VARKON Version 1.15 1997-10-16
Generic is Latin for family and a generic model is thus a model that
describes not only a single member of the family but the entire family
in more general terms. A generic screw for example has all the properties
of a screw but no specific length or diameter. A screw with specific length
and diameter (a member of the family) is an explicit screw. In object oriented
terminology generic and explicit translates to class and
instance.
VARKON uses the generic model as its primary representation but automatically
creates and updates an explicit model as well. This process is called evaluation....
generic model ---> evaluation ---> explicit model
The generic model is based on the MBS modeling language which is unique
to the VARKON system. Parameters are used to model things that differ between
members of a family and a MODULE with statements is used to model
common properties. The length and diameter of a screw for example could
be modeled with two FLOAT parameters. When an explicit model is
called for VARKON evaluates (executes) its main MODULE using the
current values of all parameters and creates the corresponding explicit
model automatically.
VARKON's explicit model is stored in a database called GM (GeometricModel).
GM is a structured database of object oriented type where geometry as well
as parameters and owner-membership relations are stored.
VARKON supports interactive generic modeling as well as text based modeling
using MBS directly. In interactive mode VARKON looks like any modern CAD-system
with menus, toolbars, multiple graphic windows and so on. All changes made
to the generic model are evaluated immediately and the impact on the explicit
model is automatically shown in the graphics window. The user can switch
to text based modeling at any time by clicking a button in the toolbar.
VARKON then opens an edit window and presents the model in the form of
MBS statements. When the user returns to interactive modeling VARKON compiles
any changes and shows the new result immediately.
MBS is the text based form of a MODULE but VARKON's uses a binary
Abstract Syntax Tree as its internal representation in order to make evaluation
as fast as possible. The compiling of MBS into its internal form and the
decompiling of the internal form back to MBS again is fully transparent
to the user but for those who need a separate compiler one is also included
in the system.
MBS is not only the basic representation of a generic model. It can also
be used to create one! A MACRO MODULE written in MBS can
be used to automatically add, update, delete and reevaluate a generic model.
MACRO MODULE ---> generic model ---> evaluation ---> explicit
model
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