hspec如何运作？

Question

来自here

type Selector a = (a -> Bool)

shouldThrow :: (HasCallStack, Exception e) => IO a -> Selector e -> Expectation
action `shouldThrow` p = do
  r <- try action
  case r of
    Right _ ->
      expectationFailure $
        "did not get expected exception: " ++ exceptionType
    Left e ->
      (`expectTrue` p e) $
        "predicate failed on expected exception: " ++ exceptionType ++ " (" ++ show e ++ ")"
  where
    -- a string repsentation of the expected exception's type
    exceptionType = (show . typeOf . instanceOf) p
      where
        instanceOf :: Selector a -> a
        instanceOf _ = error "Test.Hspec.Expectations.shouldThrow: broken Typeable instance"

anyException :: Selector SomeException
anyException = const True

具体为什么any...Exception始终返回True以及instanceOf如何工作？有没有解释这一切的文件？