thereadme/slides/functional_options_pattern_in_go/index.md

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A Trick for Designing Friendly APIs in Go	default	center	text-center	sans CaskaydiaCove Nerd Font Mono

A Trick for Designing
Friendly APIs in Go

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The Optional Configuration Problem

// No defaults?
// What is sensible?
// What does 'true' do?
// What is '30'?
srv := NewServer("localhost", 8080, 30, true)

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"We need to add a TLS parameter."

func NewServer(host string, port int, timeout int) *Server {
    // ...
}

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"Just use a struct"

srv := NewServer(Config{
    Host: "localhost",
    Port: 0, 
})

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"Nah bro, use functional options."

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1. Define the option

type Server struct {
    host string
    port int
}

// Option is a function that modifies the Server
type Option func(*Server)

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2. Create closures

// WithPort captures the 'port' value
func WithPort(port int) Option {
    // It returns a closure
    return func(s *Server) {
        s.port = port
    }
}

func WithHost(h string) Option {
    return func(s *Server) {
        s.host = h
    }
}

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3. Get variadic

```go func NewServer(host string, port int) *Server { return &Server{ host: host, port: port, } } ```

```go func NewServer(opts ...Option) *Server {

s := &Server{
    host: "localhost", 
    port: 80,
}

for _, opt := range opts {
    opt(s)
}

return s

}

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# Almost as good as C#

```go
srv := NewServer(
    WithHost("api.google.com"),
    WithPort(9090),
    WithTimeout(30 * time.Second),
)

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Extensibility? We got you.

Support TLS... ```go func WithTLS(cert, key string) Option { return func(s *Server) { s.tls = true s.cert = cert s.key = key } } ```

Breaks nothing. ```go srv := NewServer(WithPort(8080))

srv2 := NewServer( WithPort(8080), WithTLS("cert.pem", "key.pem"), )

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# So why use this pattern?

<div class="text-left mx-auto w-max text-xl leading-10 flex flex-col">
    <div>
        <v-click>
            <carbon:checkmark-filled class="text-green-500 mr-2"/> Defaults are easy (just call New)
        </v-click>
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        <v-click>
            <carbon:checkmark-filled class="text-green-500 mr-2"/> Usage is self-documenting
        </v-click>
    </div>
    <div>
        <v-click>
            <carbon:checkmark-filled class="text-green-500 mr-2"/> New options don't break old code
        </v-click>
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# <span class="text-[#00ADD8]">Go</span> forth and build friendly APIs!