Misleading compiler error for missing (sub-)arbitrary

[imRentable]

When I alter the README example to use an additional java.time.Instant field, like this:

import java.time.Instant

import com.danielasfregola.randomdatagenerator.magnolia.RandomDataGenerator

object MyApp extends App with RandomDataGenerator {

  case class Example(text: String, n: Int, inst: Instant)

  val example: Example = random[Example]
}

Then I get the following error messages:

• Error: could not find implicit value for evidence parameter of type org.scalacheck.Arbitrary[Example]
• Error: not enough arguments for method random: (implicit evidence$1: reflect.runtime.universe.WeakTypeTag[Example], implicit evidence$2: org.scalacheck.Arbitrary[Example])Example.

This is misleading because an Arbitrary[Example] can be created but not an Arbitrary[Instant]. If I add an implicit Arbitrary[Instant] in scope, it compiles just fine and yields the expected result.

Can this be improved?

[DanielaSfregola]

I am not sure it can, as this is a common issue of the typeclass derivation coming from the compiler - shapeless has similar behavior/issues.

Still, I'll have a thing - in the meanwhile suggestions are welcome.

Cheers,
D.

[stsatlantis]

@imRentable https://github.com/tek/splain can provide better error messages, maybe it helps in your case as well.

[imRentable]

@stsatlantis Thanks for the info. I tried it but unfortunately it does not provide useful additional information in this case. However, cool plugin. :)