There's a slightly cleaner way to do it. Of course it also uses
`unsafePerformIO` under the hood, but you can let Test.QuickCheck.Monadic
worry about that :)
Something like this: https://gist.github.com/etrepum/d450420a7fd8c2e73aec
import Data.Either (isLeft)
import Control.Exception (try, evaluate, SomeException)
import Test.QuickCheck (Property, quickCheck)
import Test.QuickCheck.Monadic (monadicIO, run, assert)
isFailure :: a -> IO Bool
isFailure = fmap isLeft . tryEval
where
tryEval :: a -> IO (Either SomeException a)
tryEval = try . evaluate
prop_empty_list :: Int -> Property
prop_empty_list idx = monadicIO (run (isFailure ([] !! idx)) >>= assert)
prop_unexpected_success :: Int -> Property
prop_unexpected_success idx = monadicIO (run (isFailure ([()] !! idx)) >>=
assert)
main :: IO ()
main = mapM_ quickCheck [ prop_empty_list, prop_unexpected_success ]
On Sat, Jun 7, 2014 at 1:11 PM, Niklas Hambüchen
On 07/06/14 19:56, Rafael Almeida wrote:
Let's say we want to quickcheck the function !!. Passing a negative index to it is an error. However, how can you make a check that it indeed errors in such situation?
Hello!
You can do so with the divine unsafePerformIO-try-evaluate-force combo.
-- Code also at https://gist.github.com/nh2/1ce734759b196c3483fa {-# LANGUAGE ScopedTypeVariables #-}
import Control.Exception import Control.DeepSeq import Data.List (isPrefixOf) import System.IO.Unsafe (unsafePerformIO) import Test.QuickCheck
main :: IO () main = quickCheck $ property $ \(list :: [Char], n :: Int) -> n >= length list ==> throwsIndexError (list !! n)
throwsIndexError :: (NFData a) => a -> Bool throwsIndexError expr = unsafePerformIO $ do res <- try $ evaluate (force expr) case res of Left (ErrorCall msg) -> return $ "Prelude.(!!): index too large" `isPrefixOf` msg Right _ -> return False -- no exception
-- End of code
`force` makes sure that the expression is evaluated all the way to the bottom, and nothing is left lazily unevaluated. If you don't desire that, you can leave `force` out.
We use `isPrefixOf` because for some reason, the exception contains a newline.
In case you are not familiar with deepseq, the `NFData a` requirement makes sure that the expression can be evaluated all the way down. If you don't want/need to use `force`, you can drop that one.
The use of `unsafePerformIO` is OK here because it is referentially transparent: We use it only to wrap a pure function, and perform no further IO in it.
Hope that helps! Niklas _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe