I have this type with a value member called "Url". When encoding this type to json, I want to prepend the HOST value in front of the Url. What is the best way to achieve that?

In this example, the Println statement prints {"url":"/thisurl"}, I want it to print {"url":"http://myhost.com/thisurl"}

package main

import "fmt"
import "encoding/json"

type Post struct {
  Url string `json:"url"`
}

const (
  HOST = "http://myhost.com"
)

func main() {
  post := Post{"/thisurl"}
  marshaled, _ := json.Marshal(post)

  fmt.Println(string(marshaled))
   //{"url":"/thisurl"}
}

Update

I could re-assign post.Url right before the json.Marshal line

post.Url = fmt.Sprintf("%s%s", HOST, post.Url)

But that feels a little messy, if I have to remember to do a reassignment every time I want to encode to json.

I don't want to change the value of post.Url, I simply want to change how it is represented as json.

If you want to change the (un)marshalling behaviour then use a custom type that implements json.Marshaler (and possibly json.Unmarshaler).

package main

import (
    "encoding/json"
    "fmt"
)

type Post struct {
    URL URLString `json:"url"`
}

const (
    Host = "http://myhost.com"
)

type URLString string

func (u URLString) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf(`"%s%s"`, Host, u)), nil
}

func main() {
    post := Post{"/thisurl"}
    marshaled, _ := json.Marshal(post)

    fmt.Println(string(marshaled))
    //{"url":"http://myhost.com/thisurl"}
}

Playground

(also note, using all caps for constants is not idiomatic, and golint recommends URL, MyURL, HTTP, ID vs Url, MyUrl, or Id).