Hi all, I'm trying to use the quick-check library for checking some properties of a user defined data type. Bellow the target data type: data Feature = Feature Id Name FeatureType GroupType Children Properties | FeatureError where: Id = String Name = String FeatureType = int GroupType = int Children = [Feature] Propertyes = [String] I've written the following quick-check property: prop_AlternativeFeature :: Feature -> Feature -> QuickCheck.Property prop_AlternativeFeature fm fc = length (children fc) == 0 ==> length (checkAlternativeFeature fm fc) > 0 When I try to check such property, the result is: ERROR "./EshopModelChecking.hs":11 - Type error in instance member binding *** Term : arbitrary *** Type : Feature *** Does not match : Gen Feature I think that I need to write some arbitrary or generator functions, but I didn't realize how to do that with the availalble quick-checking documentation. Any help will be welcome. Thanks in advance. Rodrigo.