Go Packages, Modules, and Concurrency

March 13, 2025

In my previous post, I wrote a simple program. Now, let’s take the next steps by exploring Go’s package system, module management, and concurrency features.

Go Packages

Go code is organized into packages. A package is a collection of related Go source files. The main package is special because it defines an executable program.

To create and use a package:

Inside your project, create a new folder called greetings:
```
mkdir greetings
```

Create a file greetings/greetings.go:

package greetings

import "fmt"

func Hello(name string) string {
    return fmt.Sprintf("Hello, %s!", name)
}

Use this package in main.go:

package main

import (
    "fmt"
    "my-go-project/greetings"
)

func main() {
    message := greetings.Hello("Kostas")
    fmt.Println(message)
}

Run the program:
```
go run main.go
```
Expected output:
```
Hello, Kostas!
```

Go Modules

Go modules simplify dependency management. If you haven’t initialized a module yet, do it with:

go mod init my-go-project

To add third-party packages, use go get. For example:

go get github.com/fatih/color

Now, you can use this package in your code:

package main

import (
    "fmt"
    "github.com/fatih/color"
)

func main() {
    color.Cyan("This is a colored message!")
}

Run the program, and you’ll see colored output!

Go Concurrency

One of Go’s strengths is its built-in support for concurrency using goroutines and channels.

Goroutines

A goroutine is a lightweight thread managed by Go.

package main

import (
    "fmt"
    "time"
)

func sayHello() {
    fmt.Println("Hello from goroutine!")
}

func main() {
    go sayHello() // Start a goroutine
    time.Sleep(time.Second) // Wait to see the output
    fmt.Println("Main function finished")
}

Channels

Channels allow goroutines to communicate safely.

package main

import "fmt"

func main() {
    messages := make(chan string)

    go func() {
        messages <- "Hello from goroutine!"
    }()

    fmt.Println(<-messages) // Receive and print message
}