Hi Stuart,
Data.Bimap is a data structure that represents a bidirectional mapping between two key types. A bimap has two type parameters; each value in the bimap is associated with exactly one value of the opposite type. In other words, it's a bijection between (subsets of) its argument types.
Very handy, I actually wanted something similar myself a few days ago and came up with: http://www.cs.york.ac.uk/fp/darcs/firstify/Yhc/Core/Firstify/Mitchell/BiMap.... A few design differences from your one: * I called my module BiMap" rather than "Bimap" - I debated this with a collegue, and we settled on the capital M, but it was a very close call. * I have no L/R and Either variants. I decided it was better to be a superset of Data.Map, and then have lookupRev/memberRev functions for the R versions. Do you think anyone is likely to use the Either variant, without a constant Left or Right in the code? Is this an artefact of how its easier to code, rather than to use? * I didn't push my Bimap to completion, because I am too lazy, and hoped others would. Only the methods I needed were implemented. Many thanks for doing this work! Thanks Neil