Simplify optics hierarchy

[serras]

In #2851 I proposed to align the optics hierarchy in Arrow 2.0 to those in other communities. In particular, get rid of some confusion like Traversal being essentially a Setter and Every being what other call Traversal. I think this is a must have, otherwise Arrow stays the weird one in the crowd.

However, I think there's room for further simplification, based on the way people use Optics. More concretely, I propose to reduce the API to the four types marked in the picture above. The documentation is then simplified by simply talking about "focusing", and having no distinction between "set" and "get":

• Traversal focuses in any amount of elements, perfect for working with collections,
• Optional is 0 or 1, perfect for focusing on one choice out of a sealed hierarchy,
• Lens in exactly 1, perfect for focusing on fields,
• Prism is an Optional which can be "reversed", which is "the other side" of a sealed hierarchy.

In addition, I propose to change the derivation of optics for nullable fields. Right now for amount: Int? we generate an Optional, but I think people find it surprising that it only "sets" when the field was already not null. Instead of this, I propose to generate a Lens, and introduce a new combinator,

fun <S, A> Lens<S, A?>.notNull(): Optional<S, A>

so "going under a null" is explicit in the same way that you need to call every to "go under a list".

An additional advantage is that this streamlined hierarchy allows us to reduce the binary size, a common complaint when using Optics. The key problem is that before we generated extension fields for every possible combination,

Iso<S, A>.field: Lens<S, A>
Lens<S, A>.field: Lens<S, A>
// ... 6 more

but now we only have 4, so we've effectively halved the size.

I'm happy to work on this, both on the library side and the compiler plug-in side.

[nomisRev]

Thanks for the proposal @serras 🙏

Looks good to me! We should focus on the practical, and what people are using. I like your proposal for notNull(), and it would indeed fit nicely into the DSL together with every, filter, index & at.
We should probably also re-evaluate the documentation to be more practically oriented. It's perhaps the oldest documentation in Arrow 😅

but now we only have 4, so we've effectively halved the size.

🥳, I think we can also ditch the extension for Prism<S, A>.field since lens compose prism is Optional, right? So we only need to DSL to be projected over Lens, Optional and Traversal. This would reduce it a bit further.

We should also fix this issue while tackling this, #2668.

[serras]

I'm working on it at #2873

[serras]

This is merged and it's now available in 2.0 snapshots