An import path can describe how to obtain the package source code using a revision control system such as Git or Mercurial. The go tool uses this property to automatically fetch packages from remote repositories. For instance, to use github.com/google/go-cmp/cmp in your program:

package main

import (
	"fmt"

	"example/user/hello/morestrings"
	"github.com/google/go-cmp/cmp"
)

func main() {
	fmt.Println(morestrings.ReverseRunes("!oG ,olleH"))
	fmt.Println(cmp.Diff("Hello World", "Hello Go"))
}
Now that you have a dependency on an external module, you need to download that module and record its version in your go.mod file. The go mod tidy command adds missing module requirements for imported packages and removes requirements on modules that aren't used anymore.

$ go mod tidy
go: finding module for package github.com/google/go-cmp/cmp
go: found github.com/google/go-cmp/cmp in github.com/google/go-cmp v0.5.4
$ go install example/user/hello
$ hello
Hello, Go!
  string(
- 	"Hello World",
+ 	"Hello Go",
  )
$ cat go.mod
module example/user/hello

go 1.16

require github.com/google/go-cmp v0.5.4
$
Module dependencies are automatically downloaded to the pkg/mod subdirectory of the directory indicated by the GOPATH environment variable. The downloaded contents for a given version of a module are shared among all other modules that require that version, so the go command marks those files and directories as read-only. To remove all downloaded modules, you can pass the -modcache flag to go clean:

$ go clean -modcache
$

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