Title: Declarative Sorting
Presenter: Steven Pemberton

XForms is a declarative programming language for writing applications for the web and elsewhere. 

One of the strengths of the language is how data is described: types, constraints, and relationships between values declaratively describe the structure, validity, and interconnection of data, as global properties.

A central part of this is invariants that describe relationships between values, such that if a value changes or is changed, its related values specified in the invariants get updated automatically. This is much like how spreadsheets work, though more general. A major advantage of this approach is that much administrative detail is taken out of the hands of the programmer, and left to the computer: the programmer specifies the relationships, the computer does the work.

Sorting data is one of the major tasks of many programs, and this is usually done programmatically: the data is, or becomes, unsorted, and a function is then called to order the data suitably.

On the other hand, being sorted is often just a necessary property of the data for the rest of the program to function properly, which could just be stated declaratively, leaving the work to ensure it remain sorted to the computer.

This paper discusses the issues involved, and how the question was approached when adding declarative sorting to XForms, as well as touching on implementation details and advantages.