I know the short-term answer is "use TH" to derive folds if I want them, but I think such an important concept should probably be part of the language.
The fold function is an example of a generic program, which can be defined using generic programming libraries. Since the fold has to know about the recursive structure of a datatype, not all (actually, very few) generic programming libraries can be used to define a fold. An example of a recent library that can define folds is multirec (developed by our own group, blatant self promotion): http://hackage.haskell.org/cgi-bin/hackage-scripts/package/multirec A description of the library can be found in http://www.cs.uu.nl/research/techreps/UU-CS-2008-019.html Older generic programming approaches such as PolyP could define the fold too, be it only for so-called regular (non mutually recursive) datatypes. The multirec library defines folds for mutually recursive datatypes. The released version of multirec doesn't include the TH files for generating the boilerplate code (for example, embedding-projection pairs for datatypes) necessary for using the library. However, the head has TH support for this. All the best, Johan Jeuring
Tim Newsham http://www.thenewsh.com/~newsham/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe