Smalltalk - Reflection

Reflection

Smalltalk-80 is a totally reflective system, implemented in Smalltalk-80 itself. Smalltalk-80 provides both structural and computational reflection. Smalltalk is a structurally reflective system whose structure is defined by Smalltalk-80 objects. The classes and methods that define the system are themselves objects and fully part of the system that they help define. The Smalltalk compiler compiles textual source code into method objects, typically instances of CompiledMethod. These get added to classes by storing them in a class's method dictionary. The part of the class hierarchy that defines classes can add new classes to the system. The system is extended by running Smalltalk-80 code that creates or defines classes and methods. In this way a Smalltalk-80 system is a "living" system, carrying around the ability to extend itself at run time.

Since the classes are themselves objects, they can be asked questions such as "what methods do you implement?" or "what fields/slots/instance variables do you define?". So objects can easily be inspected, copied, (de)serialized and so on with generic code that applies to any object in the system.

Smalltalk-80 also provides computational reflection, the ability to observe the computational state of the system. In languages derived from the original Smalltalk-80 the current activation of a method is accessible as an object named via a pseudo-variable (one of the six reserved words), thisContext. By sending messages to thisContext a method activation can ask questions like "who sent this message to me". These facilities make it possible to implement co-routines or Prolog-like back-tracking without modifying the virtual machine. The exception system is implemented using this facility. One of the more interesting uses of this is in the Seaside web framework which relieves the programmer of dealing with the complexity of a Web Browser's back button by storing continuations for each edited page and switching between them as the user navigates a web site. Programming the web server using Seaside can then be done using a more conventional programming style.

When an object is sent a message that it does not implement, the virtual machine sends the object the doesNotUnderstand: message with a reification of the message as an argument. The message (another object, an instance of Message) contains the selector of the message and an Array of its arguments. In an interactive Smalltalk system the default implementation of doesNotUnderstand: is one that opens an error window (a Notifier) reporting the error to the user. Through this and the reflective facilities the user can examine the context in which the error occurred, redefine the offending code, and continue, all within the system, using Smalltalk-80's reflective facilities.

Another important use of doesNotUnderstand: is intercession. One can create a class that does not define any methods other than doesNotUnderstand: and does not inherit from any other class. The instances of this class effectively understand no messages. So every time a message is sent to these instances they actually get sent doesNotUnderstand:, hence they intercede in the message sending process. Such objects are called proxies. By implementing doesNotUnderstand: appropriately, one can create distributed systems where proxies forward messages across a network to other Smalltalk systems (a facility common in systems like CORBA, COM+ and RMI but first pioneered in Smalltalk-80 in the 1980s), and persistent systems where changes in state are written to a database and the like. An example of this latter is Logic Arts' VOSS (Virtual Object Storage System) available for VA Smalltalk under dual open source and commercial licensing.