I've uploaded a package named [rad](http://hackage.haskell.org/package/rad) to Hackage for handling reverse-mode [automatic differentiation](http://en.wikipedia.org/wiki/Automatic_differentiation) in Haskell.  

Internally, it leverages a [trick](http://www.ittc.ku.edu/~andygill/papers/reifyGraph.pdf) from Andy Gill's Kansas Lava to observe sharing in the tape it records for back propagation purposes, and uses type level branding to avoid confusing sensitivities.

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I've tried to keep the API relatively close to that of Barak Pearlmutter and Jeffrey Mark Siskind's [fad](http://hackage.haskell.org/package/fad) package, but I couldn't resist making a couple of minor tweaks here and there for generality.

I still need to go through and finish up the remaining unimplemented fad combinators, figure out a nice way to build a reverse-mode AD tower, validate that I didn't screw up my recollection of basic calculus, and provide a nice API for using this approach to get local reverse mode checkpoints in an otherwise forward mode AD program, but I am quite happy with how things have progressed thus far.

\[Edit: I've uploaded minor bug fixes for exp and (\*\*)\]