Typeclasses requiring only read access, and private constructors

[jatcwang]

Many typeclasses require only read access to fields (e.g.Show, diffx's Diff, JSON Encoders), but due to the existence of CaseClass#construct, magnolia derivation will fail if the primary constructor is not accessible.

Proposal:

• When a library author writes def combine[T](ctx: CaseClass[Typeclass, T]): Derived[MyTypeClass[T]], allow them to use def combine[T](ctx: ReadOnlyCaseClass[Typeclass, T]): Derived[MyTypeClass[T]] instead, where ReadOnlyCaseClass will not have the construct methods.
• Magnolia.gen will detect that ReadOnlyCaseClass is used and not try to resolve constructors

This applies to abstract as well, as currently sealed abstract case class is quite a common pattern in < 2.12 due to scala/bug#7884 (Should be fixed in 2.13.2?)

WDYT?

[joroKr21]

I think it makes sense, but I'm not sure if the encoding you are proposing has the best UX.

[propensive]

I'm not sure either, but I can't think of anything that's obviously better. The change shouldn't be too difficult, with this approach.

[jatcwang]

Adding a super class to CaseClass should be binary compatible (even if I move some methods like parameters to the parent ReadOnlyCaseClass).

[joroKr21]

We haven't promised binary compatibility yet anyway.

[jatcwang]

As a downstream user I'd prefer to not deal with any MethodNotFoundExceptions ;) It will probably be ok.... 🤞

[propensive]

I did bump the version number from 0.12.x to 0.13.x in the last (as-yet unannounced) release on the basis that it included a small API change, but in general I haven't been careful about it...

[joroKr21]

Yeah if we really want to make sure, then we should cut a 1.0 release and add MiMa.

[propensive]

I'd like to leave that a while. I'm a bit reluctant to give a binary compatibility guarantee while I at least expect to change a few things... my priority right now is to make it easier to publish both Magnolia and ~20 other libraries...