Packages & Modules - Learn Go - ggcoder.com

Packages and Modules in Go

Go organizes code into packages. Each .go file must declare the package it belongs to. A package is a directory containing one or more .go files. A module is a collection of packages managed by a go.mod file. Modules allow you to manage dependency versions and share code.

The Package System

package main is special -> it defines an executable program with a main() function. All other packages are libraries. The package name usually matches the directory name. Packages are the basis of encapsulation and reuse in Go.

package_structure.go

1// math/math.go2package math34// Package level constants5const Pi = 3.1415967// Package level variables8var counter int910// Unexported function (lowercase)11func add(a, b int) int {12	return a + b

Exported vs Unexported

Go has no public/private keywords. Instead, it uses naming conventions. Names starting with an uppercase letter are exported (public). Names starting with a lowercase letter are unexported (private). This applies to functions, types, variables, constants, and methods.

export_rules.go

1package user23// Exported type (can be imported and used elsewhere)4type User struct {5	Name  string  // Exported field6	Email string  // Exported field7	age   int     // Unexported field8}910// Exported function11func NewUser(name, email string, age int) *User {12	return &User{

Importing Packages

Use the import statement to use code from other packages. You can import one package, multiple packages at once, or use aliases. Blank import _ "pkg" is for side effects (e.g., database drivers). Dot import (.) is not recommended as it reduces clarity.

imports.go

1package main23import (4	// Single import5	"fmt"67	// Alias import8	m "math"910	// Local package import11	"mymodule/user"12

go.mod and go.sum

The go.mod file defines your module and its dependencies. It contains the module path (name), Go version, and require directives. The go.sum file is a lockfile containing checksums of each dependency version. go mod tidy downloads necessary dependencies and removes unused ones.

go.mod

1// go.mod example2module github.com/alice/myapp34go 1.2156require (7	github.com/gin-gonic/gin v1.9.18	github.com/lib/pq v1.10.99)1011require (12	github.com/google/uuid v1.3.0 // indirect (transitive dependency)

go get and go install

go get downloads a package and its dependencies, updating go.mod. go get ./... downloads all dependencies of the current project. go install builds and installs a binary executable to $GOBIN. go install tool@latest installs the latest version of a tool.

go_get_install.sh

1// Terminal commands23// Add a dependency4go get github.com/lib/pq56// Add a specific version7go get github.com/lib/pq@v1.10.989// Remove a dependency (use go get and then tidy)10go get github.com/lib/pq@none11go mod tidy12

Internal Packages

Place packages in an internal/ directory to restrict who can import it. Only packages in the parent module can import from internal/. This is a good way to encapsulate implementation details that should not be public.

internal_packages.go

1// Project structure:2// mymodule/3// ├── go.mod4// ├── main.go5// ├── api/6// │   └── handler.go7// └── internal/8//     └── database/9//         └── db.go1011// internal/database/db.go12package database

Workspace Mode

go work allows you to work with multiple modules at once. Create a go.work file listing local modules. This is useful when developing related modules, such as a backend and shared library.

go.work

1// go.work2go 1.2134use (5	./backend      // Local module6	./shared       // Local module7	./cli-tool     // Local module8)910// Terminal:11// go work init ./backend ./shared  # Create go.work12// go work use ./cli-tool           # Add another module13// go work edit -dropuse=./cli-tool # Remove a module1415// When using go.work, local modules are used instead of go.mod versions16// This allows testing changes across multiple modules before publishing

Tip

go mod tidy should be run regularly to keep go.mod and go.sum clean. Always commit go.sum to version control to ensure build consistency.

Key Takeaways

Điểm Chính

Uppercase names are exported, lowercase names are unexportedTên uppercase được export, tên lowercase không được export
go.mod defines module path and dependenciesgo.mod định nghĩa đường dẫn module và phụ thuộc
internal/ directory restricts imports to the same moduleThư mục internal/ giới hạn import cho cùng một module
go get downloads dependencies, go install builds executablesgo get tải phụ thuộc, go install xây dựng các file thực thi

Practice

Test your understanding of this chapter

Quiz

Which naming convention makes a function accessible from other packages?

Quy ước đặt tên nào làm cho một hàm có thể truy cập từ các package khác?

Quiz

What is the purpose of go.sum file?

Tập tin go.sum có mục đích gì?

True or False

You can import from the internal/ directory of other modules in your go.get dependencies.

Bạn có thể import từ thư mục internal/ của các module khác trong các phụ thuộc go.get.

True or False

Running go mod tidy will remove dependencies from go.mod that are no longer used in the code.

Chạy go mod tidy sẽ xóa các phụ thuộc từ go.mod mà không còn được sử dụng trong code.

Code Challenge

Add a package to your module

Thêm một package vào module của bạn

go  github.com/goreleaser/goreleaser

1package user23// Exported type (can be imported and used elsewhere)4type User struct {5 Name string // Exported field6 Email string // Exported field7 age int // Unexported field8}910// Exported function11func NewUser(name, email string, age int) *User {12 return &User{

1// go.mod example2module github.com/alice/myapp34go 1.2156require (7 github.com/gin-gonic/gin v1.9.18 github.com/lib/pq v1.10.99)1011require (12 github.com/google/uuid v1.3.0 // indirect (transitive dependency)

1// Terminal commands23// Add a dependency4go get github.com/lib/pq56// Add a specific version7go get github.com/lib/pq@v1.10.989// Remove a dependency (use go get and then tidy)10go get github.com/lib/pq@none11go mod tidy12

1// Project structure:2// mymodule/3// ├── go.mod4// ├── main.go5// ├── api/6// │ └── handler.go7// └── internal/8// └── database/9// └── db.go1011// internal/database/db.go12package database

1// go.work2go 1.2134use (5 ./backend // Local module6 ./shared // Local module7 ./cli-tool // Local module8)910// Terminal:11// go work init ./backend ./shared # Create go.work12// go work use ./cli-tool # Add another module13// go work edit -dropuse=./cli-tool # Remove a module1415// When using go.work, local modules are used instead of go.mod versions16// This allows testing changes across multiple modules before publishing