# Modules and Packages — Go (Golang)

Source: https://www.skillbyai.com/en/go/i-modules

> Organising code and dependencies.

## go.mod, imports and visibility

A **module** is a collection of packages with a `go.mod` file that declares its path, the Go version, and dependencies (with exact versions recorded in `go.sum`). A **package** is a directory of `.go` files sharing a package name; you import it by module path plus directory. Identifiers starting with an **uppercase** letter are exported (visible to other packages); lowercase ones are private to the package. `go get` adds dependencies and `go mod tidy` cleans them up.

## A small module layout

Typical structure; names are examples.

```text
shop/
  go.mod                 module example.com/shop
  go.sum                 checksums of dependencies
  cmd/shop/main.go       package main: wires things together
  internal/orders/       package orders: only importable inside this module
    orders.go            func New(...) *Service   (exported: capital N)
    store.go             func validate(...)       (unexported)
  internal/orders/orders_test.go
```

## Use internal/ for private packages

Packages under internal/ cannot be imported by other modules, which keeps your public API small.

**Quiz:** How is an identifier exported from a Go package?

- [ ] By putting it in main.go
- [ ] With the export keyword
- [ ] By listing it in go.mod
- [x] By starting its name with an uppercase letter

*Answer:* By starting its name with an uppercase letter. Capitalisation controls visibility.