Mixing params, mixing macros

This bit of Dotty news isn’t as dramatic as many of the ones I mention here — it’s not a big new feature or anything — but it’s still something that many of us have been hoping for for a long time. There’s now an open PR to allow you to mix conventional and “given” (what we used to call “implicit”) parameter lists freely in a function declaration.

This is a big deal because of dependent types, one of Scala’s subtler but more important features. In theory (and mind, I haven’t played with this yet, so I could be wrong), it means that you can write a function like this:

def foo(a: A)(given b: B)(c: b.C): D = ...

That is, the type of parameter c depends on the type of given parameter b. This means you can directly build relatively complex chains of types in your function calls, where some of the links are implicit; this requires a fair amount of working-around in Scala 2.

In the same general category of “reducing Scala 2 pain points in power features”, there’s also a draft PR, which tries to make it possible to define macros in the same project in which you use them.

This is one of those details that has long been a mild annoyance: the fact that macros have to be in a separate project generally introduces extra sbt ceremony to applications that use them; it’s one of the many reasons why macros are kind of scary in Scala 2.

Given that macros are intended to be much more solid in Scala 3 — they’re getting properly spec’ed, they are much more carefully designed, and are no longer tied to compiler internals — I expect folks to use them a good deal more. (Certainly I look forward to using them myself, whereas I’ve traditionally avoided them due to the “experimental” label.) So making it easier to use them is a clear win.

Usual caveats: there are just PRs — not merged to Dotty master, much less started on the road to official Scala 3. (The macro one isn’t even a full PR yet: it explicitly requires more work before it’s fully functional.) So nothing is certain here. But they’re both helpful enhancements, so I hope they make it in…