In https://travis-ci.org/reflex-frp/reflex/builds/616304027 we notice that hlint suggests: linting file src/Reflex/Spider/Internal.hs... reflex-0.6.2.4/src/Reflex/Spider/Internal.hs:497:13: Warning: Use traverse_ Found: WeakBag.traverse (coincidenceSubscribedSubscribers subscribed) $ invalidateSubscriberHeight outerHeight Perhaps: traverse_ (coincidenceSubscribedSubscribers subscribed) $ invalidateSubscriberHeight outerHeight 1 hint This suggestion isn’t relevant because we don’t have a “traverse_” equivalent for WeakBag.traverse. Looking at the code in Hlint (src/Hint/Monad.hs), it becomes clear that it’s a dumb search for any usage of “traverse”, regardless of being in the Traversable typeclass. What’s less clear is why this used to not be a problem before recently. This error appears to coincide with an update of hlint from v2.2.2 to v2.2.4. To fix this, tell Hlint to ignore “Use traverse_” hints in Reflex. This can be removed once Hlint is taught to distinguise between traverse from the Traversable typeclass and from somewhere else.