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Jan 13, 2023
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Document the pattern for evolving case classes in a compatible manner #2662

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Document the patter for evolving case classes in a compatible manner
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Update _overviews/tutorials/binary-compatibility-for-library-authors.md 
Co-authored-by: Jakub Kozłowski <kubukoz@gmail.com>
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sideeffffect and kubukoz authored Dec 21, 2022
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Expand Up @@ -179,7 +179,7 @@ Sometimes it is desirable to be able to change the definition of a case class (a

To achieve that, follow this pattern:
* make the constructor `private`
* define `private` `unapply` function in the companion object (note that by doing that the case class looses the ability to be used in a pattern match)
* define a private `unapply` function in the companion object (note that by doing that the case class loses the ability to be used in a pattern match)
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@bjornregnell bjornregnell Dec 22, 2022 •
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Well, usage of instances in patterns is possible e.g. typed patterns such as
case p: Person =>
so it is "only" constructor patterns and extractor patterns that are not possible, if I'm correct?

So case Person(n,a) => would not work as unapply is private, but other kind of patterns work. So I think this should be clarified as the reader might not know about all the unapply mechanics under the hood...

The terminology of all the 15, or so, different kinds of patterns is available here: https://scala-lang.org/files/archive/spec/2.13/08-pattern-matching.html

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Edit: I guess extractor patterns could be made to work if the library author provides custom extractors?

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Anyway, I think it would be good to explain the implications on pattern matching in more detail...

* define `withXXX` methods on the case class that create a new instance with the respective field changed
* define custom `apply` factory method(s) in the companion object (these can use the private constructor)

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