Principles for Implicits in Scala 3

[lihaoyi]

@odersky

Thanks for explaining! To give better error messages here, the compiler would have to know the complete set of sources in a project and its dependencies. I have the impression that’s made possible by the fact that in Rust package management and compilation are tightly integrated. The Scala and Java world is different in that respect.

I think @mdedetrich is correct in saying that error messages are the current biggest problem with implicits, and not the superficial spelling/syntax. The fact that when you get an implicit wrong, you get zero help from the compiler about what to do, and in many cases the compiler actively misleads you: that is what confuses newbies the most, and even experienced people.

This can be seen from @tarsa’s comments, @Mocuto’s comments, and even @Ichoran’s comments on another thread Proposal: Implied Imports - #11 by soronpo and /u/kag0’s comment on reddit. I’ll chip in my own experience and say it applies to me as well: when learning how to use Futures, when learning how to use SLICK, the imports-are-wrong-and-error-messages-awful property was far more demoralizing than things like the choice of syntax or interference with currying

Better error messages should definitely be possible in the compiler just by scanning the classpath; no package manager involved. People generally don’t have “forgot to add an SBT dependency to pull in implicit from other ivy artifact” problem, people have a “forgot to add an import for an implicit already on the classpath, often in the same library” problem. All this stuff is already on the classpath, we just need to scan for it in the compiler and surface it. I think @tarsa makes a reasonably compelling argument that Rust’s usability comes from the error reporting, and not from the superficial syntax.

Error messages like:

methodA is not a member of ClassA, but there is an implicit extension foo.bar.ClassAImplicits.extensions available

implicit scala.concurrent.ExecutionContext is not found, but there are available implicit instances at scala.concurrent.ExecutionContext.Implicits.global and scala.concurrent.ExecutionContext.Implicits.parasitic available

Would make much more of a difference than changing the syntax. In fact, in the last case you can even see people have manually tried to implement this missing feature using @implicitNotFound annotations, when really this should be the standard auto-generated error message for any implicit that is not found.