Looks good. The control flow of "handle" looks a bit confusing, but overall I definitely like the direction this is going.
The decision to go with concepts is interesting. It's more moving parts than I would have expected from a language like Go, which places a high value on minimalism. I would have expected concepts/typeclasses would be entirely left out (at least at first), with common functions like hashing, equality, etc. just defined as universals (i.e. something like "func Equal<T>(a T, b T) bool"), like OCaml does. This design would give programmers the ability to write common generic collections (trees, hash tables) and array functions like map and reduce, with minimal added complexity. I'm not saying that the decision to introduce typeclasses is bad—concepts/typeclasses may be inevitable anyway—it's just a bit surprising.
The decision to go with concepts is interesting. It's more moving parts than I would have expected from a language like Go, which places a high value on minimalism. I would have expected concepts/typeclasses would be entirely left out (at least at first), with common functions like hashing, equality, etc. just defined as universals (i.e. something like "func Equal<T>(a T, b T) bool"), like OCaml does. This design would give programmers the ability to write common generic collections (trees, hash tables) and array functions like map and reduce, with minimal added complexity. I'm not saying that the decision to introduce typeclasses is bad—concepts/typeclasses may be inevitable anyway—it's just a bit surprising.