rapid_check_4_result_tree_build.hs

resultTree :: (Show a, Testable t) => Shrink a -> a -> (a -> t) -> Property
resultTree shrinker arg prop =
  Property $ Gen $ \rand ->
    let shrinkTree = buildTree shrinker arg   -- Build the shrink tree
        resultTree = fmap toResult shrinkTree -- Transform it to a result tree
        toResult x =                          -- To compute a result tree
          addCounterExample x $               -- Add the outer arg to all failures
            runProp (property (prop x)) rand  -- Inside the sub result tree
    in joinTree resultTree                    -- At the end, join the result tree