Understanding go.mod

The go.mod file declares a module path, a Go version and any module dependencies needed for the project.

For example, the lines module example.com/tools and go 1.23 identify a module and its minimum language/module semantics baseline. Dependencies are listed using require; replace and exclude have specialized uses.

The go directive is not the version of every dependency, nor does it automatically install a toolchain. Newer versions of Go may maintain additional toolchain metadata. Prefer normal Go commands over hand-editing a dependency’s version without understanding the graph.

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

Example go.mod

module example.com/tools

go 1.23

To inspect the actual file, use go mod edit -json.

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

Go code

package main

import (
	"fmt"
	"runtime"
)

func main() {
	fmt.Println("Go version:", runtime.Version() != "")
}

Output

Go version: true

The executable inspects whether the runtime provides a nonempty version string. The exact version depends on the installed toolchain; the printed boolean remains true.

• Do not assume go.mod can be executed like source code.
• The go directive concerns language/module behavior; it is not a list of installed packages.

Open go.mod and identify module, go and require directives. Explain how they differ.

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

You have practiced Understanding go.mod 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.