Creating a new module

// hello.go
package hello

func Hello() string {
    return "Hello, world."
}

// hello_test.go
package hello

import "testing"

func TestHello(t *testing.T){
    want := "Hello, world."
    if got := Hello(); got != want {
        t.Errorf("Hello() = %q, want %q, got, want")
    }
}

$ go test

$ go mod init example.com/hello

go mod init write a go.mod file.

Adding a dependence

package hello

import "rsc.io/quote"

func Hello() string {
    return quote.Hello()
}

$ go test

go list -m lists the current module and all its dependencies

Upgrading dependencies