Go Functional Options Pattern

The functional options pattern is a popular way to instantiate structs in Go.

It has two parts:

• New(options ...func(*MyStruct)) *MyStruct
• Options WithSomething(s Something) func(*MyStruct)

Option functions return a function which configures your structure, they typically look like:

func WithSomething(s Something) func(*MyStruct) {
  return func(m *MyStruct) {
    m.something = s
  }
}

Then the inside of your New function looks like:

func New(options ...func(*MyStruct)) *MyStruct {
  // Default config here.
  m := &MyStruct{}
  // ...

  // Apply options.
  for _, option := range options {
    option(m)
  }

  return m
}

Example: Http Client

package main

import (
  "fmt"
  "net/http"
  "time"
)

type Option func(*http.Client)

func New(opts ...Option) *http.Client {
  c := &http.Client{
    Timeout: 5 * time.Second,
  }
  for _, opt := range opts {
    opt(c)
  }
  return c
}

func WithTimeout(t time.Duration) func(*http.Client) {
  return func(c *http.Client) {
    c.Timeout = t
  }
}

func main() {
  c := New(WithTimeout(time.Second))
  res, err := c.Head("http://hoani.net")
  if err != nil {
    panic(err)
  }
  fmt.Printf("status %v\n", res.Status)
}