[Haskell-cafe] Am I misunderstanding something about `StableName` and `hashStableName`?

20 Nov 2016

      Hi,

I've been playing around with `System.Mem.StableName` and `hashStableName`
and
stumbled across a behavior that seems to be rather weird.

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

Running this program without an argument only prints aus the minimum and
maximum
hash values that where produced. So surprises here:

    $ ./Test
    ---------------------------
    1
    10000

Here is how things get a bit weird when I make the progam print out the hash
values while they are being produced I get a completely different result.
Not
only that but I get a whole lot of hash collisions, since the hashes
produced
only seem to be within the range of 1 and 260.

    $ ./Test print
    [...]
    128
    127
    126
    125
    124
    123
    122
    121
    120
    119
    118
    117
    116
    115
    114
    ---------------------------
    1
    260

Am I missing something fundamental here, because this behavior seems to be
rather confusing.

Kind regards,
raichoo