R. S. Bird, J. Gibbons and S-C. Mu. In Algebraic and Coalgebraic Methods in the Mathematics of Program Construction, LNCS 2297, pp. 281-307, January 2002.
We argue for the benefits of relations over functions for modelling programs, and even more so for modelling specifications. To support this argument, we present an extended case study for a class of optimization problems, deriving efficient functional programs from concise relational specifications.