This is a module I hacked together quickly after having read the following blog post: http://blog.notdot.net/archives/30-Damn-Cool-Algorithms,-Part-1-BK-Trees.html I thought the data structure sounded cool so I thought it would be an interesting excerise to implement it. BK-trees can apparently perform very good in some circumstances. The paper "Fast Approximate String Matching in a Dictionary" (Baeza-Yates, Navarro 1998) recommends them over other structures for doing approximate search. http://citeseer.ist.psu.edu/1593.html The original paper can be found here: http://portal.acm.org/citation.cfm?id=362003.362025 Henning Günter <h.guenther@tu-bs.de> generously supplied two algorithms for computing the levenshtein edit distance. The better one of the two is used in the list instance for the Metric class.