Simple, Naïve, and Wrong: More than you wanted to know about Case Classes

Design
10

In Scala 3, you declare a class open for OOP-style extension (as opposed to extension).

➜  scalac -d /tmp mycase.scala badkext.scala
➜  scalac -source future -d /tmp mycase.scala badkext.scala
there were 1 feature warning(s); re-run with -feature for details
➜  scalac -feature -source future -d /tmp mycase.scala badkext.scala
-- Feature Warning: badkext.scala:2:23 ----------------------------------------------------------------------------------------------------------
2 |class SpecialK extends K(27)
  |                       ^^^^^
  |                       Unless class K is declared 'open', its extension in a separate file should be enabled
  |                       by adding the import clause 'import scala.language.adhocExtensions'
  |                       or by setting the compiler option -language:adhocExtensions.
  |                       See the Scala docs for value scala.language.adhocExtensions for a discussion
  |                       why the feature should be explicitly enabled.
1 warning found
➜  cat mycase.scala badkext.scala

case class K(k: Int)

class SpecialK extends K(27)

Enforcement looks like

➜  scalac -Werror -feature -source future -d /tmp mycase.scala badkext.scala
-- Error: badkext.scala:2:23 --------------------------------------------------------------------------------------------------------------------
2 |class SpecialK extends K(27)
  |                       ^^^^^
  |                       Unless class K is declared 'open', its extension in a separate file should be enabled
  |                       by adding the import clause 'import scala.language.adhocExtensions'
  |                       or by setting the compiler option -language:adhocExtensions.
  |                       See the Scala docs for value scala.language.adhocExtensions for a discussion
  |                       why the feature should be explicitly enabled.
1 error found

The new support for such extensions looks like

➜  cat mycase.scala
open case class K(k: Int)

or

➜  cat badkext.scala
import scala.language.adhocExtensions
class SpecialK extends K(27)

It’s worth adding that a FAQ for someone (possibly a learner or with shallow knowledge of the language) who just wants to use a feature might look different from an expert document that might represent a project’s code policies.

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