[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:                
        Type:  bug               |      Status:  new           
    Priority:  normal            |   Component:  libraries/base
     Version:  7.1               |    Keywords:  System.Random 
    Testcase:                    |   Blockedby:                
          Os:  Linux             |    Blocking:                
Architecture:  x86               |     Failure:  None/Unknown  
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Changes (by wgmitchener):

 * cc: garrett.mitchener@… (added)


Comment:

 I ran into this a while back and forgot to post a bug-- sorry.  It's
 nothing so exotic.  If you go down to the function randomIvalDouble in
 Random.hs line 400 or so (and I'm looking at the one from
 http://darcs.haskell.org/packages/random), here's where something is going
 wrong:

        case (randomIvalInteger (toInteger (minBound::Int32), toInteger
 (maxBound::Int32)) rng) of
          (x, rng') ->
             let
              scaled_x =
                 fromDouble ((l+h)/2) +
                 fromDouble ((h-l) / realToFrac int32Range) *
                 fromIntegral (x::Int32)
             in
             (scaled_x, rng')

 ...
 int32Range = toInteger (maxBound::Int32) - toInteger (minBound::Int32)

 But if the pseudo-random integer x happens to be minBound, which is
 -2^31^, then the scaled_x comes out

 1/2 - 2^31^ / (2^32^ - 1)  = 2^31^ ( 1/2^32^ - 1/(2^32^ - 1) ) =
 -1.164e-10 < 0

 which is outside the required range.  An easy fix might be to add 1 to the
 min bound used to generate x.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5133#comment:4>
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