KnownFields not supported in Node.Decode

[tux21b]

The current API allows strict parsing into structs by setting Decoder.KnownFields(true). Elements can implement UnmarshalYAML(value *Node) error in order to influence the behavior of the decoding, and then can call node.Decode to get the default behavior. Unfortunately, node.Decode creates a new decoder internally without the known fields option enabled (the API silently looses the option) and there is no way to set it again.

[unsacrificed]

Moreover there is no way to check KnownFields value inside UnmarshalYAML(value *Node) error.

[Emyrk]

Will this be addressed? It seems pretty serious as right now if you implement your own Unmarshal function, you lose this property. It is important for my application to keep it.

[maja42]

It's been two years since the last update, but the issue still exists.
Is there any particular reason why this can't / hasn't been done yet?

[drevell]

Here's a possible workaround. I'm not too familiar with Node/Kind/Content/etc so use with due caution.

type MyStruct struct {
	MyField `yaml:"my_field"`
}

func (s *MyStruct) UnmarshalYAML(n *yaml.Node) error {
	if err := extraFields(n, []string{"my_field"}); err != nil {
		return err
	}

	// do unmarshaling as normal ...
}

func extraFields(n *yaml.Node, knownFields []string) error {
	if n.Kind != yaml.MappingNode {
		return fmt.Errorf("got yaml node of kind %d, expected %d", n.Kind, yaml.MappingNode)
	}
	m := map[string]any{}
	if err := n.Decode(m); err != nil {
		return err
	}

	for k := range m {
		if !slices.Contains(knownFields, k) {
			return fmt.Errorf("unexpected field %s", k)
		}
	}
	return nil
}

Things I haven't considered that you might want to:

• Efficiency for large numbers of fields, use set intersection or something
• Efficiency again: everything gets unmarshaled twice
• Position reporting: n.Line and n.Column will be the position of the parent yaml Node
• Stringifying n.Kind rather than showing an integer in the error message
• Use reflection to get the yaml name of each struct field so it doesn't have to be duplicated in the call to extraFields()