Random type class a failure and ignore it.
Actually it is not true that the state has to be copied. Using the lazy ST monad we can implement this interface and internally use mutable ST arrays. See for example http://web.archive.org/web/20090108050217/http://www.augustsson.net/Darcs/MT... It ends up with this function that generates the infinite lazy sequence of words from a seed word. This can be then made to fit the next :: g -> (Int, g) interface, with g = [W]. mersenneTwister :: W -> [W] mersenneTwister s = L.runST (mersenneTwisterST s) For reference: import Control.Monad.ST.Lazy as L type W = Word32 mersenneTwisterST :: W -> L.ST s [W] So yes this looses a small constant factor because of the extra lazy list (which could be reduced somewhat), so it's not suitable for the absolutely max performance interface, but it certainly allows the use of traditional mutable PRNGs with the nice interface with decent performance. Certainly none of these PRNGs support split, but that's a separate issue. Overall, I think the Random type class is salvageable given a few changes. In the end, the Random module probably needs both an ST-monadic interface and a pure one. People can use the ST one if it happens to be more convenient or if they need to squeeze the last drop of performance out. Duncan