Structural Constraints in XForms

XForms! XForms defines relationships called “invariants.” When one
member of an invariant relationship changes, the other member(s) are
updated automatically (similar to the way spreadsheets
work).

Currently XForms only allows invariants to be expressed between
simple values (values that can be calculated with a simple
expression), while structural changes can only be detected using
XForms events.

Could the invariant mechanisms of XForms be extended to
handle invariants that express structure, without resorting to events?
Yes, by considering structural invariants to be just a higher-level
form of invariant, where the work of the structural invariant is to
rebuild networks of lower-level simple invariants. Structural
invariants can then be merged into the general XForms invariant
recalculation, by treating the rebuild phase of the XForms update
mechanism as a higher-level version of the current recalculate.

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XForms is a declarative programming language for writing applications
for the web and elsewhere. One of the central aspects of XForms is
invariants that describe relationships between values, such that if a
value changes or is changed, its related values specified in the
invariant get updated automatically.

This is much like how spreadsheets work, though is more general. A
major advantage of this approach is that much administrative detail is
taken out of the hands of the programmer, and done automatically: the
programmer specifies the relationships, and the computer does the
work.

However, XForms in its current incarnation only allows invariants to
be placed between simple content values, even though there are
important relationships that could be specified over data structures
as a whole.

This paper explores what the possibilities are for extending the
XForms invariant mechanism to more general cases.