How do packages and modules organize Go code?

Question

Accepted Answer

Go organizes code with **packages** (collections of source files in a directory) and **modules** (collections of packages with versioned dependencies). Together they structure projects and manage dependencies.

## Packages — a directory of related code

```go
// every .go file starts by declaring its package
package main         // "main" is special — produces an executable

// or a library package (named after its directory, conventionally)
package mathutil

func Add(a, b int) int { return a + b }   // Exported (capitalized) — usable outside
func helper() {}                           // unexported (lowercase) — package-private
```

All files in one directory belong to one package. **Capitalization controls visibility**: capitalized names are exported (public across packages), lowercase are private to the package.

## Importing and using packages

```go
import (
    "fmt"                          // standard library
    "strings"
    "github.com/user/repo/mathutil" // a third-party or local module package
)

fmt.Println(strings.ToUpper("hi"))
mathutil.Add(2, 3)                  // use Exported names via package.Name
```

## Modules — versioned dependency management

```bash
go mod init github.com/user/myapp   # create a module → makes go.mod
go get github.com/gin-gonic/gin     # add a dependency (records it in go.mod)
go mod tidy                          # add missing & remove unused dependencies
go build                             # build using the module's dependencies
```

```text
go.mod  — declares the module path, Go version, and direct dependencies (+ versions)
go.sum  — cryptographic checksums of dependencies (integrity/security)
```

A **module** (introduced in Go 1.11) is the unit of dependency management — `go.mod` lists exact dependency versions, making builds reproducible. `go.sum` verifies dependency integrity.

## The init function and main

```go
package main

func init() {    // runs automatically when the package loads (before main)
    // setup
}
func main() {    // entry point of an executable (package main only)
}
```

## Project structure conventions

```text
myapp/
  go.mod
  main.go              package main
  internal/            packages here are PRIVATE to this module (can't be imported externally)
  pkg/ or other dirs/  reusable packages
```

The `internal/` directory is special — its packages can only be imported within the same module, enforcing encapsulation at the module level.

## Why it matters

Packages and modules are how Go projects are structured and how dependencies are managed — essential for any real project.

Understanding packages (a directory of files, capitalization-based exported/unexported visibility, the special `main` package) is fundamental to organizing code and controlling its public API.

Modules (`go mod`, `go.mod`/`go.sum`) provide reproducible, versioned, integrity-checked dependency management — the modern standard that replaced the old GOPATH approach.

Knowing the import system, the `internal/` convention for module-private code, and the tooling (`go mod init/tidy`, `go get`) is necessary for building, sharing, and maintaining Go projects, and reflects practical fluency with the Go ecosystem.