Integrating Pointer Variables into One-Way Constraint Models
Pedro Szekely, Brad Vander Zanden, Brad A. Myers, Dario A. Giuse

Pointer variables have long been considered useful for
constructing and manipulating data structures in traditional
programming languages.  This paper discusses how pointer
variables can be integrated into one-way constraint models and
indicates how these constraints can be usefully employed in user
interfaces. Pointer variables allow constraints to model a wide
array of dynamic application behavior, simplify the
implementation of structured objects and demonstrational systems,
and improve the storage and efficiency of constraint-based
applications.  This paper also presents two incremental
algorithms--one lazy and one eager--for solving constraints with
pointer variables.  Both algorithms are capable of handling 1)
arbitrary systems of one-way constraints, including constraints
that involve cycles; and 2) editing models that allow multiple
changes between calls to the constraint solver.  These algorithms
are also fault-tolerant in that they can gracefully handle and
recover from formulas that crash due to programmer error.
Constraints that use pointer variables have been implemented in a
comprehensive user interface toolkit, Garnet, and our experiences
with applications written in Garnet have proven the usefulness of
pointer-variable constraints.  Manylarge scale applications have
now been implemented using these constraints.