> For the complete documentation index, see [llms.txt](https://calvinfeng.gitbook.io/gonotebook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://calvinfeng.gitbook.io/gonotebook/hello-world/01-02-go-packages.md).

# Go Packages

Code is organized by packages in Go. They are similar to Python's modules and JavaScript's npm modules.

## What is `main`

The most common package in Go is `main` because main package is served as an entry point to any Go program.

For example:

```go
package main

import "fmt"

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

HelloWorld()
```

This will not compile and throw an error. You must call your functions inside the `main` function.

```go
package main

import "fmt"

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

func main() {
  HelloWorld()
}
```

### Non-main Packages

We can write our own packages. However, we cannot run a non-main package directly. We must import it into the `main` package. For example, create a new folder in `helloworld` and call it `foo`. Inside the folder, make a file called `foo.go`.

```go
package foo

func SayHello() {
  fmt.Println("Foo says hello")
}
```

Now we can import the package into the main program and run `foo.SayHello`.

```go
// => hello_world/main.go
package main

import (
  "fmt"
  "go-academy/helloworld/foo"
)

func main() {
  fmt.Println("Hello World, this is Go!")
  SayHello()
  SayBye()
  foo.SayHello()
}
```