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 <mail@nh2.me> wrote:

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
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