The ability to require that a struct field contains a certain value

[rtheunissen]

Is your feature request related to a problem? Please describe.
Yes. We model lists as structs like field: { "0": "a", "1": "b" } which means we can not use list.Contains` to validate that the field contains a certain value.

Describe the solution you'd like
We are looking for a way to practically do struct.Contains in a way that allows us to unify the field. In this case, field: struct.Contains("a") would unify but field: struct.Contains("c") would not. Alternatively, a way to use cue currently to achieve this.

Describe alternatives you've considered
So far nothing has come up as a viable solution.

Additional context
Add any other context or screenshots about the feature request here.

[WaywardWizard]

I think this is already well supported by cue

Try;

#contains2: {"2": string}
field: #contains2

Probably, unifying a definition - #contains2 will close your field struct, so instead you may want to use an embedding

field: {#contains2, "0": "a", "1": "b"}

I think there is a users group to get support for questions you may have. Im looking for it now myself...

[WaywardWizard]

For your reference - support/usage questions can go in the github discussions. That will be best as it will not clutter the issue log.

[mvdan]

Indeed, try Discord or Slack for questions, which are linked from the website. You can also use discussions here on GitHub. Closing for now as you got an answer in any case.

[rtheunissen]

@WaywardWizard

#containsC: { "c": string }

field: { "0": "a", "1": "b", "2": "c" }

field: #containsC

field."0": field not allowed:
    -:3:13
    -:5:10
    -:7:8

https://cuelang.org/play/?id=BdZubTaNYLG#w=function&i=cue&f=eval&o=cue

[mvdan]

Keep the struct open: https://cuelang.org/play/?id=O1dTQ-fi74D#w=function&i=cue&f=eval&o=cue

[rtheunissen]

@mvdan apologies for continuing this discussion on a closed issue, but since you commented, I think I am misunderstanding something fundamental here. When I change this to something that the struct does not contain as a value, cue doesn't seem to mind? I would like for there to be an error if the value is not in the struct, using some expression following the selector for the struct field in question, i.e. field: ...something here...

https://cuelang.org/play/?id=wA7Jk_i6q99#w=function&i=cue&f=eval&o=cue

[mvdan]

Use a required field: https://cuelang.org/play/?id=gL1wxi-ArDx#w=function&i=cue&f=export&o=cue

[rtheunissen]

That would be a constraint on the key, not the value.
https://cuelang.org/play/?id=vLOLYLgIfni#w=function&i=cue&f=export&o=cue

[mvdan]

My apologies, I misunderstood what you meant by "value" earlier. You want to require that at least one field in the struct contain the value "c". I'm not sure that that's easy to do currently in CUE. I will reopen for the time being because I might need @myitcv to chime in here with his experience.

My immediate attempt is a comprehension (https://cuelang.org/play/?id=zzVA4tm5wWQ#w=function&i=cue&f=export&o=cue) but it doesn't work as the definition by itself already results in an error. It might be that #575 is required for such an approach to work.

[rtheunissen]

#575 would be a general solution that would solve this case and many others. Alternatively, a Contains method in the struct package, assuming it is not currently possible in cue somehow.

[myitcv]

@rtheunissen thanks for raising. I've converted your issue to a discussion for now. But it might be that, beyond #575, we create an issue if we find a bug or alternative feature request.

The kind of thing you need can be achieved via something like (play):

import "list"

field: { "0": "a", "1": "b", "2": "c" }
field: x={
  _mustContainC: list.Contains([for _, v in x {v}], "c") & true
  ...
}

But what does not seem to be possible right now is to extract that to a definition (play):

import "list"

field: { "0": "a", "1": "b", "2": "c" }
field: #mustContainC
#mustContainC: x={
  _mustContainC: list.Contains([for _, v in x {v}], "c") & true
  ...
}

which gives:

#mustContainC._mustContainC: conflicting values true and false:
    -:6:18
    -:6:60

I think I've reasoned to myself that such behaviour is a bug. Hence I've raised #3713