Go build cannot find main Module; see ‘go help modules’

Description:

In the process of learning golang, in $GOPATH/src/github.com/xxx/hello directory performs go build and go test will be submitted to the following error

go: cannot find main module; see ‘go help modules’

Solution:

Since GO111MODULE=on was set earlier, use the following command to solve the problem

$ go mod init hello
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A go. Mod file is generated, and go Build succeeds

The reason:

In order to improve the download speed of dependent packages, we used GoProxy and then started it with the following command

$ go env -w GO111MODULE=on
$ go env -w GOPROXY=https://goproxy.cn,direct
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This changes the default way go manages dependencies to go Module mode, which relies on a go.mod file that describes the packages and versions the project depends on (similar to NPM package.json, Maven’s POM.xml).

And there is no go.mod file in the directory, so go doesn’t know what the main module is, so it can’t compile

Delve into

When the go language was born, it did not provide package management tools. Instead, it used go Get to download dependent packages and put them in $GOPATH/SRC. It did not use version control, because it would pull the code from the master branch every time. The code for the package is placed under SRC /github.com/xx/xx

Versions after Go 1.11 introduced the Go Modules, which use versions of packages marked in go.mod, the code for which is placed under PKG /mod

Use Go Modules or GOPATH

Go uses an environment variable, GO111MODULE, to decide whether to use Go Modules or GOPATH, which has three values and different semantics across versions