Packages and Modules

# CHAPTER 17

Packages and Modules #

1. Introduction #

2. Learning Objectives #

• Understand what a Package is in Go.

• Export and Unexport variables and functions using Capitalization.

• Create and import your own custom packages.

• Understand Go Modules (go.mod).

• Download third-party packages using go get.

3. What is a Package? #

4. Visibility Rules (Exported vs Unexported) #

• Exported (Public): If a variable, struct, or function starts with a Capital Letter, it can be accessed by *other* packages. (e.g., func CalculateTax(), type User struct).

• Unexported (Private): If it starts with a lowercase letter, it is strictly private to its own package. Other packages cannot see or use it. (e.g., func calculateTax(), var dbPassword string).

*(This is why we use fmt.Println and not fmt.println! Println is Exported by the fmt package).*

5. Creating Custom Packages #

Project Structure:

my-app/
├── go.mod
├── main.go
└── mathutils/
    └── calc.go

1. Create calc.go inside the mathutils folder:

package mathutils

// Exported function (Capital 'A')
func Add(a, b int) int {
    return a + b
}

// Unexported function (Lowercase 's') - Cannot be used outside this package
func subtract(a, b int) int {
    return a - b
}

2. Import and use it in main.go:

package main

import (
    "fmt"
    "my-app/mathutils" // Import based on the module name + folder path
)

func main() {
    result := mathutils.Add(10, 5) // Works!
    fmt.Println(result)

    // mathutils.subtract(10, 5) // ERROR! subtract is unexported (private)
}

6. Go Modules (go.mod) #

The generated go.mod file tracks:

1. 1. The module's exact name.

1. 2. The Go version required.

1. 3. Any third-party dependencies you install.

7. Downloading External Dependencies (go get) #

// Example: Downloading the popular Gorilla Mux router
go get github.com/gorilla/mux

When you run this:

1. 1. Go downloads the code from GitHub.

1. 2. It adds the version to your go.mod file.

1. 3. It generates a go.sum file (which locks the exact version cryptographically for security).

You can then use it in your code:

import "github.com/gorilla/mux"

8. Common Mistakes #

• Circular Dependencies: If Package A imports Package B, and Package B imports Package A, the Go compiler will throw a fatal error. You must refactor your code to flow in one direction.

• Lowercasing Struct Fields: If you create an exported struct (type User struct) but its fields are lowercase (name string), other packages can instantiate the User but cannot access the name field! Always capitalize fields you want to be public (Name string).

9. Best Practices #

• Package Naming: Package names should be short, single-word, lowercase names. Avoid snakecase or camelCase (use mathutils, not MathUtils or mathutils).

• Organize by Domain: Group files by what they *do* (e.g., package auth, package payment), not by their architectural role (e.g., avoid package models, package controllers).

10. Exercises #

1. 1. Create a module using go mod init my-project.

1. 2. Create a subfolder stringutils and write an exported function Reverse(s string) string.

1. 3. Call it from main.go.

11. MCQs with Answers & Explanations #

12. Interview Preparation #

1. 1. How does Go handle Encapsulation (Access Modifiers) without using public or private keywords?

1. 2. Explain the role of go.mod and go.sum in a Go project.

1. 3. What is a Circular Dependency, and why does Go strictly forbid it?

13. Summary #

14. Next Chapter Recommendation #