Example provides use case marshalling map with custom key as a structure to json

marshal_custom_key.go

package main

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

type key struct {
	this string
	another string
	t chan int
}

func(k key) MarshalText() ([]byte, error) {
	return []byte(fmt.Sprint(k)), nil
}

var _ encoding.TextMarshaler = new(key)

func main() {
	key1 := key{
		this: "1",
		another: "1",
	}
	key2 := key{
		this: "2",
		another: "2",
	}

	mm := make(map[key]string)
	mm[key1] = "1"
	mm[key2] = "2"

	mm[key{}] = "empty"

	b, err := json.Marshal(mm)
	if err != nil {
		panic(err)
	}

	fmt.Println(string(b))
}