language: support optional markers for dynamic fields

[disintegrator]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.4.3 darwin/arm64

Does this issue reproduce with the latest release?

Yes

What did you do?

https://cuelang.org/play/?id=QTJz8OOP282#cue@export@cue

#Geos: ["UK", "AU"]
#PerGeo: {
  _value: _

  for _, geo in #Geos {
    (geo)?: _value
  }
}

out: #PerGeo & { _value: >= 0.0 }

What did you expect to see?

I was expecting to get the same output as the following: https://cuelang.org/play/?id=U2gP5VJNJS-#cue@export@cue

out: {}

or more accurately

What did you see instead?

The fields that resulted from the comprehension were not marked as optional. The optional marker was ignored and I got the following:

out: {
	UK: >=0.0
	AU: >=0.0
}

---

Keywords: string interpolation, field name expression, optional fields, structs

[disintegrator]

A smaller reproducer from @rogpeppe: https://tip.cuelang.org/play/?id=gGtOGipIe0B#cue@export@json

[disintegrator]

Another reproducer from @rogpeppe and @mvdan: https://tip.cuelang.org/play/?id=MTJhnnEvkU8#cue@export@cue

[myitcv]

Marking as a feature request, because this is simply not supported yet. Arguably there should be a syntax error in the linked example until it is supported.

[disintegrator]

Marking as a feature request, because this is simply not supported yet. Arguably there should be a syntax error in the linked example until it is supported.

Marking it as a feature request makes sense to but I do think it's a bug to allow an unsupported syntax to pass silently.

[mpvl]

Marked this as v0.6. With the eminent changes to optional field representations, this will be easy to make work. So rather than throwing an error, we will make this work.

For now, you can write this as:

[geo]: _value

[disintegrator]

Thanks @mpvl! I'll keep on eye on it.

[myitcv]

@disintegrator - just to flag this got completed in c3138e3. Our bot doesn't do a good job of notifying when things get done!

[disintegrator]

@myitcv amazing, thank you all!

[jpluscplusm]

@myitcv thanks to any and all involved in adding this feature :-) May I ask which release you anticipate it'll be available in?

[myitcv]

@jpluscplusm it's already present in tip at the commit indicate, which means it's available in the pseudo version v0.6.0-0.dev.0.20230323113716-c3138e362ec8. The next version we will be tagging based on tip is an alpha of v0.6.0.