My guess is that you've disabled the monomorphism restriction somehow... but you need the MR to type-check this code. Without the MR, `new` gets a type `forall a. a`, which gets specialized differently in `new`'s two occurrences, meaning that the type information from the second occurrence doesn't affect the first one... which you need it to in order to type-check the `null new` call.
Hm. I see the MR is off at the command prompt by default, but on in compiled modules. The offending code was in a module starting: module CLTutorial9 where import Prelude hiding (lookup) import Data.Set(Set, insert, empty, member, fromList, toList, union,intersection, size, singleton, (\\)) import qualified Data.Set as S import Test.QuickCheck import Data.Char I haven't knowingly done any thing to turn off the MR.