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GNU Smalltalk User's Guide

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2.2.1 Introduction

The standard Smalltalk-80 programming environment supports symbolic identification of objects in one global namespace--in the Smalltalk system dictionary. This means that each global variable in the system has its unique name which is used for symbolic identification of the particular object in the source code (e.g. in expressions or methods). Most important global variables are classes defining the behavior of objects.

In a development dealing with modelling of real systems, polymorphic symbolic identification is often needed. This means that it should be possible to use the same name for different classes or other global variables. Let us mention class Module as an example which would mean totaly different things for a programmer, for a computer technician and for a civil engineer or an architect.

This issue becomes inevitable if we start to work in a Smalltalk environment supporting persistence. Polymorphism of classes becomes necessary in the moment we start to think about storing classes in the database since after restoring them into the running Smalltalk image a mismatch with the current symbolic identification of the present classes could occur. For example you might have the class Module already in your image with the meaning of a program module (e.g. in a CASE system) and you might attempt to load the class Module representing a part of the computer hardware from the database for hardware configuration system. The last class could get bound to the #Module symbol in the Smalltalk system dictionary and the other class could remain in the system as unbound class with full functionality, however, it could not be accessed anymore at the symbolical level in the source code.

Objects which have to be identified in the source code of methods or message sends by their names are included in Smalltalk which is a sole instance of SystemDictionary. Such objects may be identified simply by stating their name as primary in a Smalltalk statement. The code is compiled in the Smalitalk environment and if such a primary is found it is bound to the corresponding object as receiver of the rest of the message send. In this way Smalltalk as instance of SystemDictionary represents the sole symbolic name space in the Smalltalk system. In the following text the symbolic name space will be called simply environment to make the text more clear.


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