Using Uber's FX framework with optional dependencies in Go.

fx_optional.go

package main

import (
	"fmt"

	"go.uber.org/fx"
)

type IConfigSmall interface {
	GetUri() string
}

type ConfigSmall struct {
	url string
}

func (c *ConfigSmall) GetUri() string {
	return c.url
}

type IConfigLarge interface {
	GetUri() string
	GetPort() string
}

type ConfigLarge struct {
	url  string
	port string
}

func (c *ConfigLarge) GetUri() string {
	return c.url
}

func (c *ConfigLarge) GetPort() string {
	return c.port
}

func NewCS() IConfigSmall {
	return &ConfigSmall{url: "https://proficloud.io"}
}

func NewCL() LargeResult {
	cfg := &ConfigLarge{url: "https://proficloud.io/large", port: "8080"}
	return LargeResult{
		ConfigSmall: cfg,
		ConfigLarge: cfg,
	}
}

type LargeResult struct {
	fx.Out

	ConfigSmall IConfigSmall
	ConfigLarge IConfigLarge
}

type Params struct {
	fx.In

	ConfigSmall IConfigSmall
	ConfigLarge IConfigLarge `optional:"true"`
}

func main() {
	fmt.Println("Hello, 世界")
	fx.New(
		// Try commenting out one of the following fx.Provide statements and observe
		//fx.Provide(NewCS),
		fx.Provide(NewCL),
		fx.Invoke(func(lifecycle fx.Lifecycle, p Params) {
			fmt.Printf("parameters small: %v\n", p.ConfigSmall.GetUri())
			if p.ConfigLarge != nil {
				fmt.Printf("parameters large: %v\n", p.ConfigLarge.GetPort())
			} else {
				fmt.Println("fallback to default value")
			}
		}),
	).Run()
}