Back at the beginning of 2011, when I first started doing the experiments in generic programming that would eventually turn into shapeless, I had no idea that five years later it would have evolved into such a widely used library. I am profoundly grateful to the people who have trusted me and added shapeless as a dependency to their own projects: the vote of confidence that this represents is a huge motivator for any open source project. I am also hugely grateful to the many people who have contributed to shapeless over the years: eighty one at the time of writing. Without their help shapeless would be a far less interesting and useful library.

These positives notwithstanding, shapeless has suffered from one of the common failings of open source projects: a lack of comprehensive, accurate and accessible documentation. The responsibility for this lies squarely at my door: despite acknowledging the lack I have never been able to find the time to do anything about it. To some extent shapeless has been saved from this by Travis Brown’s heroic Stack Overflow performance and also by the many people who have given talks about and run workshops on shapeless (in particular I’d like to highlight Sam Halliday’s “Shapeless for Mortals” workshop).

But Dave Gurnell has changed all that: we now have this wonderful book length treatment of shapeless’s most important application: type class derivation via generic programming. In doing this he has pulled together fragments of folklore and documentation, has picked my brain, and turned the impenetrable tangle into something which is clear, concise and very practical. With any luck he will be able to make good on my regular claims that at its core shapeless is a very simple library embodying a set of very simple concepts.