PoC for Generic Typeclasses in Dotty
(This is very early, just a preliminary experiment, but I wanted to bookmark it for myself and might as well bring it to other folks’ attention. If you’re interested in Dotty and Shapeless, this one’s worth taking a look at.)
Martin has just committed a proof-of-concept experiment in generic typeclass derivation for Dotty. This sort of thing is pretty much the heart and soul of Shapeless — taking a complex type (most often an ADT, sometimes a very complex one), transforming it to a “generic” form, and providing typeclass instances for the various shapes of that generic. This is how Shapeless lets you magically create picklers and suchlike for arbitrary case classes.
It’s a terribly important topic for Dotty — many of us count on Shapeless, whether we realize it or not, for the more sophisticated libraries that let us do really complex actions in low-boilerplate ways. Since Dotty throws out Scala 2 macros, and is replacing them with new tools for metaprogramming, it hasn’t been immediately obvious whether Shapeless-like tools could be built.
The current experiment shows two potential ways to write Shapeless-style generics in Dotty. Martin specifically calls out the second approach, which is worth reading through if you’ve been paying attention to Dotty — it’s a nice deep example of Dotty code, including enums, inlining, and match types. It demonstrates simple generic-style typeclasses for pickling and equality.
(Related: Dotty 0.10.0 has just been released, and the documentation is essential for understanding the ins and outs of this code. I particularly call to your attention the sections on inlining and match types, both of which are new in this release, and which are essential for understanding this code — they’re both heavily used.)
It’s far from production-quality yet: the typeclasses themselves are boilerplatey, and this experiment doesn’t yet handle real case class hierarchies. But it’s a fine initial proof of concept, illustrating some possible ways forward for this crucial Scala capability. Personally, I think it’s very exciting, showing that Dotty’s new metaprogramming systems should suffice for one of the more important use cases…
