How to unmarshal a string or a struct value?

[livingsilver94]

For example, take a look at docker-compose file. build can be a string or a struct, and that's perfectly valid. To unmarshal such a situtation, I thought I could try to unmarshal a string before, and then a struct if the former errored out. Unfortunately I found out that TypeError exists, but it's internal:

https://github.com/goccy/go-yaml/blob/v1.9.8/internal/errors/error.go#L225

Could you expose it, or suggest me an alternative way to unmarshal a value which may be of two different types?

[fstamour]

I would also like to know, I have the exact same situation

[fstamour]

I managed to parse my stuff correctly using a struct with 2 fields: one string and one struct.

Then I implemented the InterfaceUnmarshaler interface for it, something like that (ironically, #446 was pretty much the documentation I was missing to understand how to do this):

type A {
  Name string
}

type Data struct {
  A A
  B string
}

func (data *Data) UnmarshalYAML(unmarshal func(interface{}) error) error {
        // Try to unmarshal as a string
	errUnmarshalAsString := unmarshal(&data.B)
	if errUnmarshalAsString == nil {
		// Success
		return nil
	}

	// Try to unmarshal as a map
	errUnmarshalAsStruct := unmarshal(&data.A)
	if errUnmarshalAsStruct != nil {
		// Both tries failed
		err := fmt.Errorf("Failed to unmarshal \"data\" as either a string or as struct A:\n %w\n %w\n",
			errUnmarshalAsString, errUnmarshalAsStruct)
		log.Print(err)
		return err
	}

	// Success
	return nil
}