This is copied from my email: -------------- I can have a "normal" method in a class, that has default values for some of its parameters: class A { def someMethod(a, b, c= null) { ... } } This allows me to do: def a= new A() a.someMethod(1, 2, 3) a.someMethod(1, 2) // third one 'c' defaults to null If I had not defined the third param of 'someMethod' as accepting a default value, the second call would have failed with a "No signature...." exception. So far, so good. Now, if I want to do something similar with a method added through the MetaClass: A.metaClass.someMeta = {a, b, c= null -> println "this is someMeta $a $b $c" } a.someMeta(1, 2, 3) // good a.someMeta(1, 2) // fails!!! 1) Observation number one: default-value parameters work for normal closures: def someClo = {a, b, c= 789 -> println "clo clo $a $b $c" } someClo(44, 55) // this one works, printing 789 for 'c' 2) Observation number two: when adding through the MetaClass you lose that feature...