! cue vet ./yaml.cue
cmp stderr expect-stderr

-- expect-stderr --
phrases: error in call to encoding/yaml.Validate: phrases.quote1.text: incomplete value (!=""):
    ./yaml.cue:19:10
    ./yaml.cue:11:17
-- yaml.cue --
import "encoding/yaml"

// Phrases defines a schema for a valid phrase.
#Phrases: {
	phrases: {
		[string]: #Phrase
	}

	#Phrase: {
		lang:         #LanguageTag
		text:         !=""
		attribution?: !="" // must be non-empty when specified
	}
	#LanguageTag: =~"^[a-zA-Z0-9-_]{2,}$" | false
}

// phrases is a YAML string with a field phrases that is a map of Phrase
// objects.
phrases: yaml.Validate(#Phrases)

phrases: """
    phrases:
      # A quote from Mark Twain.
      quote1:
        lang: en
        attribution: Mark Twain

      # A Norwegian proverb.
      proverb:
        lang: no
        text: Stemmen som sier at du ikke klarer det, lyver.
    """