add init options for legacy code

legacy.go

package main

type AnyOther struct{}

type Module interface {
	Classify(*AnyOther)
}

type SomeModule struct {
	Param string
}

func CreateInternalSomeModule() Module {
	return &SomeModule{Param: "some string"}
}

func (m *SomeModule) Classify(a *AnyOther) {
}

var modules = []Module{
	CreateInternalSomeModule(),
}

func Create() {
	// do something
}

func main() {
	// original exported method
	// we cannot customize the Param variable
	Create()
}

with_options.go

package main

import "fmt"

// --- legacy code ---
type AnyOther struct{}

type Module interface {
	Classify(*AnyOther)
}

type SomeModule struct {
	Param string
}

func CreateInternalSomeModule() Module {
	return &SomeModule{Param: "some string"}
}

func (m *SomeModule) Classify(a *AnyOther) {
}

var modules = []Module{
	CreateInternalSomeModule(),
}
// --- legacy code ---

type Option interface {
	Apply(Module)
}

type ModuleOption struct {
	Param string
}

func (o *ModuleOption) Apply(m Module) {
	if o.Param != "" {
		m.(*SomeModule).Param = o.Param
	}
	fmt.Println(m.(*SomeModule))
}

func create(opts ...Option) {
	for _, opt := range opts {
		switch opt.(type) {
		case *ModuleOption:
			opt.Apply(modules[0])
		}
	}
}
func main() {
	// original exported method
	create()
	// add options
	// so the original method still work
	create(&ModuleOption{Param: "other string"})
}