applyPatches produces same result, but should it also preserve identity?

🙋‍♂ Question

Consider the following code:

https://codesandbox.io/s/immer-sandbox-forked-t7ghd9?file=/src/index.ts

While patchedState is indeed deep equal to producedState, there is a difference in that produce preserves object identity, but applyPatches does not.

Is this a property that would be useful? I arrived at this during a different route, which may be my real question: when an unshift happens, instead of an add op for path ["items", 0], which I expected, instead the patches include a replace op for the first two items in the array, and then an add op for position ["items", 2]. Applying these patches will result in object identity being broken.

I am experimenting with using Immer for change tracking, and this makes life more complicated: I want to track whether I need to update paths if their indices have shifted, but I don't get sufficient information from the patch to determine that.

Link to repro

https://codesandbox.io/s/immer-sandbox-forked-t7ghd9?file=/src/index.ts

Environment

We only accept questions against the latest Immer version.

• Immer version: 9.0.12
• Occurs with setUseProxies(true)
• Occurs with setUseProxies(false) (ES5 only)

Start with the linked CodeSandbox reproduction and compare the identity behavior of produce and applyPatches for the unshift example. Review the generated replace and add operations for items, then establish whether preserving object identity and providing enough information about shifted indices are intended requirements. Done means a documented decision or an agreed implementation scope.