Packages

Every Go file so far has started with package main. Time to see what that first line actually controls, and how real projects are organized beyond a single file.

Packages: Go's unit of code organization

Every .go file belongs to a package, declared on its first line. Files in the same directory must belong to the same package, and that directory is the package — Go has no separate "module" keyword for grouping files the way some languages do; the folder structure does the job.

// file: shapes/rectangle.go
package shapes
 
type Rectangle struct {
	Width, Height float64
}
 
func (r Rectangle) Area() float64 {
	return r.Width * r.Height
}

Another file, shapes/circle.go, would also start with package shapes and can use Rectangle directly — no import needed for files in the same package.

Only package main produces a runnable program; every other package name marks a library that other code imports. Recall from the very first lesson: only capitalized names (Rectangle, Area) are visible outside the package — that's the mechanism controlling what a package exposes to its importers.

Importing your own package

// file: main.go
package main
 
import (
	"fmt"
	"myproject/shapes"
)
 
func main() {
	r := shapes.Rectangle{Width: 3, Height: 4}
	fmt.Println(r.Area())
}

The import path (myproject/shapes) isn't a filesystem path — it's the module path defined in go.mod, plus the subdirectory. Which brings us to modules, next.