Add withWrap / mapWrap methods to endpoint builder

[kubukoz]

Follow-up to #1097 - currently, there are no methods for customizing the wrap method of the endpoint (which is the method that essentially wraps an endpoint input into an operation).

There might be an (arguably obscure) use-case for customizing wrap, so let's consider adding those, or use this ticket to document the reason against that.

Note: this isn't blocking for 0.18 because adding these concrete methods can be done without breaking BC.

[Baccata]

Can you formulate such a usecase ?

In the meantime, here's my train of thought :

• A map modifying function would be of the shape I => I
• The only way to do that "lawfully" (ie without unsafe casts) would be via a SchemaVisitor (or more generically, a PolyFunction[Schema, A =>> A => A])
• The wrap method's canonical use-site is client-side interpreters, which are written in terms of SchemaVisitors (in part)
• This kinda indicates that if there is such a transformation needs occurring, a good location for it would probably within the interpreter.

[kubukoz]

Can you formulate such a usecase ?

not at the moment, I just wanted to have an issue or discussion as a record of us in case we don't consider this functionality useful ;)

Agree on the next two points. I was thinking in terms of a transformation on a concrete endpoint, but that doesn't seem useful for anything outside of testing with a very specific endpoint.

However, on these:

The wrap method's canonical use-site is client-side interpreters, which are written in terms of SchemaVisitors (in part)
This kinda indicates that if there is such a transformation needs occurring, a good location for it would probably within the interpreter.

I mostly agree but you may not always be writing an interpreter of your own - you might want to wrap an existing one. Having to change a SchemaVisitor means you need to stop wrapping and do the internals yourself. However, this usecase is theoretical enough that I don't really think it's going to be an issue.

[Baccata]

This layer got an overhaul, the modification of the wrap methods should be easier without the need of specific methods.

As a result, I'm closing this issue.