Theory and applications of inverting functions as folds

  • Authors:
  • Shin-Cheng Mu;Richard Bird

  • Affiliations:
  • Programming Research Group, Oxford University, Wolfson Building, Parks Road, Oxford OX1 3QD, UK;Programming Research Group, Oxford University, Wolfson Building, Parks Road, Oxford OX1 3QD, UK

  • Venue:
  • Science of Computer Programming - Special issue on mathematics of program construction (MPC 2002)
  • Year:
  • 2004

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Abstract

This paper is devoted to the proof, applications, and generalisation of a theorem, due to Bird and de Moor, that gave conditions under which a total function can be expressed as a relational fold. The theorem is illustrated with three problems, all dealing with constructing trees with various properties. It is then generalised to give conditions under which the inverse of a partial function can be expressed as a relational hylomorphism. Its proof makes use of Doornbos and Backhouse's theory on well-foundedness and reductivity. Possible applications of the generalised theorem is discussed.