Using type alias in custom JSON marshalling

custom_marshalling.go

package main

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

type Foo struct {
	One string
	Two string
	// Three is One + Two
}

func (f *Foo) MarshalJSON() ([]byte, error) {

	// Alias the original Foo Type to avoid
	// recursing through this MarshalJSON func
	// further down
	type ShadowFoo Foo

	return json.Marshal(&struct {
		Three      string
		*ShadowFoo // Use the alias
	}{
		// Feed data into the anonymosu struct above
		Three: fmt.Sprintf("%s + %s", f.One, f.Two), // synthesized data

		// Without casting 'f' as the same type declared in
		// the anonymous struct, you get the following error:
		//
		// 		"cannot use f (type *Foo) as type *ShadowFoo in field value"
		//
		ShadowFoo: (*ShadowFoo)(f),
	})
}

func main() {
	data := Foo{One: "1", Two: "2"}

	// Output: {"Three":"1 + 2","One":"1","Two":"2"}
	_ = json.NewEncoder(os.Stdout).Encode(&data)
}