Consider the following tests for the distributivity law between reverse and ++,
import Test.QuickCheck
test :: [Int] -> [Int] -> Bool
test xs ys = reverse (xs ++ ys) == reverse xs ++ reverse ys
test2 :: (Eq a) => [a] -> [a] -> Bool
test2 xs ys = reverse (xs ++ ys) == reverse xs ++ reverse ys
Note for lists of Int that
*Main> quickCheck test
*** Failed! Falsifiable (after 5 tests and 3 shrinks):
[1]
[0]
However the test for lists of equatable items,
*Main> quickCheck test2
+++ OK, passed 100 tests.
What makes the second test pass ?
Update On compiling with main = quickCheck test2, the subsequent error on ambiguous type variable hints the problem (as already depicted in answers),
No instance for (Eq a0) arising from a use of `test2'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
When you actually evaluate test2, GHCi has to pick a type a to use. Without more information, GHCi's extended default rules make it default to (), for which the law is true.