Factor out backing and construct into a separate lightweight package?

[oschulz]

We have duplicate implementations of what backing and construct do scattered all over the ecosystem, explicitly (e.g. ConstructionBase, Functors, ...) and implicitly in the internals of many packages.

The approach and implementation in ChainRulesCore is, to my knowledge, very solid and obviously extensively field-tested. Should we factor this out into a seperate package to make it more directly accessible? ChainRulesCore itself isn't that lightweight a dependency anymore, but a package with just backing and construct could be, I think.

[oxinabox]

I don't particularly see a pressing need to remove this duplication.
Further for construct it is not well tested as nothing uses our accumulation functionality AFAIK.
and for backing it's not really part of the public API.

If there was such a need, I would lean towards depending on ConstructionBase.

[oschulz]

I don't particularly see a pressing need to remove this duplication.

With less duplication, more people might be encouraged to specialize these for their types (e.g. types with cache fields, etc.).

backing it's not really part of the public API

Oh, I didn't realize that. I have been using it in more complex pullbacks - what's the recommended way to get at the "contents" of a tangent?

[oxinabox]

what's the recommended way to get at the "contents" of a tangent?

There isn't one. I also have been using it in more complex tangents.
Possibly we should decide it is part of the public API, and write guidelines for it's use etc.
In theory one isn't required to ever use it, since the getproperty and iterate overloads should be enough.
When are you finding problems with it?
(The problems i find are mostly to do with type-stability)

[oschulz]

In theory one isn't required to ever use it, since the getproperty and iterate overloads should be enough.

I've been using it a few times where I need to deal with or forward the "contents" of a tangent in a generic fashion, like here https://github.com/oschulz/ForwardDiffPullbacks.jl/blob/2ef833bc63886a4be4904e18f49066847c022edd/src/fwd_back.jl#L42 and here https://github.com/oschulz/ValueShapes.jl/blob/ca3b61eabec20445ea0798df3d50a148584e216e/src/named_tuple_shape.jl#L449 and in some testing code.

I guess custom code for doing math with tangents will be hard(er) to write without it, too. In general it makes sense to me to have a function to get the "undecorated" contents of tangents.

[oxinabox]

For that use case you can use NamedTuple(tangent), I hope.

[oschulz]

For that use case you can use NamedTuple(tangent), I hope.

Thanks, I'll give that a try.