Exhaustive properties in Haskell QuickCheck?

Question

Consider the following QuickCheck program in Haskell

{-# LANGUAGE TemplateHaskell #-}
import Test.QuickCheck
import Test.QuickCheck.All

prop_trivial :: Bool
prop_trivial = 42 == (6 * 7)

-- Wacky boilerplate to make all tests run.
return []
runTests = $quickCheckAll
main = do
  runTests

This works with ghc version 7.8.3 and QuickCheck 2.7.6. Problem is that it repeats the test 100 times. I look around for a mitigation and find exhaustive in the QuickCheck docs here. Groovy! I change my prop_trivial to the following:

prop_trivial = exhaustive $ property $ 42 == (6 * 7)

which type-checks and compiles, but fails:

=== prop_trivial from /blahblahblah/FooTest.hs:6 ===
*** Failed! Falsifiable (after 1 test): 
False

I'm a bit stuck on how to understand and debug this result; the docs are a bit too thin for me to figure out what's going on.

Answer 1

It seems that you can use once to modify a property to run only once.

Some example code:

{-# LANGUAGE TemplateHaskell #-}
import Test.QuickCheck
import Test.QuickCheck.All

prop_trivial :: Int -> Bool
prop_trivial x = x == x
prop_trivial2 = once prop_trivial
prop_true = True
prop_true2 = once True

-- Wacky boilerplate to make all tests run.
return []
runTests = $quickCheckAll
main = do
  putStrLn $ "exhaustive prop_trivial  = " ++ show (exhaustive prop_trivial)
  putStrLn $ "exhaustive prop_trivial2 = " ++ show (exhaustive prop_trivial2)
  putStrLn $ "exhaustive prop_true     = " ++ show (exhaustive prop_true)
  putStrLn $ "exhaustive prop_true2    = " ++ show (exhaustive prop_true2)
  runTests

Output:

exhaustive prop_trivial  = False
exhaustive prop_trivial2 = False
exhaustive prop_true     = True
exhaustive prop_true2    = False
=== prop_trivial from qc2.hs:5 ===
+++ OK, passed 100 tests.

=== prop_trivial2 from qc2.hs:7 ===
+++ OK, passed 1 tests.

=== prop_true from qc2.hs:8 ===
+++ OK, passed 100 tests.

=== prop_true2 from qc2.hs:9 ===
+++ OK, passed 1 tests.

exhaustive prop only returns True if testing of prop is known to be exhaustive - note the difference between exhaustive True and exhaustive $ once True.