Lacking a definition. A val, var, def, or type declaration in a class or trait can be left without a definition, making it abstract. If a class contains any members that are abstract, then it must be declared to be abstract. Abstract types, whether class or trait, can not be instantiated. Only subtypes that have defined all the abstract members can be instantiated.
The concept of writing code so that certain parts of what happens are not concrete until the usage point. Functionality can be abstracted by passing in functions. Types can be abstracted through subtyping or type parameters.
An actor is an object that receives messages and acts on them. It should be able to process those messages in a ...