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Use yaml:",inline" as Catch All

UPDATE: yaml.v2 is bork and does not work with this. You will get an error every time you try to marshal your structs as JSON. Use v3 instead (like the Kind project does).

People seeing the inline tag parameter for unmarshaling structures in Go might be confused. It is a catch all from gopkg.in/yaml.v3 package that reads everything from that point on into the map or struct. It is extremely valuable when dealing with files that might contain fields that you do not want to upset or overwrite. It allows any YAML file to be parsed without fear. In fact, it is so powerful it is really a reason to choose YAML for your configurations over JSON all by itself. There is currently nothing equivalent to it in the JSON Go world.

Inline the field, which must be a struct or a map, causing all of its fields or keys to be processed as if they were part of the outer struct. For maps, keys must not conflict with the yaml keys of other struct fields.

I first learned of this trick by inspecting the code of the amazing, award-winning Kubernetes utility, Kind. Kind uses it to modify the ~/.kube/config file without fear of destroying the users existing YAML contexts and other configurations.

Note that this does not work with github.com/ghodss/yaml (which has other advantages). You must always use the yaml tag and not the json one. But this is usually not a problem. In fact, since JSON is YAML you can just use the YAML library for everything and forget about all the JSON encoding stuff completely.

Here’s a sample from the “lab”:

package main

import (
	"fmt"
	"log"
	"os"

	"gopkg.in/yaml.v3"
)

// this works
type Foo struct {
	One int            `yaml:"two"`
	Two int            `yaml:"one"`
	O   map[string]any `yaml:",inline"`
}

func main() {
	foo := new(Foo)
	buf, _ := os.ReadFile("foo.json")
	if err := yaml.Unmarshal(buf, foo); err != nil {
		log.Print(err)
	}
	fmt.Println(foo)
}

Produces

&{2 1 map[other:[1 2 3] some:true]}

And yes this works fully with YAML reference (*) and anchors (&).

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