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.)
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