Your First Go Program

The package main Declaration

Every Go source file starts with a package declaration. To produce a standalone, runnable program, the file must belong to 'package main' -- this tells the Go tool that this package is an entry point rather than a reusable library. If you name it anything else, 'go build' will produce a library archive instead of an executable.

Note: Library code (meant to be imported, not run directly) uses a different package name, like 'package mathutils'.

Example: The package main Declaration

package main

import "fmt"

func main() {
	fmt.Println("package main marks this as a runnable program")
}

The main Function

Inside package main, Go looks for a function literally named main with no parameters and no return value -- this is where execution begins. Everything that happens when you run the program is triggered from inside, or reachable from, this single function.

Example: The main Function

package main

import "fmt"

func main() {
	fmt.Println("Execution starts here, inside func main()")
}

Importing and Using fmt

The fmt package (short for format) is part of Go's standard library and provides functions for formatted input/output, most commonly fmt.Println for printing a line and fmt.Printf for printing with format verbs like %s or %d. You bring it into scope with an import statement at the top of the file.

Note: An unused import is a compile-time error in Go, not just a linter warning -- this keeps codebases free of dead imports.

Example: Importing and Using fmt

package main

import "fmt"

func main() {
	name := "Gopher"
	fmt.Printf("Hello, %s! Welcome to Go.\n", name)
}

Running Your Program

The 'go run' command compiles your source file into a temporary binary and immediately executes it, printing the output to your terminal. This is the fastest way to try out small programs while learning, without manually managing build artifacts.

Note: 'go run' is great for quick iteration; use 'go build' when you want a reusable binary file.

Example: Running Your Program

go run hello.go