4. 建造者模式（Builder）

问题：对象有很多可选参数，构造函数参数列表越来越长。

核心：用链式调用一步步构建对象，最终 build() 出成品。

class HttpRequest {
  constructor() {
    this.method = 'GET';
    this.url = '';
    this.headers = {};
    this.body = null;
  }
}

class HttpRequestBuilder {
  #req = new HttpRequest();

  method(m) { this.#req.method = m; return this; }
  url(u)    { this.#req.url = u;    return this; }
  header(k, v) { this.#req.headers[k] = v; return this; }
  body(b)   { this.#req.body = b;   return this; }

  build() { return this.#req; }
}

const req = new HttpRequestBuilder()
  .method('POST')
  .url('/api/users')
  .header('Content-Type', 'application/json')
  .body({ name: 'Hok Keung' })
  .build();

package builder

type Request struct {
  Method  string
  URL     string
  Headers map[string]string
  Body    any
}

type RequestBuilder struct {
  req *Request
}

func NewRequestBuilder() *RequestBuilder {
  return &RequestBuilder{req: &Request{
    Method:  "GET",
    Headers: make(map[string]string),
  }}
}

func (b *RequestBuilder) Method(m string) *RequestBuilder {
  b.req.Method = m
  return b
}

func (b *RequestBuilder) URL(u string) *RequestBuilder {
  b.req.URL = u
  return b
}

func (b *RequestBuilder) Header(k, v string) *RequestBuilder {
  b.req.Headers[k] = v
  return b
}

func (b *RequestBuilder) Body(body any) *RequestBuilder {
  b.req.Body = body
  return b
}

func (b *RequestBuilder) Build() *Request {
  return b.req
}