02-package-and-module

Package and Module

A package is a collection of source files in the same directory that are compiled together. A module is a collection of related Go packages that are released together.

Steps

1. Create a Go module

go mod init <module_name>

1. If you want to use your module from other module. You can set github.com/<github user/org name>/<github repo name> to your module name.
2. This command will generate a go.mod file.

Example:

go mod init github.com/nakamasato/kubernetes-operator-basics/05-golang-basics/02-package-and-module
go: creating new go.mod: module github.com/nakamasato/kubernetes-operator-basics/05-golang-basics/02-package-and-module

I specified the path to the current directory.

go.mod file will be created.

module github.com/nakamasato/kubernetes-operator-basics/05-golang-basics/02-module

go 1.19

2. Hello World

1. Create a main.go.

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

2. Run main.go

   go run main.go
   Hello, World
   

3. Use your own package

1. Add mypackage. Create a mypackage/mypackage.go

   package mypackage
   
   func GetName() string {
       return "MyName"
   }

2. Use mypackage in the module.
   1. Import the pacakge in main.go

      import (
          "fmt" // Packages in the standard library do not have a module path prefix
          "<module_name>/mypackage" // e.g. "github.com/nakamasato/kubernetes-operator-basics/05-golang-basics/02-module/mypackage"
      )

      • Standard library: https://pkg.go.dev/std
   2. Use GetName function in mypackage package.

      func main() {
          name := mypackage.GetName()
          fmt.Printf("Hello, %s\n", name)
      }

3. Run main.go

   go run main.go
   Hello, MyName
   

4. Use an external package

1. Import a package

   import "github.com/google/go-github/v47/github"

2. Use it.

   client := github.NewClient(nil)
   // list all organizations for user "willnorris"
   orgs, _, err := client.Organizations.List(context.Background(), "willnorris", nil)
   if err != nil {
   	fmt.Println("Error")
   	os.Exit(1)
   }
   for i, org := range orgs {
   	fmt.Println(i, *org.Login)
   }

3. Run go mod tidy go.mod is updated:

   require github.com/google/go-github/v47 v47.1.0
   
   require (
       github.com/google/go-querystring v1.1.0 // indirect
       golang.org/x/crypto v0.0.0-20210817164053-32db794688a5 // indirect
   )
   

4. Run main.go

   go run main.go
   

   Hello, MyName
   0 diso
   1 activitystreams
   2 indieweb
   3 webfinger
   4 todogroup
   5 maintainers
   6 perkeep
   7 tailscale
   

Practice

1. Packages, variables, and functions.
   1. Packages
   2. Imports
   3. Exported names
   4. Functions
   5. Functions continuedn
   6. Multiple results
   7. Named return values
   8. Variables
   9. Variables with initializers
   10. Short variable declarations
   11. Basic types
   12. Zero values
   13. Type conversions
   14. Type inference
   15. Constants
   16. Numeric Constants
2. Flow control statements: for, if, else, switch and defer
   1. For
   2. For continued
   3. For is Go's "while"
   4. Forever
   5. If
   6. If with a short statement
   7. If and else
   8. Exercise: Loops and Functions
   9. Switch
   10. Switch evaluation order
   11. Switch with no condition
   12. Defer
   13. Stacking defers