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Packages and Modules

Status

This note is complete, reviewed, and considered stable.

Go Modules​

Go Modules are the official dependency management system introduced in Go 1.11 and became the default in Go 1.16+. They provide a way to manage versioned dependencies and project structure.

Key Concepts​

  • A module is a collection of Go packages stored in a file tree with a go.mod file at its root.

  • Each module defines:

    • Its own module path (usually a repository URL like github.com/user/project)
    • Its dependencies (along with their versions)

Important Files​

  • go.mod: Defines the module and its dependencies
  • go.sum: Records checksums of modules to verify integrity

Commands​

  • go mod init <module-path>: Initializes a new module
  • go mod tidy: Adds missing and removes unused dependencies
  • go get <package>@version: Adds or upgrades a dependency
  • go build, go run, etc., will automatically download necessary modules

Example​

go mod init github.com/abhishek/myapp

This creates:

module github.com/abhishek/myapp

go 1.21

package main​

In Go, every file must declare a package. The main package is special. It tells the Go compiler that this package is executable, not a shared library.

Rules​

  • Only the main package can produce an executable binary.
  • The main package must contain a function called main().

Usage​

Typical for programs meant to be run (e.g., CLI apps, web servers, etc.)

package main

import "fmt"

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

If we change package main to something else (like package utils), Go will not generate an executable for it. It will be a reusable library.

func main()​

Purpose​

The main() function is the entry point of a Go application.

Rules​

  • It must have the signature func main()
  • It cannot take arguments or return values
  • We can define other functions and call them from main()
  • There can only be one main() function per main package

Behavior​

  • When we run go run . or build and execute the binary, Go looks for main.main() to start execution.

Example​

package main

import "fmt"

func greet(name string) {
    fmt.Printf("Hello, %s!\n", name)
}

func main() {
    greet("Abhishek")
}