raichoo via Haskell-Cafe wrote:
First of all the documentation of `hashStableName` says the following:
The Int returned is not necessarily unique; several StableNames may map to the same Int (in practice however, the chances of this are small, so the result of hashStableName makes a good hash key).
OK, sounds fine, let's put this to the test. So I wrote a little program.
module Main where
import Control.Monad
import System.Mem.StableName import System.Environment
main :: IO () main = do args <- getArgs res <- forM [0..10000] $ \i -> do x <- makeStableName i let h' = hashStableName x unless (null args) $ print h' return h'
putStrLn "---------------------------" print $ minimum res print $ maximum res
There is nothing in this program that keeps the stable names alive. It appears, from your experiments, that once a stable name is garbage collected, its ID, which also serves as its hash value, may be reused for another stable pointer. Consider this variant of your main function: main = do res <- forM [0..10000] $ fmap hashStableName . makeStableName performGC res <- forM [0..10000] $ fmap hashStableName . makeStableName performGC res <- forM [0..10000] $ fmap hashStableName . makeStableName print (minimum res, maximum res) This produces `(1,10001)` as output in my tests. I'm not sure how exactly `print` affects garbage collections. Overall, I believe the documentation of `hashStableName` is mostly correct, but it would make sense to stress that the statement is only valid for stable names that are currently alive at a particular point in time, not globally for the whole run of a program. Cheers, Bertram