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FUTURE COMPUTING ENVIRONMENTS FOR MICROSIMULATION MODELING 214 â¢ Data Desk,76 while not following a strict construction kit approach, provides very effective tools for the construction of data subsets. Combined with powerful multiple views into data, it allows construction and experimentation at the data level similar in feel to the above programs. These programs and similar products all use a construction kit paradigm77 to model a real world environment on the computer. All of the programs are successful, for reasons in addition to sharing the construction kit paradigm: â¢ Each program uses a visual interface, with the object and environmental building blocks taken from the real world objects and environment being modeled. â¢ The objects in each program look and behave like their real world counterparts. â¢ The programs provide a highly interactive environment within which a user can iterate back and forth rapidly and effectively between designing the model and activating it. The graphical user interface is a very important part of an environment based on a construction kit paradigm. Generally, models are effectively viewed visually as a set of interlinked components, where the linkages can be quite complex.78 Allowing a user to manipulate these components, or objects, in visual space provides an effective way to define more complex structures on the basis of simpler ones.79 Object-Oriented Computing Within the past several years, programming language developments have increasingly embraced the notion of object-oriented programming. The basic notion in such languages is that of a design with objects that receive messages as inputs and produce other messages as outputs. Existing objects can be used to define new, more complex objects, which inherit various properties of the objects underlying them. The current shift toward object- oriented languages is 76 DataDesk was written by Paul Velleman (Cornell University) and is distributed by Odesta Corp., Northbrook, Ill. 77 The construction kit paradigm is sometimes called a plug-and-play environment, referring to consumer electronics components that are so simple to assemble and use that âalmost anyoneâ can put one together. 78 Graphical user interfaces are not strictly necessary for defining effective interfaces for certain types of models. For example, the TROLL interface (Maling, 1971) was command line and equation oriented and was effective because of the good delineation of the implementation of functions to define and support aggregate econometric models' activities. 79 Graphical user interfaces are of course applicable far beyond the needs of specific application areas, and their utility is now generally recognized throughout the computing industry. For an interesting retrospective piece on their taxonomy and development, see Johnson et al. (1989).