What is the best way to represent cyclic data structures in Haskell?

There used to be some work on direct cyclic representations at UCL: Dynamic Cyclic Data Structures in Lazy Functional Languages Chris Clack, Stuart Clayman, David Parrott Department of Computer Science, University College London, October 1995 The link to the project, from http://www.cs.ucl.ac.uk/staff/C.Clack/research/report94.html is broken, but the paper seems to be online at http://www.cs.ucl.ac.uk/teaching/3C11/graph.ps If you want to go for this direct cyclic representation, the interplay with lazy memo functions is also interesting: J. Hughes, Lazy memo-functions, Functional Programming Languages and Computer Architecture, J-P. Jouannaud ed., Springer Verlag, LNCS 201, 129-146, 1985. (lazy memo-functions remember previous input-output pairs based only on pointer-info, which is sufficient to write functions over cyclic structures that, instead of infinitely unrolling the cycles, produce cyclic results) (not online?-( Claus