quickCheckAll always return “True”

问题:

I'm trying to use QuickCheck following another answer. I test like this:

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


last' :: [a] -> a
last' [x] = x
last' (_:xs) = last' xs

prop_test x = last' x == last x

check = do
        putStrLn "quickCheck"
        quickCheck (prop_test :: [Char]-> Bool)

check2 = do
        putStrLn "quickCheckAll"
        $quickCheckAll

Then I load it in winGHCI and call check and check2. I get

quickCheck
*** Failed! (after 1 test): 
Exception:
  list.hs:(7,1)-(8,23): Non-exhaustive patterns in function last'
""

which I think it's reasonable. However, I get this from check2

quickCheckAll
True

I'm confused because no matter how I change the last' function, even wrong, quickCheckAll always return True.

What's wrong with my code? How can I fix this?

回答1:

From the Test.QuickCheck.All docs:

To use quickCheckAll, add a definition to your module along the lines of

return []
runTests = $quickCheckAll

and then execute runTests.

Note: the bizarre return [] in the example above is needed on GHC 7.8; without it, quickCheckAll will not be able to find any of the properties.

Adding return [] before your check makes it work for me.

回答2:

To use quickCheckAll, you need a function which reads:

return [] runTests = $quickCheckAll

The other comment mentions this, but doesn't point out that it will still always return true unless the function is located below all of your quickCheck functions!