packages-and-modules.md

📦 Packages and Modules

• 📦 Packages and Modules
  • 👀 Fast Lookup
  • 📚 Concepts
  • 🛠️ Let's Create a Simple Calculator Program
  • 🔐 Public and Private Identifiers

👀 Fast Lookup

• Package: Collection of Go files in the same directory
• Module: Collection of packages with go.mod file
• go mod init: Initialize a new module
• Import path: modulename/packagename format
• Public identifiers: Start with uppercase letter (exported)
• Private identifiers: Start with lowercase letter (unexported)
• Main package: Special package for executable programs
• Package declaration: package name at top of each .go file

📚 Concepts

• Package is a collection of Go files in the same directory.
• Module is a collection of packages.
• Program is a collection of main package and its dependencies.
• Process is a running instance of a program.

🛠️ Let's Create a Simple Calculator Program

go mod init simplecalc

This will create a go.mod file in current directory. simplecalc is the name of the module.

simplecalc/            # This is the root directory of the module (where go mod init simplecalc was executed)
├── go.mod             # Module definition file
├── main.go            # Main package file (main.go)
└── mymath/            # Directory for mymath package
    ├── add.go         # package mymath
    ├── subtract.go    # package mymath
    └── ...            # (Other files in the mymath package)

package main

import (
  "fmt"
  "simplecalc/mymath" // simplecalc is the module name, mymath is the package name
)

func main() {
  sum := mymath.Add(1, 2)
  fmt.Println("1 + 2 =", sum)
}

🔐 Public and Private Identifiers

In Go, identifiers (names of variables, functions, types, etc.) are public if they start with a capital letter and private if they start with a lowercase letter.

package mymath

func Add(a, b int) int {
  return a + b
}

func subtract(a, b int) int {
  return a - b
}

In the above code, Add is a public function and subtract is a private function.