So the other day I was writing some code, and I ended up wanting to have a collection of data indexed in more than one way. In other words, I wanted fast lookup with several different keys. Initially I built something using two Data.Map objects to represent the two lookup keys, but then I needed up needing more keys per value than that, so I decided I should write a small library to abstract the whole thing. What I ended up writing is this: http://www.hpaste.org/fastcgi/hpaste.fcgi/view?id=25782 It sorta kinda works, but man, take a look at the first line. That's *a lot* of type-system abuse to make it go. And it strikes me that if you had two keys, both of which happen to have the same type (e.g., String)... what then? On the other hand, it's not like you can go lookup :: KeyID -> Key -> Container -> Maybe Value since the type of both the container and the key depend on which key you want to access. Man, this is way, way harder than I realised! Does anybody have a less-insane way of doing this?