safe, human readable nested JSON encoding in Go

potential_json.go

package main

import (
	"encoding/json"
)

// PotentialJSON acts as a JSON encoding wrapper for []byte. The encoding is
// tentative:
//
// * if the contents already happen to be valid JSON, then it is represented
// verbatim. I.e.: not encoded at all.
//
// * otherwise the default encoding (i.e. base64) is used.
//
// This is useful for human readable "nested-json" output.
type PotentialJSON []byte

func isValidJSON(b []byte) bool {
	var v interface{}
	return json.Unmarshal(b, &v) == nil
}

func (p PotentialJSON) MarshalJSON() ([]byte, error) {
	b := []byte(p)
	if isValidJSON(b) {
		return b, nil
	} else {
		return json.Marshal(b)
	}
}

potential_json_test.go

package main

import (
	"encoding/json"
	"os"
)

func ExamplePotentialJSON() {
	type TestType struct {
		A, B, C, D PotentialJSON
	}
	b, _ := json.MarshalIndent(TestType{
		A: PotentialJSON(`this is not JSON`),
		B: PotentialJSON(`"this is JSON"`),
		C: PotentialJSON(`null`),
		D: PotentialJSON(`{"nestedJSON": true}`),
	}, "", "  ")
	os.Stdout.Write(b)
	// Output:
	// {
	//   "A": "dGhpcyBpcyBub3QgSlNPTg==",
	//   "B": "this is JSON",
	//   "C": null,
	//   "D": {
	//     "nestedJSON": true
	//   }
	// }
}