There are a pair of research languages called Harmony and Boomerang [1] that does this with what they call "lenses". There are also some easy-reading introductory slides [2,3].

[1] http://www.seas.upenn.edu/~harmony/

[2] http://www.cis.upenn.edu/~bcpierce/papers/lenses-etapsslides...

[3] http://www.cis.upenn.edu/~bcpierce/papers/harmful-mfps.pdf

Here's the link: http://stanford.edu/~jacobt/reversible.html

I don't think anything happened to it since the hackathon. Judging by own experience with hackathons, I wouldn't expect too much in the way of code quality. (Reading some of my old hackathon code still makes me shudder :)).