I'm learning the ropes of QuickCheck >= 2.6 but I don't understand what a shrink is. From looking at the type signature shrink looks more like expand! Please illuminate me :)
When QuickCheck finds an input that violates a property, it will first try to find smaller inputs that also violate the property, in order to give the developer a better message about the nature of the failure.
What it means to be „small“ of course depends on the datatype in question; to QuickCheck it is anything that comes out from from the shrink function.
It is best explained in a QuickCheck session:
Prelude Test.QuickCheck> let prop l = all (/= 5) l Prelude Test.QuickCheck> quickCheck prop *** Failed! Falsifiable (after 10 tests and 2 shrinks): [5]
So here QuickCheck was able to give the smallest counter-example, but judging from the comments, it first had a larger list in mind and then reduced it using shrink. To have a closer look at what is happening, we use verboseCheck:
Prelude Test.QuickCheck> verboseCheck prop Passed: [] Passed: [0] Passed: [-2,1] Passed: [-2,2,-2] Passed: [-4] Failed: [-1,-2,5,4,2] *** Failed! Passed: [] Failed: [5,4,2] Passed: [] Passed: [4,2] Failed: [5,2] Passed: [] Passed: [2] Failed: [5] Passed: [] Passed: [0] Passed: [3] Passed: [4] Falsifiable (after 6 tests and 3 shrinks): [5]
QuickCheck tries a few lists for which the proposition holds, and then finds [-1,-2,5,4,2]. Now it reduces the list, by trying sublists of it. You can convince yourself in GHCi that shrink [-1,-2,5,4,2] == [[],[5,4,2],[-1,-2,2],... and the second entry is the first that still fails the test. QuickCheck then continues with that and shrinks further: shrink [5,4,2] == [[],[4,2],[5,2],..., and further shrink [5,2]
[[],[2],[5],.... Lastly it tries to shrink [5] further, but none of shrink [5] ==
[[],[0],[3],[4]] fail the proposition, so the final count-example is [5].