Getting Started with Go Packages

What is Go?

Go is a compiled, statically typed programming language developed by Google. Many modern applications, including Docker, Kubernetes, and Caddy, are written in Go.

Go packages allow developers to organize and reuse Go code in a simple and maintainable manner. Using and declaring Go packages are essential tasks that are performed when writing Go applications. This guide will demonstrate and help you to understand how to create and use Go packages.

Before You Begin

If you haven’t installed Go on your server yet, follow the instructions from the Install Go on Ubuntu guide before proceeding.

If you prefer to experiment with Go without installing it first, you can run the examples found in this guide using the Go Playground.

Packages

Packages provide the capability to organize and reuse source code.

Declaring a Package

In a text editor, create a hellogopher.go file in your GOPATH and add the following content to create a simple “Hello world” program:

File: hellogopher.go

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package main

import "fmt"

func main() {
        fmt.Println("Hello Gopher!")
}

The first statement declares the package main using the package keyword.

Every Go source file must declare a package. This tells Go which package the source code file is part of. Any Go app that’s intended to run on the command line declares the package main. The main function that we define after declaring package main is where execution of the Go code begins. The main function takes no arguments and returns no values.

Run the program:

go run hellogopher.go

Hello Gopher!

Import a Package

Import the fmt package using the import keyword:

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import "fmt"

The fmt package includes functionality for formatting output to the screen such as the Println function:

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fmt.Println("Hello Gopher!")

Grouping Imports

To have the example program print the current time as well as a greeting, add the time package. You could add each import on its own line:

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import "fmt"
import "time"

Instead of using import on each line, replace the import statements with an import grouping that includes both the fmt and time packages.

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import (
        "fmt"
        "time"
)

If your text editor includes a Go language plugin, it may adjust the text automatically.

In the main function, right after the greeting is printed, use the Now() function in the time module to print out the current time:

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fmt.Println(time.Now())

We print out the current time by printing out the time value returned from the Now function of the time package. The time package and the fmt package are part of Go’s robust standard library.

After making these two changes, the Hello Gopher program now looks like this:

File: hellogopher.go

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package main

import (
        "fmt"
        "time"
)

func main() {
        fmt.Println("Hello Gopher!")
        fmt.Println(time.Now())
}

Run the modified program:

run hellogopher.go

Hello Gopher!
2018-04-29 14:41:08.715214 -0700 PDT m=+0.000391168

Aliasing a Package

Go provides the ability to alias imported packages. For example, in the Hello Gopher program we can use an alias, f, to refer to the fmt package like so:

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package main

import (
        f "fmt"
        "time"
)

func main() {
        f.Println("Hello Gopher!")
        f.Println(time.Now())
}

We alias the fmt package with the name f, by placing the alias name in front of the package name inside the import grouping.

Notice that all Println function calls to the fmt package have now been replaced to call the fmt package’s alias, f.

Create a Package

Create a package called greetings, which will contain functionality to print out a greeting to the screen.

Go packages follow a convention where Go source files that implement a particular package must be housed in a directory whose name matches the package’s name. In this example, our package is called greetings, and we create a directory called greetings for the Go source file that implements the greetings package.

Create and change to a new directory for the greetings package:

mkdir -p $GOPATH/src/greetings && cd $GOPATH/src/greetings

Create a source file called greetings.go and place it inside the greetings directory with the following contents:

File: $GOPATH/src/greetings/greetings.go

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package greetings

import (
  "fmt"
)

func PrintGreetings() {
        fmt.Println("Hello Gopher!")
}

func printGreetingsUnexported() {
        fmt.Println("Hello Gopher! (from unexported)")
}

The first line of code in greetings.go declares the greetings package, using the package keyword.

The PrintGreetings function in the greetings package is responsible for printing out the greeting. In Go, capitalizing the first letter of a function name has a special significance – it indicates that a function is an exported function. An exported function is a function that can be called from outside a package.

Function names that begin with a lowercase letter are only accessible by Go source files within the same package. The printGreetingsUnexported function also prints out a variation of the “Hello Gopher” greeting, but it cannot be called outside of the greetings package, since it is an unexported function.

A Go package can contain multiple Go source files. Let’s create another Go source file named magic.go inside the greetings directory with the following contents:

File: magic.go

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package greetings

import "fmt"

var magicNumber int

// PrintMagicNumber : Prints the internal variable magicNumber
func PrintMagicNumber() {
        fmt.Println("The magic number is...", magicNumber)
}

// PrintTheUnexportedGreetings : Prints the unexported greeting
func PrintTheUnexportedGreetings() {
        printGreetingsUnexported()
}

func init() {
        magicNumber = 108
}

The first line of code declares that magic.go belongs to the greetings package.

In the same manner by which we declare exported and unexported functions by capitalization of the first letter, we can follow the same rules when declaring package variables.

For example, the magicNumber variable is an unexported package variable of type int (integer):

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var magicNumber int

This means that it can be accessed by any Go source file within the greetings package, but the magicNumber variable cannot be accessed outside of the greetings package.

The PrintMagicNumber function prints out the value of the magicNumber variable.

The magicNumber package variable gets initialized in the init function, which is a special function in the Go language. The init function is used to initialize the state of a package. Go will automatically call the init function defined here, prior to calling the main function of a command line Go program.

We use the PrintTheUnexportedGreetings function, an exported function, to call the unexported printGreetingsUnexported function. This allows Go source files outside of the greetings package to call the printGreetingsUnexported function.

Although printGreetingsUnexported is defined in a different source file than magic.go, the PrintTheUnexportedGreetings function is able to call it because both source files belong to the greetings package.

Use a Custom Package

At this point we’ve created our own custom package, and now it’s time to use the functionality defined in the package.

Create a new directory called usegreetings inside the $GOPATH/src directory and create a source file called usegreetings.go with the following contents:

File: $GOPATH/src/usegreetings/usegreetings.go

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package main

import (
        "greetings"
)

func main() {
        greetings.PrintGreetings()
        greetings.PrintMagicNumber()
        greetings.PrintTheUnexportedGreetings()
}

The usegreetings.go source file implements a command line program, so we have to declare the main package and function which is the primary entry point of the command line Go program.

Notice that we have included the greetings package in the import grouping.

Run the program:

go run usegreetings.go

Hello Gopher!
The magic number is... 108
Hello Gopher! (from unexported)

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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