According to the YAML specification [^1]:
- > An anchored node need not be referenced by any alias nodes
This means that it's OK to declare anchors but don't have any alias
referencing them. However users could want to avoid this, so a new
option (e.g. `forbid-unused-anchors`) is implemented in this change.
It is disabled by default.
[^1]: https://yaml.org/spec/1.2.2/#692-node-anchors
Although accepted by PyYAML, `{*x: 4}` is not valid YAML: it should be
noted `{*x : 4}`. The reason is that a colon can be part of an anchor
name. See this comment from Tina Müller for more details:
https://github.com/adrienverge/yamllint/pull/550#discussion_r1155297373
Even if it's not a problem for yamllint, let's fix our tests to include
valid YAML snippets.
According to the YAML specification [^1]:
- > It is an error for an alias node to use an anchor that does not
> previously occur in the document.
The `forbid-undeclared-aliases` option checks that aliases do have a
matching anchor declared previously in the document. Since this is
required by the YAML spec, this option is enabled by default.
- > The alias refers to the most recent preceding node having the same
> anchor.
This means that having a same anchor repeated in a document is
allowed. However users could want to avoid this, so the new option
`forbid-duplicated-anchors` allows that. It's disabled by default.
- > It is not an error to specify an anchor that is not used by any
> alias node.
This means that it's OK to declare anchors but don't have any alias
referencing them. However users could want to avoid this, so a new
option (e.g. `forbid-unused-anchors`) could be implemented in the
future. See https://github.com/adrienverge/yamllint/pull/537.
Fixes#395Closes#420
[^1]: https://yaml.org/spec/1.2.2/#71-alias-nodes
amimas
Adrien Vergé