What is the purpose of 'package objects' in Scala?

Instruction: Explain the role of package objects and how they are used in Scala applications.

Context: This question evaluates the candidate's knowledge of Scala's package objects, including their benefits and typical use cases.

Updated

In Scala 2, a package object supplies package-level values, methods and type aliases that cannot otherwise be declared directly at the top level. Its public members are accessed as members of the package.

package example {
  package object metrics {
    type Count = BigInt
    def doubled(value: Count): Count = value * 2
  }
}
package client {
  import example.metrics.{Count, doubled}
  @main def packageExample(): Unit = {
    val count: Count = BigInt(7)
    assert(doubled(count) == 14)
  }
}

The example also compiles in Scala 3's compatibility support. Package objects are conventionally placed in package.scala, within the relevant package directory; that filename is a convention, not the language mechanism that makes the declaration a package object. External consumers can import or qualify its members as normal package members. Package definitions

For new Scala 3 code, define methods, values and type aliases directly at package level. Package objects remain available, while the reference documentation describes their eventual deprecation and removal; they are not already unavailable merely because the migration page is headed “Dropped.” Scala 3 package-object migration

Keep package APIs cohesive and avoid unexpected shared mutable state. Moving existing public definitions to top-level source files can change generated JVM names and binary compatibility, so migration requires compatibility checks beyond confirming that source code still compiles.

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