quickcheck-dynamic-3.0.0: src/Test/QuickCheck/DynamicLogic/CanGenerate.hs

module Test.QuickCheck.DynamicLogic.CanGenerate (canGenerate) where

import System.IO.Unsafe
import Test.QuickCheck

-- | @canGenerate prob g p@
--   returns @False@ if we are sure @Prob(g generates x satisfying p) >= prob@
--   otherwise @True@ (and we know such an x can be generated).
canGenerate :: Double -> Gen a -> (a -> Bool) -> Bool
canGenerate prob g p = unsafePerformIO $ tryToGenerate 1
  where
    tryToGenerate luck
      | luck < eps = return False
      | otherwise = do
          x <- generate g
          if p x
            then return True
            else tryToGenerate (luck * (1 - prob))

    -- Our confidence level is 1-eps
    eps = 1.0e-9