Spec writing: put common values like senders, room IDs, aliases, etc in a shared location

[turt2live]

At the moment each example is responsible for trying to be consistent with all its $refs, which is a bit annoying. It'd be nice to be able to say %(roomId) or similar and have the spec generator automatically populate that field with a known value, allowing for consistency.

[KitsuneRal]

For YAML parts, anchors look like a natural answer that works right now. Just tried it on banning.yaml: assuming

            example: {
                "reason": "Telling unfunny jokes",
                "user_id": &user_id1 "@cheeky_monkey:matrix.org"
              }

in /ban and

            example: {
                "user_id": *user_id1
              }

in /unban, the rendered spec has the resolved anchor in both places. Using a non-existent anchor leads to a very readable "yaml.composer.ComposerError: found undefined alias 'no_such_anchor'" error message.