Hello, Forgive me, I have but a fuzzy idea of what Arrows are. I was thinking it might be possible to make lists arrows instead of monads. This would allow (++) to be one of those special arrow computations. It would have some quite nice properties. For example (++ [a]) could be a O(1) operation instead of a O(n). Also, the trivial implementation of a queue would be O(1) for push and pop operations instead of the more complicated amortized O(1) implementation. Am I missing the point or is what I said above a possibility? David J. Sankel