Go module

last modified August 24, 2023

Go module tutorial shows how to work with modules in Golang.

$ go version
go version go1.18.1 linux/amd64

We use Go version 1.18.

A module is a collection of Go packages stored in a file tree.

go.modgo.sumgo.modgo.sum

Modules allow us to define precise dependency requirements and design reproducible builds for multiple environments.

go

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.

go list -mgo getgo buildgo test

The go mod init

go mod initgo.mod
$ 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

com.zetcode/first
$ cat go.mod
module com.zetcode/first

go 1.17

go.mod

The go get command

go getgo.modgo.sum
$ 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

go.modgo.sum
~/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

go
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"
)

stat
$ 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
...

go list -m all

Vendoring

vendor
$ go mod vendor

go mod vendorvendor/modules.txt
$ 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

vendor
$ ls vendor/github.com/
bykof

It contains our dependency.

In this article we have talked about modules in Go.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.