A “Fifteen Puzzle” in Fran

This note describes a Fran implementation of the classic “Fifteen Puzzle”, and in doing so conveys by example the Fran style of programming interactive behavior. It is good design practice is to decompose a problem into simpler pieces, solve the pieces independently, and compose the solutions. This practice, which is the essence of modularity, makes the tasks of programming and maintenance more manageable mentally, and sometimes gives rise to reusable components. As this note demonstrates, Fran promotes modularity by providing a high-level vocabulary in which to express the data exchanged by components of the overall solution.