Dotty: An Alternative to Implicits
On the Dotty News Watch front: the “Alternative to Implicits” PR, which has been on the table for a while (and has changed names a couple of times) is nearing completion.
Usual caveats: this isn’t official yet — it’s a proposal that Martin has been working on for some time. But it might get merged to master soon. (Which doesn’t mean it is a shoo-in for inclusion in Dotty, but it’s at least under serious consideration.) Also, all of the below is simply my interpretations, and should not be taken as gospel truth: see the linked pages for more authoritative discussion.
This one is pretty big and important — arguably the most conspicuous language change for Scala 3. The high concept is replacing most (maybe all) uses of the implicit keyword with higher-level constructs — in particular, introducing syntax to more-explicitly support typeclasses in Scala. The proposal doesn’t change semantics all that much, but it reduces the overloading of implicit with alternatives that hopefully will express the intent of code better, and reduce the boilerplate around typeclass instances a bit.
Useful pages for reading into this proposal:
- Motivation for the proposal.
- Syntax of the new
instancekeyword. This is particularly useful as a way to define typeclass instances. - Replacing implicit parameters with context parameters, and new syntax for implicit functions. Basically, implicit parameter lists now get called out as something more different from ordinary ones, reducing ambiguity.
- Replacing the remaining usages of the
implicitkeyword. - Migration and Discussion. In particular, discussion about the alternatives for implicit / context parameters.
It’s fascinating stuff — refocusing Scala’s most distinctive and controversial feature around its distinctive use cases.
Extensive discussion and debate can be found on the PR itself. This has evolved a lot over the past month or so, and has received a lot of discussion — if you want to understand how it got to its current state, it’s worth reading that discussion.
