How to write a YAML schema that defines complex mapping key

Unlike JSON, YAML allows an object's keys to be anything, like arrays or objects. (reference)

Is it possible to write a schema for YAML that defines what the keys should be like? For example

mapping:
  # accepted: each key must be an array of ints
  ? - 2
    - 3
  : okay

  # not accepted
  ? - hello
    - world
  : not okay

mapping:
  # accepted: each key must be an object
  # with a field called "value", whose value is a string
  ? value: hello world
  : okay

  # not accepted
  ? hello: world
  : not okay

I'm familar with writing JSON schemas, which also work for YAML as long as the target yaml doesn't use any such "advanced" features. But I don't know if something like this, which is beyond JSON, is possible.

Solution

Is it possible to write a schema for YAML that defines what the keys should be like?

Technically yes, but not with every implementation.

You can argue based on the YAML spec's definition of a schema that implementations in statically typed languages (e.g. SnakeYAML for Java, go-yaml for Go, NimYAML for Nim, ...) use the native types they deserialize into as schema. So if you deserialize into, e.g., Map<ComplexKey, Value> that would be a schema that defines the structure of keys by means of the type ComplexKey.

However, your use-case is possibly a different one. For example, a common definition of schema is „a structural definition I can validate a YAML file against with a command-line tool or editor plugin“. If this is your use-case, then what I described in the previous paragraph is of no use to you – as you would need to compile a tool based on that for each schema you want to validate against.