This happens with both nhc98 1.16 and 1.18: $ cat Q.hs module Q () where import Control.Monad (liftM2) newtype A a = A a $ nhc98 -c Q.hs nhc98comp: isRealData InfoClass 1 Prelude.Monad IEnone \/ b . (1 b) [38,39,40,41] [] [(5,([],[])),(25,([],[])),(26,([],[]))] Thanks Ian