[GHC] #5133: Random instance for Float can generate values out of requested range

[GHC]

#5133: Random instance for Float can generate values out of requested range
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    Reporter:  richardsenington  |        Owner:  rrnewton    
        Type:  bug               |       Status:  new         
    Priority:  normal            |    Milestone:              
   Component:  libraries/base    |      Version:  7.1         
    Keywords:  System.Random     |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Linux             |     Blocking:              
Architecture:  x86               |      Failure:  None/Unknown
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Comment(by scpmw):

 Well, the way this was discovered was a program assuming `floor ((fst
 (random gen) :: Float) * fromIntegral x) < x`. That might well be why
 other languages have chosen this approach as well.

 I read that paper as well after commenting... The interesting question is
 how often the extra bits of information would actually have a positive
 effect in programs. And whether it's worth it to have the algorithm loop a
 few times for that.

 In my humble opinion, the cleanest solution would be to determine how many
 fraction bits we need (e.g. Float -> 23, Double -> 52). Then we can
 generate a random number of the appropriate range, which  maps 1:1 to a
 floating-point representation. Not *all* possible floating point
 representations, obviously, but at least it would be fast, uniform and in
 the [0, 1) range.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5133#comment:8>
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