A disclaimer first: I'm not an expert on QuickCheck and I had no experience with monadic checking before your question, but I see stackoverflow as an opportunity to learn new things. If there's an expert answer saying this can be done better, I'll remove mine.
Say you have a function test
that can throw exceptions using exitWith
. Here's how I think you can test it. The key function is protect
, which catches the exception and converts it to something you can test against.
import System.Exit
import Test.QuickCheck
import Test.QuickCheck.Property
import Test.QuickCheck.Monadic
test :: Int -> IO Int
test n | n > 100 = do exitWith $ ExitFailure 1
| otherwise = do print n
return n
purifyException :: (a -> IO b) -> a -> IO (Maybe b)
purifyException f x = protect (const Nothing) $ return . Just =<< f x
testProp :: Property
testProp = monadicIO $ do
input <- pick arbitrary
result <- run $ purifyException test $ input
assert $ if input <= 100 then result == Just input
else result == Nothing
There are two disadvantages to this, as far as I can see, but I found no way over them.
I found no way to extract the
ExitCode
exception from theAnException
thatprotect
can handle. Therefore, all exit codes are treated the same here (they are mapped toNothing
). I would have liked to have:purifyException :: (a -> IO b) -> a -> IO (Either a ExitCode)
I found no way to test the I/O behavior of test. Suppose
test
was:test :: IO () test = do n <- readLn if n > 100 then exitWith $ ExitFailure 1 else print n
Then how would you test it?
I'd appreciate more expert answers too.