Unmarshal yaml while keeping extra parameters

Question

I've been trying to write a custom UnmarshalYAML function that will unmarshal a yaml to a struct and also keep unexpected parameters saving them into a map into the struct itself.

I've tried writing this (https://go.dev/play/p/azf2hksriQ1) but, of course, I get an infinite loop when I try to unmarshal the struct inside the UnmarshalYAML.

Does anyone have any suggestion on how to make this work? Thank you very much!

Answer 1

You can use a simple wrapping technique where you define a type alias for your original type (so yaml.Unmarshaler won't call your UmarshalYAML method) and create a struct which embeds all known and unknown parameters.

type Foo struct {
    Param1     string         `yaml:"param1"`
    Param2     []int          `yaml:"param2"`
    // NOTE: we explicitly ignore this parameter so it won't be 
    // eventually unmarshaled 
    Extensions map[string]any `yaml:"-"` 
}

// UnmarshalYAML implements the yaml.UnmarshalYAML interface.
func (f *Foo) UnmarshalYAML(value *yaml.Node) error {
    // Here we are wrapping Foo, so it will be unmarshaled
    // using without calling foo.UnmarshalYAML
    type Wrapped Foo
    // NOTE: for yaml it's required to provide inline
    // option if we would like to embed
    var val struct {
        Wrapped    `yaml:",inline"`
        Extensions map[string]any `yaml:",inline"`
    }
    if err := value.Decode(&val); err != nil {
        return err
    }
    *f = Foo(val.Wrapped)
    f.Extensions = val.Extensions
    return nil
}

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