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Deriving an applicative heapsort algorithm

The development of sorting routines is a well trodden path.” [DF88, p. 66] We proceed by program transformation to derive a version of the Heapsort algorithm from a non-deterministic specification. The object language is Haskell and, therefore, the style of the resulting Heapsort program is necessarily applicative. Our development is supported by the program transformation system Ultra. Although a valuable tool, several shortcomings of Ultra are identified.