unique identifiers as a separate library

[Simon Marlow]

ajb at spamcop.net wrote:
> G'day all.
> 
> Quoting Sebastian Fischer <sebf at informatik.uni-kiel.de>:
> 
>> I have wrapped up (a tiny subset of) GHC's uniques into the package
>> `uniqueid` and put it on Hackage:
>>
>>     http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uniqueid
> 
> First off, thanks for this.
> 
>> The main difference is due to my fear of depending on the foreign
>> function `genSymZh` which I replaced by a global counting IORef.
> 
> Why not depend on this instead?
> 
> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/value-supply

Looking at the code for this, I'm somewhat suspicious that it actually 
works with GHC:

   -- The extra argument to ``gen'' is passed because without
   -- it Hugs spots that the recursive calls are the same but does
   -- not know that unsafePerformIO is unsafe.
   where gen _ r = Node { supplyValue  = unsafePerformIO (genSym r),
                          supplyLeft   = gen False r,
                          supplyRight  = gen True r }

even if that extra Bool argument is enough to fool Hugs, I wouldn't count 
on it being enough to fool GHC -O2!  You probably want to use 
unsafeInterleaveIO like we do in GHC's UniqSupply library.

Also, I'd replace the MVar with an IORef and use atomicModifyIORef for speed.

Cheers,
	Simon