QuickCheck.hs

-- | Importing this module allows you to use a QuickCheck property as an example
-- for a behavior. Use the 'property' function to indicate a QuickCkeck property.
-- Any output from the example to stdout is ignored. If you need to write out for
-- debugging, you can write to stderr or a file handle.
--
-- > describe "cutTheDeck" [
-- >   it "puts the first half of a list after the last half"
-- >      (property $ \ xs -> let top = take (length xs `div` 2) xs
-- >                              bot = drop (length xs `div` 2) xs
-- >                          in cutTheDeck xs == bot ++ top),
-- >
-- >   it "restores an even sized list when cut twice"
-- >      (property $ \ xs -> even (length xs) ==> cutTheDeck (cutTheDeck xs) == xs)
-- >   ]
--
module Test.Hspec.QuickCheck (
  property, prop
) where

import System.IO.Silently
import Test.Hspec.Core
import qualified Test.QuickCheck as QC

-- just for the prop shortcut
import qualified Test.Hspec.Monadic as DSL

data QuickCheckProperty a = QuickCheckProperty a

property :: QC.Testable a => a -> QuickCheckProperty a
property = QuickCheckProperty

-- | Monadic DSL shortcut, use this instead of @it@
prop :: QC.Testable t => String -> t -> DSL.Specs
prop n p = DSL.it n (QuickCheckProperty p)


instance QC.Testable t => Example (QuickCheckProperty t) where
  evaluateExample (QuickCheckProperty p) = do
    r <- silence $ QC.quickCheckResult p
    let r' = case r of
              QC.Success {}           -> Success
              f@(QC.Failure {})       -> Fail (QC.output f)
              g@(QC.GaveUp {})        -> Fail ("Gave up after " ++ quantify (QC.numTests g) "test" )
              QC.NoExpectedFailure {} -> Fail ("No expected failure")
    return r'