Remap enumerating/looping RFC

[binarylogic]

We need an RFC that obtains consensus on our approach to enumerating/looping in Remap. There are a few open issues that require this (#5377, #5784, #5783, #5852), and we've had a number of discussions floating around (#5852 (comment)). Once we decide on an approach we should update the referenced tickets to reflect it.

Note: this is not blocking 0.12. We will be shipping 0.12 without this feature.

[jszwedko]

Just putting this here so I don't lose it again: https://github.com/josevalim/nested-data-structure-traversal

Examples of mapping / iteration in a bunch of languages.

[binarylogic]

That project is very interesting! For posterity, Lua and Python are my favorite due to their immediately obvious and simple nature. I get that we'd need to discuss low-level changes to the language, but from a pure readability perspective those are my favorite.

[JeanMertz]

ref #8381

[dbcfd]

Given that arrays have a push capability, could a simple first pass just be

acc = []
for obj in .array {
  obj.new_field = 1
  push(acc, obj)
}

More complicated functions could be built later, but this seems like it would solve a lot of use cases.

This would be the iterate without index case in #5875

For example, to iterate with index

idx = 0
acc = []
for obj in .array {
  obj.idx = idx
  idx = idx + 1
  push(acc, idx)
}

[dbcfd]

Additionally, iterating an object could be accomplished with a tuple object

for kv in .object {
  .key = kv.0
  .value = kv.1
  }

[binarylogic]

Closing in favor of #8381