working directory is not part of a module

程式碼參考：https://www.studyiris.com/doc/irisDoc/Installation.html

    package main
    import "github.com/kataras/iris"
    func main() {
        app := iris.New()
        app.Get("/", func(ctx iris.Context){})
        app.Run(iris.Addr(":8080"))
    }

報錯如下：

main.go:2:8: cannot find module providing package github.com/kataras/iris: working directory is not part of a module
 

包的依賴我已經下載好了，存放到$GOPATH/pkg/mod下（注意有github.com就放到github.com目錄下）。

一直無非解決，直到這篇文字：https://www.jianshu.com/p/c666ebdb462b

一般網上的資料都是建議在工程的根目錄下執行go mod init projectName命令。在執行go mod化之後，所有的引用都不再是以GOPATH為相對路徑的引用了，而是變成了以go.mod中初始化的專案名為起始的引用。

$ go mod init main
go: creating new go.mod: module main
$ ls
go.mod main.go
$ go run main.go 
go: finding module for package github.com/kataras/iris/v12
go: found github.com/kataras/iris/v12 in github.com/kataras/iris/v12 v12.1.8
go: finding module for package github.com/shurcooL/sanitized_anchor_name
go: found github.com/shurcooL/sanitized_anchor_name in github.com/shurcooL/sanitized_anchor_name v1.0.0
Now listening on: http://localhost:8080
Application started. Press CMD+C to shut down.