-- const [] for a HeadList quux :: a -> HeadList a quux = HeadList . (const []) *Main> HeadList "hello... no wait" >>= quux HeadList {getHeadLi

-- const [] for a HeadList quux :: a -> HeadList a quux = HeadList . (const []) *Main> HeadList "hello... no wait" >>= quux HeadList {getHeadList = "*** Exception: Prelude.head: empty list Such breakage is verboten. This monad instance does not fly.