This is DList, 0.1 I've cabalised, and packed up a small difference lists module. In case you've not used them, they are a Haskell idiom for implementing O(1) append and snoc, using functions to represent lists. I use them from time to time, and thought it a good idea to finally pack them into a library for all to share. Home: http://www.cse.unsw.edu.au/~dons/dlist.html Docs: http://www.cse.unsw.edu.au/~dons/dlist/Data-DList.html Source: http://www.cse.unsw.edu.au/~dons/code/dlist/ I've also described the entire process of releasing this library here: http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/11#release-a-library-today Cheers, Don P.S. If you love Haskell, you'll write a library today!