In Go, a package is a way to organize and encapsulate code. Packages help you structure your application into reusable and maintainable components. Go packages can contain functions, variables, constants, and types, and they provide a level of visibility control (public and private scope) based on naming conventions.

Here are some key points about Go packages:

1. Standard Library Packages: Go provides a rich set of standard library packages, like fmt, net/http, os, and math, among others.

2. Custom Packages: You can create your own packages to modularize your code.

3. Importing Packages: Packages are imported using the import keyword. Multiple packages can be imported by enclosing them in parentheses.

4. Package Initialization: When a package is imported, all its init functions are called (in the order they appear), and all global variables are initialized.

5. Visibility: If an identifier (function, variable, or type name) starts with an uppercase letter, it is exported (public). Otherwise, it's unexported (private to the package).

6. Package Alias: When importing a package, you can give it an alias, which can be useful for avoiding name conflicts or shortening long package names.

7. Package Main: Every executable Go application must contain a main package and a main() function as the entry point.

Workshop Exercise: Test the Concept

For this exercise, we'll create a simple Go project to explore packages. Specifically, we'll build a main package and a custom package, and then import the custom package into the main package.

1. Create the directory structure

    mkdir -p go_packages_demo/mycalculator
   

2. Write the custom package

   Navigate into the mycalculator directory and create a file called calculator.go.

    // mycalculator/calculator.go
    package mycalculator
   
    // Add returns the sum of two integers.
    func Add(a int, b int) int {
        return a + b
    }
   
    // Subtract returns the result of a - b.
    func Subtract(a int, b int) int {
        return a - b
    }
   

3. Write the main package

   Navigate back to go_packages_demo directory and create a file called main.go.

    package main
   
    import (
        "fmt"
        "go_packages_demo/mycalculator"
    )
   
    func main() {
        sum := mycalculator.Add(5, 3)
        difference := mycalculator.Subtract(10, 4)
   
        fmt.Printf("Sum: %d\n", sum)
        fmt.Printf("Difference: %d\n", difference)
    }
   

4. Initialize a Go Module

    go mod init go_packages_demo
   

   This will create a go.mod file in the project directory.

   Now, the import path "go_packages_demo/mycalculator" is relative to the root of the Go module, which is go_packages_demo.

5. Run the Code

   Navigate back to the go_packages_demo directory and run:

    go run main.go
   

   You should see the following output:

    Sum: 8
    Difference: 6
   

In this exercise, we created a custom package named mycalculator with Add and Subtract functions. Then, we imported this package into our main package and used the Add and Subtract functions.

This is a very simple example, but it should give you an idea of how Go packages can be used to organize and modularize your code.

References

1. Go Standard Library

2. Understanding Package Visibility in Go