The bug is in the "variant" function in QuickCheck. I replaced
variant :: Int -> Gen a -> Gen a
variant v (Gen m) = Gen (\n r -> m n (rands r !! (v+1))
where
rands r0 = r1 : rands r2 where (r1, r2) = split r0
with
variant :: Int -> Gen a -> Gen a
variant v (Gen m) = Gen (\n r -> m n (rands r !! v'))
where
v' = abs (v+1) `mod` 10000
rands r0 = r1 : rands r2 where (r1, r2) = split r0
and now everything works fine. "10000" seems like a reasonable value
here, but one could hard-code a lower or higher value as well.
Can someone make sure this gets fixed in the next version of
QuickCheck? I'm not sure who the maintainer is.
Patrick
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