Document the pattern for evolving case classes in a compatible manner (…

…#2662)

Co-authored-by: Jakub Kozłowski <[email protected]>
Co-authored-by: Julien Richard-Foy <[email protected]>

_overviews/tutorials/binary-compatibility-for-library-authors.md

@@ -173,6 +173,141 @@ You can find detailed explanations, runnable examples and tips to maintain binar
 
 Again, we recommend using MiMa to double-check that you have not broken binary compatibility after making changes.
 
+### Changing a case class definition in a backwards-compatible manner
+
+Sometimes, it is desirable to change the definition of a case class (adding and/or removing fields) while still staying backwards-compatible with the existing usage of the case class, i.e. not breaking the so-called _binary compatibility_. The first question you should ask yourself is “do you need a _case_ class?” (as opposed to a regular class, which can be easier to evolve in a binary compatible way). A good reason for using a case class is when you need a structural implementation of `equals` and `hashCode`.
+
+To achieve that, follow this pattern:
+ * make the primary constructor private (this makes the `copy` method of the class private as well)
+ * define a private `unapply` function in the companion object (note that by doing that the case class loses the ability to be used as an extractor in match expressions)
+ * for all the fields, define `withXXX` methods on the case class that create a new instance with the respective field changed (you can use the private `copy` method to implement them)
+ * create a public constructor by defining an `apply` method in the companion object (it can use the private constructor)
+
+Example:
+
+{% tabs case_class_compat_1 %}
+{% tab 'Scala 3 Only' %}
+
+```scala
+// Mark the primary constructor as private
+case class Person private (name: String, age: Int):
+  // Create withXxx methods for every field, implemented by using the copy method
+  def withName(name: String): Person = copy(name = name)
+  def withAge(age: Int): Person = copy(age = age)
+
+object Person:
+  // Create a public constructor (which uses the primary constructor)
+  def apply(name: String, age: Int): Person = new Person(name, age)
+  // Make the extractor private
+  private def unapply(p: Person) = p
+```
+{% endtab %}
+{% endtabs %}
+This class can be published in a library and used as follows:
+
+{% tabs case_class_compat_2 %}
+{% tab 'Scala 2 and 3' %}
+~~~ scala
+// Create a new instance
+val alice = Person("Alice", 42)
+// Transform an instance
+println(alice.withAge(alice.age + 1)) // Person(Alice, 43)
+~~~
+{% endtab %}
+{% endtabs %}
+
+If you try to use `Person` as an extractor in a match expression, it will fail with a message like “method unapply cannot be accessed as a member of Person.type”. Instead, you can use it as a typed pattern:
+
+{% tabs case_class_compat_3 class=tabs-scala-version %}
+{% tab 'Scala 2' %}
+~~~ scala
+alice match {
+  case person: Person => person.name
+}
+~~~
+{% endtab %}
+{% tab 'Scala 3' %}
+~~~ scala
+alice match
+  case person: Person => person.name
+~~~
+{% endtab %}
+{% endtabs %}
+
+Later in time, you can amend the original case class definition to, say, add an optional `address` field. You
+ * add a new field `address` and a custom `withAddress` method,
+ * update the public `apply` method in the companion object to initialize all the fields,
+ * tell MiMa to [ignore](https://github.com/lightbend/mima#filtering-binary-incompatibilities) changes to the class constructor. This step is necessary because MiMa does not yet ignore changes in private class constructor signatures (see [#738](https://github.com/lightbend/mima/issues/738)).
+
+{% tabs case_class_compat_4 %}
+{% tab 'Scala 3 Only' %}
+```scala
+case class Person private (name: String, age: Int, address: Option[String]):
+  ...
+  def withAddress(address: Option[String]) = copy(address = address)
+
+object Person:
+  // Update the public constructor to also initialize the address field
+  def apply(name: String, age: Int): Person = new Person(name, age, None)
+```
+{% endtab %}
+{% endtabs %}
+
+And, in your build definition:
+
+{% tabs case_class_compat_5 %}
+{% tab 'sbt' %}
+~~~ scala
+import com.typesafe.tools.mima.core._
+mimaBinaryIssueFilters += ProblemFilters.exclude[DirectMissingMethodProblem]("Person.this")
+~~~
+{% endtab %}
+{% endtabs %}
+
+Otherwise, MiMa would fail with an error like “method this(java.lang.String,Int)Unit in class Person does not have a correspondent in current version”.
+
+> Note that an alternative solution, instead of adding a MiMa exclusion filter, consists of adding back the previous
+> constructor signatures as secondary constructors:
+> ~~~ scala
+> case class Person private (name: String, age: Int, address: Option[String]):
+>   ...
+>   // Add back the former primary constructor signature
+>   private[Person] def this(name: String, age: Int) = this(name, age, None)
+> ~~~
+
+The original users can use the case class `Person` as before, all the methods that existed before are present unmodified after this change, thus the compatibility with the existing usage is maintained.
+
+The new field `address` can be used as follows:
+
+{% tabs case_class_compat_6 %}
+{% tab 'Scala 2 and 3' %}
+~~~ scala
+// The public constructor sets the address to None by default.
+// To set the address, we call withAddress:
+val bob = Person("Bob", 21).withAddress(Some("Atlantic ocean"))
+println(bob.address)
+~~~
+{% endtab %}
+{% endtabs %}
+
+A regular case class not following this pattern would break its usage, because by adding a new field changes some methods (which could be used by somebody else), for example `copy` or the constructor itself.
+
+Optionally, you can also add overloads of the `apply` method in the companion object to initialize more fields
+in one call. In our example, we can add an overload that also initializes the `address` field:
+
+{% tabs case_class_compat_7 %}
+{% tab 'Scala 3 Only' %}
+~~~ scala
+object Person:
+  // Original public constructor
+  def apply(name: String, age: Int): Person = new Person(name, age, None)
+  // Additional constructor that also sets the address
+  def apply(name: String, age: Int, address: String): Person =
+    new Person(name, age, Some(address))
+~~~
+{% endtab %}
+{% endtabs %}
+
 ## Versioning Scheme - Communicating compatibility breakages
 
 Library authors use versioning schemes to communicate compatibility guarantees between library releases to their users. Versioning schemes like [Semantic Versioning](https://semver.org/) (SemVer) allow