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17.2.6 Object Orientation


Most of the MovieScript components discussed so far, such as Fortran90-style matrix algebra or communication, are implemented in terms of extended sets of Postscript types and operators. In the area of data-parallel computing, based on a finite set of generic operations, the distinction between language models such as Fortran, C, C++, Lisp, or PostScript is largely a matter of taste. However, with the growing structural complexity of a given domain, typically associated with irregular, dynamic computational complexity, both the compiled imperative languages such as Fortran or C/C++ as well as the interpreted functional languages such as Lisp of PostScript become impractical. The best techniques invented so far to handle such complex problems are provided by the interpretive object-oriented models.

MovieScript extends PostScript by the full object-oriented sector with all ``orthodox'' components such as polymorphism, encapsulation, data abstraction, dynamic binding, and multiple inheritance. This extension process is organized structurally in the form of a two-dimensional inheritance forest which provides a novel design platform for integrating functional and object-oriented language structures.

All the original PostScript types such as array, string, dict, and so on. are retained and included in the topmost ``horizontal'' layer of primitive types in MovieScript. This layer is further extended by new computation, graphics, and communication primitives. The design objectives of this language sector are optimized performance, structural simplicity, and enforced polymorphism of the operator set. The group of primitive types within the inheritance forest plays the role of the root class in conventional object-oriented models.

At the same time, the PostScript syntax itself is also extended within the MovieScript design to support the C++-style, ``true'' object-oriented model with dynamic binding and multiple inheritance. The derived types, constructed via the inheritance mechanism starting from the primitive functional types, extend the inheritance forest in the ``vertical'' direction towards more composite, complex, and abstract language structures. A finite set of primitive types is constructed in C and hardwired into the server design. Other primitive types and all derived types are constructed at run time at the interpreted level. Some elements of the inheritance forest model are illustrated in Figure 17.5.

Figure 17.5: Elements of the Inheritance Forest Model. The upper horizontal axis represents primitive MovieScript types such as dict, array, xtwidget, and so on. The forest of derived types extends down and follows the multiple inheritance model. Closed loops in the inheritance network are allowed and resolved by maintaining only a single copy of a degenerate superinstance. The figure illustrates the image browser class which can be thought of as being both a dictionary (of image names) and a widget (such as a selection list). An instance of a derived type is represented by a noncontiguous collection of superinstance headers and buffers, with each buffer maintaining a list of pointers to its superinstance headers, as illustrated in the figure.

The integration of the PostScript-style functional layer with the C++-style object-oriented layer, as well as the ``in large'' extensibility model which defines a suitable balance between both layers, are considered distinctive features of MovieScript. The idea is to encapsulate the structural complexity in the form of methods for derived types and to maintain a finite set of maximally polymorphic operators in the functional sector. The resulting organization is similar to the way complexity is handled by natural languages and human practices. The world is described by a large number of ``things'' (objects, words) and a relatively small number of ``rules'' (polymorphic operators, relations). We could define ``common English'' as a set of rules and a very restricted subset of objects. The ``expert English'' dialects are constructed by extending the vocabulary by more specialized and/or abstract objects with complex methods and inheritance patterns. The process of building expert extensions is graded and consistent with the human learning process.

Our claim is that the good ``in large'' computer language design should contain a nontrivial ``common English'' part, useful by itself for a broad set of generic tasks, and it should offer a graded, multiscale  extensibility model towards specialized expert dialects. Indeed, we program by building reusable associations between software entities and names. Each ``in large'' programming model unavoidably contains a large number of names. The disciplined and structured process of naming software entities is crucial for successful complexity control. In languages such as C or Fortran, the ``common English'' part is reduced to arithmetic and simple control structures such as if, for, switch, and so on. All other names are simply mapped on a huge and ever-growing linear chain of library functions. The original language syntax, based on mathematics notation, degenerates towards a poorly organized functional programming style. ``In large'' programming in such languages becomes very complex.

More abstract models such as functional, object-oriented and dataflow modular programming are more useful, but there are usually some structure versus function trade-offs in the individual language designs and the optimal choice for ``in large'' model is all but obvious. A few examples of various language models are sketched in Figure 17.6. In our approach, we consider the object-oriented techniques as the best available tool for building expert extensions (with the expert knowledge encapsulated in methods) and the functional model of PostScript as the best way of encoding the common part of the language. PostScript operators play the role of rules and Postscript primitive types represent the common vocabulary. Inheritance forest of MovieScript allows for smooth transition from common to expert types.

Figure 17.6: Computational Vertices in Various Language Models. Solid arrows indicate input/output arguments or objects. Wavy lines indicate messages sent to objects. Dark blobs represent nonsyntactic components of the language. Light blobs represent polymorphic operators, considered as syntactic identifiers/keywords. Among the models illustrated in the figure, we consider MovieScript organization of computational vertices as most adequate for ``in large'' programming. C, AVS, and PostScript have a poor encapsulation model. C and C++ are not convenient for dynamic dataflow programming as they don't offer any universal mechanism for multiobject/argument output. MovieScript vertices are constructed by superposition of the C++ style encapsulation model and PostScript-style multiobject interaction model. Large MovieScript operators are functionally similar to AVS modules but they follow a multiscale structural design which enforces software economy and reusability.

The complexity of ``expert English'' is encapsulated in methods for derived types, and general-purpose functionality of ``common English'' is exposed in terms of restricted set of polymorphic operators, processing objects of all granularities. A multiscale language development model, supported by such organization, is discussed in Section 17.2.8.

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Next: 17.2.7 Integrated Visualization Model Up: 17.2 System Overview Previous: 17.2.5 Distributed Computing

Guy Robinson
Wed Mar 1 10:19:35 EST 1995