Type State Pattern in Go

response.go

package main

import (
	"fmt"
	"io"
	"strings"
)

type Header struct {
	Key string
	Val string
}

type ResponseBuilder struct {
	StatusCode int
	Headers    []Header
	Body       io.Reader
}

type Response ResponseBuilder

func (r *Response) Send() {
	fmt.Printf("Sending response: %+v\n", *r)
}

type (
	NewResponse interface {
		Status(int) WithHeaders
	}

	WithHeaders interface {
		AddHeader(k string, v string) WithHeaders
		SetBody(io.Reader) WithHeaders
		Build() *Response
	}
)

func (r *ResponseBuilder) Status(code int) WithHeaders {
	r.StatusCode = code
	return r
}

func (r *ResponseBuilder) AddHeader(k string, v string) WithHeaders {
	r.Headers = append(r.Headers, Header{k, v})
	return r
}

func (r *ResponseBuilder) SetBody(body io.Reader) WithHeaders {
	r.Body = body
	return r
}

func (r *ResponseBuilder) Build() *Response {
	response := Response(*r)
	return &response
}

func main() {
	var builder NewResponse = &ResponseBuilder{}

	builder.
		Status(200).
		SetBody(strings.NewReader("asf")).
		AddHeader("Content", "json").
		AddHeader("Un", "").
		Build().
		Send()
}