Go module
last modified April 11, 2024
In this article we show how to work with modules in Golang.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
A module is a collection of Go packages stored in a file tree.
Modules contain two specific files: go.mod and go.sum.
The go.mod defines the module path and the dependency requirements.
The go.sum is an auto-generated dependencies lock file.
Modules allow us to define precise dependency requirements and design reproducible builds for multiple environments.
Third-party modules are downloaded from their repositories into a module cache.
These dependencies are then loaded from these copies when building applications.
The module cache is by default located in the go subdirectory of
the home directory.
The go module commands
The go tool provides several commands that are related to modules.
- go mod init - initializes new module in current directory
- go mod tidy - adds missing and removes unused modules
- go mod download - downloads modules to local cache
- go mod vendor - makes vendored copy of dependencies
- go mod graph - prints module requirement graph
- go mod verify - verifies dependencies have expected content
- go mod why - explains why packages or modules are needed
There are additional commands related to Go modules.
The go list -m lists available modules. The go get
installs dependencies and updates the go.mod file. The go build and
go test commands add new dependencies to go.mod as needed.
The go mod init
The go mod init command creates a new module rooted at the current
directory. A new go.mod file is created in the current directory;
it must not already exist.
$ go mod init [module-path]
The module path is the prefix path that is used to import all packages of that module. Developers often use the URL of the repository that hosts the source code as the module path.
$ go mod init com.zetcode/first go: creating new go.mod: module com.zetcode/first
A new module with com.zetcode/first module path is created.
$ cat go.mod module com.zetcode/first go 1.17
These are the contents of the go.mod file. We have the module path
the Go version.
The go get command
The dependencies are installed with the go get command. The command
automatically updates the go.mod file and creates the
go.sum file.
$ go mod init com.zetcode/myprog
First, we create a module.
$ go get gonum.org/v1/gonum/stat go: downloading gonum.org/v1/gonum v0.9.3 go get: added gonum.org/v1/gonum v0.9.3
We install the Gonum library. It is a set of numeric libraries for the Go language.
$ ls go.mod go.sum
We have go.mod and go.sum in the current working
directory.
~/go/pkg/mod/gonum.org/v1/gonum@v0.9.3$ ls -1 appveyor.yml AUTHORS blas cmplxs CONDUCT.md CONTRIBUTING.md CONTRIBUTORS diff doc.go dsp floats go.mod GOPHER gopher.png gopher.svg go.sum graph integrate internal interp lapack LICENSE mat mathext num optimize README.md spatial stat THIRD_PARTY_LICENSES unit version.go
Gonum library is distributed as a module; it is installed in the go
subdirectory of the user's home directory by default.
package main
import (
"fmt"
"math"
"gonum.org/v1/gonum/stat"
)
func main() {
xs := []float64{
12.44, 11.2, 34.5, 1.4, 6.7, 23.4,
}
fmt.Printf("data: %v\n", xs)
mean := stat.Mean(xs, nil)
variance := stat.Variance(xs, nil)
stddev := math.Sqrt(variance)
fmt.Printf("mean: %v\n", mean)
fmt.Printf("variance: %v\n", variance)
fmt.Printf("std-dev: %v\n", stddev)
}
We have a simple example that computes some basic statistics.
import (
"fmt"
"math"
"gonum.org/v1/gonum/stat"
)
We get the stat package of the module.
$ go run first.go data: [12.44 11.2 34.5 1.4 6.7 23.4] mean: 14.94 variance: 145.12640000000002 std-dev: 12.046841909811883
$ go list -m all com.zetcode/myprog dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9 gioui.org v0.0.0-20210308172011-57750fc8a0a6 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802 github.com/ajstarks/svgo v0.0.0-20180226025133-644b8db467af github.com/boombuler/barcode v1.0.0 github.com/davecgh/go-spew v1.1.0 ...
With the go list -m all, we list all the dependencies of the
project.
Vendoring
Vendoring is a specific way of satisfying project dependencies. Instead of using
the default module cache, the dependencies are downloaded and later loaded from
the vendor subdirectory. The directory is a subdirectory of the
project directory.
$ go mod vendor
The go mod vendor command makes a directory named vendor in the
main module's root directory. The command also creates the
vendor/modules.txt file that contains a list of vendored packages
and the module versions they were copied from.
$ go mod init com.zetcode/second $ go get github.com/bykof/gostradamus
We create a project and download a dependency. The gostradamus is a library for working with date and time.
package main
import (
"fmt"
"github.com/bykof/gostradamus"
)
func main() {
now := gostradamus.Now()
fmt.Println(now)
}
The example prints the current datetime.
$ go mod vendor
We initialize vendoring.
$ ls go.mod go.sum second.go vendor
We have the vendor directory created.
$ ls vendor/github.com/ bykof
It contains our dependency.
Source
In this article we have talked about modules in Go.
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