Default values when unmarshalling JSON and YAML in Go

unmarshal.defaults.go

package main

import (
	"encoding/json"
	"fmt"
	"os"

	yaml "gopkg.in/yaml.v2"
)

type TestStruct struct {
	SomeField  string `json:"somefield" yaml:"somefield"`
	OtherField string `json:"otherfield" yaml:"otherfield"`
	Number     int64  `json:"number" yaml:"number"`
}

// UnmarshalJSON is the implementation of the json.Unmarshaler interface.
func (t *TestStruct) UnmarshalJSON(data []byte) error {
	type innerLol TestStruct
	inner := &innerLol{
		SomeField:  "default value",
		OtherField: "other default",
		Number:     77,
	}

	if err := json.Unmarshal(data, inner); err != nil {
		return err
	}

	*t = TestStruct(*inner)
	return nil
}

// UnmarshalYAML is the implementation of the yaml.Unmarshaler interface.
func (t *TestStruct) UnmarshalYAML(unmarshal func(interface{}) error) error {
	type innerLol TestStruct
	inner := &innerLol{
		SomeField:  "default value",
		OtherField: "other default",
		Number:     77,
	}

	if err := unmarshal(inner); err != nil {
		return err
	}

	*t = TestStruct(*inner)
	return nil
}

func main() {
	jsonData := []byte(`
		{
			"somefield": "fubar",
			"otherfield": "yes"
		}
	`)

	yamlData := []byte("somefield: \"fubar\"\nnumber: 66")

	var sample1 TestStruct
	err := json.Unmarshal(jsonData, &sample1)
	if err != nil {
		fmt.Println("An error occured: %v", err)
		os.Exit(1)
	}

	var sample2 TestStruct
	err = yaml.Unmarshal(yamlData, &sample2)
	if err != nil {
		fmt.Println("An error occured: %v", err)
		os.Exit(1)
	}

	fmt.Printf("%+v\n", sample1)
	fmt.Printf("%+v\n", sample2)
}