Following code produces an internal compiler error : callable handler(a as int) as int class Action: cb as handler def constructor(callback as handler): cb = callback class A: a as Action ac = Action() def(para): newa = A(32) def constructor(p): a = ac But if you replace ac = Action() def(para): by ac as Action = Action() def(para): or if you remove the line newa = A(32) or if you remove the line a = ac (replacing them by print "hello" for example) then it compiles (though I'm not sure the generated code is correct)