Structure of a Go Program

Go programs have a predictable shape: a package declaration, optional imports, declarations and functions. Understanding this structure makes later examples much easier to read.

package main identifies the package as executable. import "fmt" makes the package available by name. func main() declares the entry-point function. Statements in its braces execute in order.

Go requires a package declaration in every source file. File-scope declarations can appear outside functions, but ordinary short variable declarations cannot appear there. Imports must be used; unused imports cause compilation errors.

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

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

Go code

package main

import "fmt"

const appName = "Go Explorer"

func main() {
	message := "Welcome"
	fmt.Println(appName)
	fmt.Println(message)
}

Output

Go Explorer
Welcome

First, the constant appName is declared at package scope. When the program starts, main() creates a local variable message, prints the application name and then prints the message.

• A local statement such as message := "Welcome" belongs inside a function.
• Do not expect imports or package declarations to generate output themselves.

Locate the package declaration, import, constant declaration, function declaration and two function-call statements in the example.

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

You have practiced Structure of a Go Program 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.