Jon Pretty

Well, ideally Arbitrary would be one of the F's we can derive instances of! The guts of my problem is a finite tree/graph generation problem. The tricky bit is mapping those into types which can feed into derivation infrastructure.

Presumably the actual combinatorics of this are well-understood, and it's the technicalities of performing the mapping in Scala that are the issue?

Exactly. We need to generate types which can feed into downstream compilation. It's doable with current mechanisms, but clunky and very time consuming.

So, I have an idea but it’s really horrible and you’re all going to question my sanity. And it’s probably wrong too. This is for tests, so generate source prior to compilation that will be compiled with the tests. Your generator can pick from the randomly generated types.

Code generation always works, but why would we do that when we could also crack this nut with a sledgehammer

Well, we need something fancier if we want to be able to shrink counterexamples.

If I understood @channingwalton's suggestion correctly, he's proposing to codegen a large variety of types in a single compilation, then have ScalaCheck from amongst them, which would be much faster than generating them on demand, running the compiler once for each.

At the very least, you could generate them in batches of, say 100.