Records: migration compatibility

[Remi Forax]

Hi Brian,

----- Mail original -----
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Mardi 23 Juillet 2019 20:32:08
> Objet: Records: migration compatibility

> In the course of exploring serialization support for records, Chris
> asked about the compatible evolution modes for records.  We have
> explored this briefly before but let's put this down in one place.
> 
> Since we are saying that records are a lot like enums, let's start with:
> 
>  A. Migrating a record-like class to a record
>  B. Migrating a record to a record-like class
> 
> (which is analogous to refactoring between an enum and a class using the
> type-safe enum pattern.)

enums is a counter example here, no ?

for enums, the only refactoring allowed is from a class to an enum,
the other direction doesn't work because all enums inherits from java.lang.Enum.

The JDK/javac implementation goes as far as
- not allowing classical class to inherits from java.lang.Enum
- using java.lang.Enum as a type in several public methods
- testing at runtime that the class of an enum has to have the enum bit AND be a direct super type of java.lang.Enum
  (in case you generate your own bytecode)

given that a record already have a different behavior at runtime than a nomrla class (the Record attribute),
a public abstract class (AbstractRecord), the migration B seems unlikely.

BTW, AbstractRecord also has the nasty side effect of not allowing inline record.

Rémi