Don't autowire no-arg constructors, or make this configurable?

[adamw]

Citing @mbore:

Actually I'm still not sure if we want to support no parameters constructors. It may cause difficult to debug mistakes like "forgot to pass filled config", for example:

import cats.effect._
import cats.effect.unsafe.implicits.global

class MutableConfig() {
  var port: Option[Int] = None
  var host: Option[String] = None
}

class Service(cfg: MutableConfig) {
  println(s"[${cfg.host}]:[${cfg.port}]")
}

object Main extends App {

  def loadConfig(): Resource[IO, MutableConfig] = Resource.pure {
    val mc = new MutableConfig()
    mc.host = Some("xyz")
    mc.port = Some(8080)

    mc
  }

  val cfg = loadConfig()

  val service = autowire[Service]()

  service.allocated.unsafeRunSync()._1

}

it works and prints [None]:[None]. I see that it may reduce boilerplate in some cases, but I'm not sure if it's worth to risk.

Something to consider - I think no-arg implementations might be common, but the argument with configuration is also a valid one.

Maybe we could somehow make this configurable - either support no-arg constructors or not? Or better, maybe the configuration could include packages from which we want to autowire, or a blacklist of packages which should never be autowired?

However, it's challenging to provide such configuration at compile-time - I suppose it would need to be somehow available at the type level?

[mbore]

Adding compile-time configuration might be also useful to define a default trait implementation that should be wired. To do so, we may introduce a tag useClass[A] or useImplementation[A] that would be passed in autowire dependencies list.

It would be worth considering additional parameters list for configuration tags, then we could use it as follows:

autowire[ServiceA](useImplementation[ServiceB], useEmptyConstructors)(serviceC, serviceD)