Why this interpolation from schemas work?

[exlee]

Recently I've been doing generation of consistent diagrams (along with cue cmd :)) and looked for more ergonomic way to use objects than #Block & {label: "Hello"}. After some inspection I was able to run the code as pasted below.

I'm thrilled it works, but I have no idea WHY it works. As in comments, it looks like string has attributes which is completely 🤯. #ball schema is no coincidence, because #ballBase couldn't fetch default values from environment, so #ball provides defaults.

If there is something I missed please point me toward the discussion but if that's a bug I'd rather not have it fixed 😉

// #ballBase is a string with attributes?
#ballBase: this = "I'm \(this.#description) ball"

// #Ball looks like a schema for a struct
#ball: this = {
  #description: _ | this.#description | *"plain"
  #color: this.#color

  // Just checking conditional interpolation
  if (#color != _|_) {
    #description: "a \(#color)"
  }
  #ballBase
}

// Concrete structs
red: {
  #color: "red"
  #ball
}
blue: {
  {
  #color: "blue"
  #ball
  }
}
just: {
  #description: "just a"
  #ball
}
// Struct below won't work. Error:
// plain.#description: invalid interpolation:
//     ./example.cue:2:19
//     ./example.cue:11:19
// plain: {
//   #ball
// }

[anntnzrb]

The interpolation works because this = "string" is a label binding pattern that binds the entire struct value to the string expression. When #ballBase: this = "I'm \(this.#description) ball" is embedded into a struct like red: {#color: "red"; #ball}, the this reference binds to the embedding struct context (the red struct), allowing \(this.#description) to access fields from that context. The evaluation order is: (1) #ball definition is processed with its fields, (2) #ballBase is embedded, (3) this.#description resolves from the parent struct's fields (set by conditionals in #ball), (4) string interpolation completes, (5) the entire struct unifies to become that string value. The pattern fails for plain: {#ball} because #description defaults to requiring this.#description but without #color defined, the conditional doesn't set it, causing an "invalid interpolation" error since the reference is unresolvable. Fix: Add an explicit default field value or define #color to satisfy the interpolation dependency (see #ball definition at line 6-7 where #description: _ | this.#description | *"plain" requires the field to exist).