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