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.
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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.