System.Random: StdGen's genRange doesn't match its next

[rturk@…]

> g <- getStdGen
> length $ filter (<0) $ take (10^6) $ unfoldr (Just . next) g
0
> genRange g
(-2147483648,2147483647)

Apparently, StdGen's next only returns non-negative values. Also, StdGen doesn't override de default method genRange. However, the System.Random docs ( http://haskell.org/ghc/docs/latest/html/libraries/base/System-Random.html#v%3Anext) promise:

The next operation returns an Int that is uniformly distributed in the range returned by genRange (including both end points), and a new generator.

Thus, StdGen's next violates the uniform distribution requirement.

[simonpj]

Good point. Rather than try to make Random.stdNext return numbers in the full range (which is dodgy, because stdNext is carefully crafted code), I changed the genRange method of Random StdGen to return the range that stdNext actually delivers.

The documentation only guarantees a 30-bit range, which is still the case.

Simon