I'm learning QuickCheck, and I'm puzzled by the behavior I'm seeing with conditional properties. After writing and loading a simple qsort function from a separate file, I typed these into ghci: let prop_minimum xs = (length xs > 0) ==> head (qsort xs) == minimum xs where types = xs :: [Integer] let prop_minimum1 xs = (length xs > 3) ==> head (qsort xs) == minimum xs where types = xs :: [Integer] These differ only in that prop_minimum1 uses a more restrictive condition. Then quickCheck prop_minimum outputs "OK, passed 100 tests" but quickCheck prop_minimum1 outputs "Arguments exhausted after 0 tests" Investigating with verboseCheck yields puzzling results. For example, verboseCheck prop_minimum says "OK, passed 100 tests", but some of the test instances it shows have length 0, contrary to the condition... and a significant fraction of the test instances have length > 3, or even length > 10! So why the problem when running quickCheck on prop_minimum1? Running verboseCheck prop_minimum1 just deepens the mystery. Now we see most of the test instances generated having 0 length, and none of them having length > 3. What's going on here? Why is the distribution of generated test instances so drastically altered? BTW, I'm using ghc version 6.10.1, running on Intel Mac OS 10.5.5.