Can I set a value conditionally based on the existence of an optional field?

[cueckoo]

Originally opened by @NReilingh in cuelang/cue#887

I would like to define a struct that accomplishes the following:

#Type {
  optional?: string
  result: bool
  result: <if optional is set then true else false>
}

Is this possible?

[cueckoo]

Original reply by @mpvl in cuelang/cue#887 (comment)

With the API or in CUE directly?

In CUE directly you can write:

#Type: {
	optional?: string
	result: optional != _|_
  }
  a: #Type
  a: optional: "foo"

Comparing against bottom is not specified in the spec, or at the very least it is poorly defined. I've written (but still need to publish) a relatively minor change, replacing this with builtins like isdefined(optional). But for now this works.

In the API you can do a LookupPath and see whether the resulting value exists.

[cueckoo]

Original reply by @NReilingh in cuelang/cue#887 (comment)

In CUE directly, yes -- this works! I almost got there on my own, but was comparing with _ instead of bottom. Thanks!