FW: [GHC] #794: System.Random: StdGen's genRange doesn't match its next

[Simon Peyton-Jones]

Would someone who knows and cares about random numbers like to fix this bug in the Random library?   

Many thanks

Simon

-----Original Message-----
From: glasgow-haskell-bugs-bounces at haskell.org [mailto:glasgow-haskell-bugs-bounces at haskell.org] On Behalf Of GHC
Sent: 12 June 2006 23:28
To: glasgow-haskell-bugs at haskell.org
Subject: [GHC] #794: System.Random: StdGen's genRange doesn't match its next

#794: System.Random: StdGen's genRange doesn't match its next
-------------------------------------+--------------------------------------
    Reporter:  rturk at science.uva.nl  |        Owner:             
        Type:  bug                   |       Status:  new        
    Priority:  normal                |    Milestone:             
   Component:  libraries/base        |      Version:  6.4.2      
    Severity:  normal                |     Keywords:             
          Os:  Unknown               |   Difficulty:  Easy (1 hr)
Architecture:  Unknown               |  
-------------------------------------+--------------------------------------
{{{
 > 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.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/794>
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