package main

import (
	"fmt"
	"log"
	"gopkg.in/yaml.v2"
)

func main() {
	t := T{99}
	data, err := yaml.Marshal(t)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s\n", data)
}

type T struct {
	X int
}

func (t T) MarshalYAML() (interface{}, error) {
	log.Printf("in T.MarshalYAML %#v", t)
	return S{t.X}, nil
}

type S struct {
	Y int
}

func (s S) MarshalYAML() (interface{}, error) {
	log.Printf("in S.MarshalYAML %#v", s)
	return "something", nil
}