Use class wrappers instead of objects in the REPL for better serialization ?

[smarter]

As suggested by @alexarchambault, see com-lihaoyi/Ammonite#736.
Scala 2 also has -Yrepl-class-based to accomplish something similar but it's not the default. @adriaanm are there known issues with -Yrepl-class-based that prevent it from being the default ?

[adriaanm]

One corner case pointed out in https://issues.scala-lang.org/browse/SI-7747 is that you can't define nested value classes.

[adriaanm]

Other than that, it's mostly inertia that's keeping the default in place AFAIK.

[smarter]

@alexarchambault How does the ammonite repl handle value classes ?

[alexarchambault]

It just doesn't handle them as is when class-wrapping is enabled (which is not the default).

Users can still force wrapping stuff in singletons if the code consists of a singleton too. Like

@ case class Foo(s: String) extends AnyVal

fails, but

@ object Defs {
    case class Foo(s: String) extends AnyVal
  }

is ok, as the latter will itself be wrapped in a singleton.

[smarter]

maybe we just need class-wrapping to be turned off when the expression defined contains a value class.

[alexarchambault]

Just discussed with @allanrenucci: one can't define macros if everything is wrapped in classes (in scala 2.x at least, AFAIK). Hence the special-case for singletons above.

[smarter]

To recap (and let me know if I've left out something), the issue with object wrappers is that deserializing them does not actually deserialize the value of their fields, it just returns the singleton instance of the object class. This is problematic when computing these fields are expensive, and serialized instances are sent over the wire (e.g., Spark). Class wrappers solve this issue but introduce their own problems which don't seem trivial to work around.

So how about simply making object wrappers do what we want ? I see two ways to achieve this:

• Change the serialization scheme for all objects to always serialize all their fields, and on deserialization set the fields which are still uninitialized in the object.
• Or, if this is too radical, add an @alwaysSerialize annotation (the reverse of @transient), serializing an object should only serialize the fields in that object with that annotation. The REPL could automatically add this annotation to all vals declared in it.

/cc @retronym who's worked on serialization for objects recently