Functional options in Go

The Go answer to optional arguments: variadic functions that mutate a config.

type Server struct {
    addr    string
    timeout time.Duration
    verbose bool
}

type Option func(*Server)

func WithTimeout(d time.Duration) Option {
    return func(s *Server) { s.timeout = d }
}

func WithVerbose() Option {
    return func(s *Server) { s.verbose = true }
}

func NewServer(addr string, options ...Option) *Server {
    server := &Server{addr: addr, timeout: 30 * time.Second}
    for _, apply := range options {
        apply(server)
    }
    return server
}

func configured() string {
    s := NewServer(":8080", WithTimeout(time.Second), WithVerbose())
    return fmt.Sprintf("%s %s %t", s.addr, s.timeout, s.verbose)
}

How it works

1. Each option is a function that edits the struct.
2. The constructor applies the defaults, then the options.
3. Adding an option later breaks no existing caller.

Keywords and builtins used here

• bool
• for
• func
• range
• return
• string
• struct
• type

The run, in numbers

Lines

28

Characters to type

596

Tokens

155

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 85 seconds.

Type this snippet

Step 1 of 2 in API shapes, step 17 of 19 in Interfaces & generics.

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