CheckResult

type CheckResult =
  | Localized
  | {
  message: Localized;
  severity?: Severity;
}
  | null
  | undefined
  | void;

What a check returns: nothing when the value is fine, otherwise a message. The message can be given in several languages ({ en: ‘Too high’, de: ‘Zu hoch’ }); the editor shows the one that fits its language best. Or use ctx.language to write it in the editor’s language.