I think your idea is a good one. It reminds me a little bit of the Haskell library SafeCopy. They use TH (macros) to generate typeclass implementations and I suppose you could possibly do something similar with protocols, but I think a new function head that just knows how to migrate to the next version would make more sense. I’d probably make separate named, versioned structs for each version so it is clear what kind of structures each migration clause is operating on. This may seem like a lot of overhead but you could tuck older versions away in a separate file.
