Shrinking

Once QuickCheck has found a counterexample, it’s the tester’s or programmer’s job to figure out why the test fails. The purpose of shrinking is to reduce the size of a counterexample as much as possible to make this more manageable.

Shrinking in action

To see what shrinking does, let’s return to the failing prop_at_zero property. We’ll contrast the same test run. When executed with quickCheck and with verboseCheck, the verbose version of quickCheck gives us a blow-by-blow account of the test run. The first gives a short report:

*Main> quickCheck prop_at_zero
*** Failed! Falsified (after 3 tests and 3 shrinks):
1
[0]

The second also shows the steps that precede the generation of that report:

*Main> verboseCheck prop_at_zero
Failed:
-2
[2,0]
Failed:
2
[2,0]
Passed:
0
[2,0]
Failed:
1
[2,0]
Passed:
0
[2,0]
Passed:
1
[]
Failed:
1
[0]
Passed:
0
[0]
Passed:
1
[]
*** Failed! Falsified (after 3 tests and 3 shrinks):
1
[0]

...