Friends GHC has embodied data type families since 6.8, and now type synonym families (aka type functions) in 6.10. However, apart from our initial papers there isn't much published material about how to *use* type families. But that hasn't stopped you: quite a few people are using them already, and of course there is a rich seam of work on using functional dependencies to express type-level computation. Ken Shan and Oleg Kiselyov and I are collaborating to write a paper for an upcoming workshop, under the general rubric of "Fun with type functions" (in homage to Thomas Hallgren's paper "Fun with functional dependencies" and Ralf Hinze's paper "Fun with phantom types"). So this message is to ask you: can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies? Simple is good. It doesn't have to be elaborate: just something that does something useful you could not have done otherwise. Pointers to email threads are fine. Don't assume we already know about them (even if we participated in the thread :-) Part of what we're interested in is that *you* found the example compelling. Many thanks Simon, Ken, Oleg PS: I'm broadcasting this message to GHC-users and Haskell-cafe, but to avoid deluging ghc-users, please reply just to us and Haskell cafe. (Interested ghc-users can follow the threads there from the archives if they want.)