Re: [Haskell-cafe] design question: decision tree from "Programming Collective Intelligence"

Hello Maybe "permutation trees" are a viable starting point? See the paper "Parsing Permutation Phrases" which appears to be on CiteSeer. Some slides are also here - the data type definitions and Functor instance for permutation trees are on page 18 (pdf index page 19): http://www.comlab.ox.ac.uk/jeremy.gibbons/wg21/meeting56/loeh-slides.pdf An alternative implementation for applicative functors is here: http://hackage.haskell.org/package/action-permutations Note the use of existentials here is pretty cunning, I didn't get very far the time I attempted to use the technique for my own purposes. Best wishes Stephen