Ticket #2535: variant-bug.patch

Thu Aug 21 20:59:48 PDT 2008  Patrick Perry <patperry@stanford.edu>
  * Fix bug in variant
  
  Currently, "variant" will throw an execption if given a negative argument.
  Moreover, the runtime of variant is linear in the argument.  Both of these
  issues manifest in the following properties, which will either hang or
  throw an exception, depending on the platform:
  
  \begin{code}
  import Test.QuickCheck
  import Text.Show.Functions
  
  prop_ok (f :: Double -> Double) =
     f (-5.5) `seq` True
  
  prop_bug (f :: Double -> Double) =
     f (-5.6) `seq` True
  
  main = do
     putStr "prop_ok:\t"  >> quickCheck prop_ok
     putStr "prop_bug:\t" >> quickCheck prop_bug
  \end{code}
  
  This patch backports the version of variant in QuickCheck2.  It avoids the
  negativity problem and makes the run-time logarithmic in the size of the
  argument.
  

New patches:

[Fix bug in variant
Patrick Perry <patperry@stanford.edu>**20080822035948
 
 Currently, "variant" will throw an execption if given a negative argument.
 Moreover, the runtime of variant is linear in the argument.  Both of these
 issues manifest in the following properties, which will either hang or
 throw an exception, depending on the platform:
 
 \begin{code}
 import Test.QuickCheck
 import Text.Show.Functions
 
 prop_ok (f :: Double -> Double) =
    f (-5.5) `seq` True
 
 prop_bug (f :: Double -> Double) =
    f (-5.6) `seq` True
 
 main = do
    putStr "prop_ok:\t"  >> quickCheck prop_ok
    putStr "prop_bug:\t" >> quickCheck prop_bug
 \end{code}
 
 This patch backports the version of variant in QuickCheck2.  It avoids the
 negativity problem and makes the run-time logarithmic in the size of the
 argument.
 
] {
hunk ./Test/QuickCheck.hs 99
-variant v (Gen m) = Gen (\n r -> m n (rands r !! (v+1)))
+variant v (Gen m) = Gen (\n r -> m n (rands r v))
hunk ./Test/QuickCheck.hs 101
-  rands r0 = r1 : rands r2 where (r1, r2) = split r0
+  rands r0 0 = r0
+  rands r0 n = let (r1,r2) = split r0
+                   (n',s)  = n `quotRem` 2
+               in case s of
+                    0 -> rands r1 n'
+                    _ -> rands r2 n'
}

Context:

[Make QuickCheck work with extensible exceptions
Ian Lynagh <igloo@earth.li>**20080805125743] 
[TAG GHC 6.8.3 release
Ian Lynagh <igloo@earth.li>**20080619124351] 
[TAG 2008-06-06
Ian Lynagh <igloo@earth.li>**20080605235803] 
Patch bundle hash:
38f20a1de98fa973219f7379d25d33b0d399dd0b