Methods as Assertions

A method definition can be viewed as a logical assertion.Wheneverwe declare a method as the implementation of an operation, we assert that if the operation is invoked on objects of the appropriate types then the method body will satisfy the specificationof the operation. This view of methods as assertions is simple but general. Among its applications are: methods defined on interfaces as well as on classes; an elementary type system for objects that handles multi-methods; and a mechanism for method dispatch based on the desired output type as well as on the types of arguments. Further, these applications are compatible with traditional execution models and implementation techniques. Logical reasoning about methods plays a role at compile time, then gets out of the way.