Your first program and Go modules

Packages and modules

A package is a folder of .go files that belong together. Every file starts with a package line. A module is a whole project: one or more packages with a go.mod file at the top. The module has a name called the module path, usually where the code lives online, like github.com/asha/expenses. For learning, any name like example.com/hello is fine.

Step by step

mkdir hello
cd hello
go mod init example.com/hello

module example.com/hello

go 1.24

Now create a file named main.go in the same folder:

package main

import "fmt"

func main() {
	fmt.Println("Namaste, Go!")
}

Every line explained

• package main: this package becomes a runnable program, not a library. Only package main can be run.
• import "fmt": use the standard package fmt (short for format), which prints and formats text.
• func main(): the function where the program starts. When main returns, the program ends.
• fmt.Println(...): call the function Println from package fmt. It prints its values separated by spaces and then a new line.
• Braces { } hold the body. The opening brace must be on the same line as func main(); putting it on the next line is a compile error.
• No semicolons: Go adds them for you at the end of lines, which is why the brace rule exists.

Printing: Println, Print and Printf

package main

import "fmt"

func main() {
	fmt.Println("Total:", 250, "rupees")
	fmt.Print("no newline ")
	fmt.Print("here\n")
	fmt.Printf("%s scored %d marks (%.1f%%)\n", "Asha", 452, 90.4)
}

go.sum

When you add a dependency (another module), Go creates go.sum next to go.mod. It stores a checksum (a fingerprint) of every dependency, so a changed or tampered download is detected. Commit both go.mod and go.sum to Git. You will add dependencies in module 5.

Common mistakes

• "go: cannot find main module": you ran go run outside a folder with go.mod. Run go mod init first, or cd into the project.
• "package hello is not a main package" style errors: a runnable program must say package main.
• Unused imports are compile errors. If you import "os" and never use it, the build fails with "os" imported and not used. Remove it (most editors do this for you).
• Capital letters matter: fmt.println does not exist; it is fmt.Println. Names starting with a capital letter are the ones a package exports.