Packages

Go code is organized into packages. A package is a folder of related .go files that share the same package name.

Declaring a package

Every file starts with its package name. The entry point of a program is package main.

package main

A reusable library would use a descriptive name, like package mathutil.

Importing

Bring in other packages with import. You then use the package name as a prefix.

import (
    "fmt"
    "strings"
)

func main() {
    fmt.Println(strings.ToUpper("go"))
}

Exported vs unexported

Visibility is controlled by capitalization. An identifier starting with an uppercase letter is exported (public) and usable from other packages. A lowercase name is unexported (private) to its package.

package mathutil

// Exported: usable elsewhere
func Add(a, b int) int {
    return a + b
}

// unexported: only inside this package
func subtract(a, b int) int {
    return a - b
}

This simple rule replaces keywords like public and private found in other languages.