In go, how to create a constructor for a type without any arguments

Given this type

type Response map[string]interface{}

I created a method NewResponse which fills in the default values:

 func NewResponse() Response {
	resp := Response{"status": 200, "msg": "Added jobs to queue"}

	resp_metadata := make(map[string]string)
	resp_metadata["base"] = "api/v1"
	resp_metadata["self"] = "/crawler/jobs/add"
	resp["metadata"] = resp_metadata
	return resp
}

which i call like NewResponse() but I would like to do Response.New() instead, so the method signature should be like this

func (Response) New() Response {

but then I always get the error not enough arguments in call to Response.New.

So, how could this be implemented?

When you write Go programs, use idiomatic Go. Then, other people will be able to read your programs. For example,

package main

import "fmt"

type Response map[string]interface{}

func NewResponse() Response {
	metadata := map[string]string{
		"base": "api/v1",
		"self": "/crawler/jobs/add",
	}
	r := Response{
		"status":   200,
		"msg":      "Added jobs to queue",
		"metadata": metadata,
	}
	return r
}

func main() {
	resp := NewResponse()
	fmt.Println(resp)
}

Output:

map[status:200 msg:Added jobs to queue metadata:map[base:api/v1 self:/crawler/jobs/add]]

Although it is definitely not idiomatic Go, you could do something like this:

type Response map[string]interface{}

func (r *Response) New() {
  *r = make(map[string]interface{})
  (*r)["hello"] = "World"
  (*r)["high"] = 5
}

func main() {
  var r Response
  r.New()
  for k, v := range r {
    fmt.Printf("%s = %v\n", k, v)
  }
}

But really, there's nothing wrong with func NewResponse() Response.

It doesn't. Go doesn't have constructors. To create an "empty" object is to create a zero value of the type of the object.

What you're trying to do is a Response method named New to be called on an existing Response object that would return a (different) Response object.

resp := Response{} or resp := make(Response) is fine if you need to create an empty Response.