New subject: Conditional properties in QuickCheck alter test data generation?

17 Dec 2008

      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.

Conditional properties in QuickCheck alter test data generation?

Kevin Van Horn

Thomas Schilling

Gwern Branwen

Duncan Coutts

Max Rabkin

Duncan Coutts

Kevin Van Horn

tags

participants (5)