A precondition filters generated test cases before the property runs. Cases that fail the condition are discarded rather than evaluated. section 9.5.1
Restricting a sorting property
QuickCheck can require lists to contain at least two elements: `length xs >= 2 ==> sort `sorts` xs`. The property still passes 100 tests, while 43 generated cases that fail the condition are discarded. section 9.5.1
Use a precondition when an input does not reflect the specification or is irrelevant to the functionality under test. Do not use it merely to improve coverage: excluding cases can hide an unhelpful generator, whereas `cover` reports whether useful cases occurred. section 9.5.1
ghci> :{
ghci| prop_sortSorts :: [Int] -> Property
ghci| prop_sortSorts xs = length xs >= 2 ==> sort `sorts` xs
ghci| :}
ghci> quickCheck prop_sortSorts
+++ OK, passed 100 tests; 43 discarded.
See the complete QuickCheck property and its discarded-case report. section 9.5.1
