json.go

package main

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

	"github.com/PaesslerAG/jsonpath"
)

func jsonPath(m interface{}, q string) (string, error) {
	v := interface{}(nil)
	err := json.Unmarshal([]byte(m.(string)), &v)
	if err != nil {
		return "", err
	}

	out, err := jsonpath.Get(q, v)
	if err != nil {
		return "", err
	}
	return out.(string), nil
}

func main() {
	t := template.Must(template.New("letter").Funcs(template.FuncMap{"jsonp": jsonPath}).Parse(`{{ jsonp . "$.username" }}`))
	err := t.Execute(os.Stdout, `{"username": "guest", "password": "secret"}`)
	if err != nil {
		fmt.Println("executing template:", err)
	}
}