go 50 lines · 7 steps

The functional options pattern in Go

Configure a struct with any subset of settings by passing composable functions instead of a giant argument list.

Explained by highlit
Intermediate functional-options closures immutability api-design structs
1package config
2 
3import "time"
4 
5type ServerConfig struct {
6 Host string
7 Port int
8 ReadTimeout time.Duration
9 WriteTimeout time.Duration
10 MaxConnections int
11 TLSEnabled bool
12 AllowedOrigins []string
13}
14 
15type Option func(*ServerConfig)
16 
17func WithHost(host string) Option {
18 return func(c *ServerConfig) { c.Host = host }
19}
20 
21func WithPort(port int) Option {
22 return func(c *ServerConfig) { c.Port = port }
23}
24 
25func WithReadTimeout(d time.Duration) Option {
26 return func(c *ServerConfig) { c.ReadTimeout = d }
27}
28 
29func WithMaxConnections(n int) Option {
30 return func(c *ServerConfig) { c.MaxConnections = n }
31}
32 
33func WithTLS(enabled bool) Option {
34 return func(c *ServerConfig) { c.TLSEnabled = enabled }
35}
36 
37func WithAllowedOrigins(origins ...string) Option {
38 return func(c *ServerConfig) {
39 c.AllowedOrigins = append([]string(nil), origins...)
40 }
41}
42 
43func (base ServerConfig) With(opts ...Option) ServerConfig {
44 clone := base
45 clone.AllowedOrigins = append([]string(nil), base.AllowedOrigins...)
46 for _, opt := range opts {
47 opt(&clone)
48 }
49 return clone
50}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Functional options let callers set only the fields they care about while everything else keeps its zero or default value.
  2. 2Returning closures over a shared config pointer keeps each option small, self-contained, and independently testable.
  3. 3Copying slices during a clone prevents callers from mutating each other's shared backing arrays.

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