Dear all, The following new paper may be of interest to some readers of this group: When is a Function a Fold or an Unfold? Jeremy Gibbons, Graham Hutton, and Thorsten Altenkirch http://www.cs.nott.ac.uk/~gmh/when.pdf http://www.cs.nott.ac.uk/~gmh/when.ps We give a necessary and sufficient condition for when a set-theoretic function can be written using the recursion operator fold, and a dual condition for the recursion operator unfold. The conditions are simple, practically useful, and generic in the underlying datatype. (To appear in the Proceedings of the 4th International Workshop on Coalgebraic Methods in Computer Science, Genova, Italy, April 2001.) Best wishes, Graham +-----------------------------------------------------------------------------+ | Dr Graham Hutton Email : gmh@cs.nott.ac.uk | | Languages and Programming Group Web : http://www.cs.nott.ac.uk/~gmh/ | | School of Computer Science and IT | | The University of Nottingham | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB | | United Kingdom | +-----------------------------------------------------------------------------+