Introduction to Packages

A package groups Go declarations that belong together. A Go program uses packages to separate responsibilities without placing every function in one file.

Each Go source file begins with a package clause. Files in the same directory normally declare the same package and share package-level declarations. The executable entry point is package main with func main(); reusable libraries use another package name.

import "fmt" lets code refer to an exported name such as fmt.Println. A package name is not necessarily its full import path: the path identifies where Go finds the package, while the declaration supplies its local name.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

Reference: How to Write Go Code.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"strings"
)

func main() {
	name := strings.ToUpper("go")
	fmt.Println("Language:", name)
}

Output

Language: GO

The main package imports fmt for printing and strings for text manipulation. strings.ToUpper produces GO, then fmt.Println displays the result.

• One directory should not mix ordinary package names such as main and helpers in its non-test Go files.
• Importing a package does not make its unexported identifiers accessible.

Import math and print the result of math.Sqrt(81).

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Introduction to Packages in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.