uber-go-style-guide-kr/README.md

# uber-go-style-guide-kr

Translated in Korean

---

<!--

Editing this document:

- Discuss all changes in GitHub issues first.
- Update the table of contents as new sections are added or removed.
- Use tables for side-by-side code samples. See below.

Code Samples:

Use 2 spaces to indent. Horizontal real estate is important in side-by-side
samples.

For side-by-side code samples, use the following snippet.

~~~
<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
BAD CODE GOES HERE
```

</td><td>

```go
GOOD CODE GOES HERE
```

</td></tr>
</tbody></table>
~~~

(You need the empty lines between the <td> and code samples for it to be
treated as Markdown.)

If you need to add labels or descriptions below the code samples, add another
row before the </tbody></table> line.

~~~
<tr>
<td>DESCRIBE BAD CODE</td>
<td>DESCRIBE GOOD CODE</td>
</tr>
~~~

-->

# Uber의 Go언어 스타일 가이드 (Uber's Go Style Guide)

- [uber-go-style-guide-kr](#uber-go-style-guide-kr)
- [Uber의 Go언어 스타일 가이드 (Uber's Go Style Guide)](#uber%ec%9d%98-go%ec%96%b8%ec%96%b4-%ec%8a%a4%ed%83%80%ec%9d%bc-%ea%b0%80%ec%9d%b4%eb%93%9c-ubers-go-style-guide)
  - [소개 (Introduction)](#%ec%86%8c%ea%b0%9c-introduction)
  - [가이드라인 (Guidelines)](#%ea%b0%80%ec%9d%b4%eb%93%9c%eb%9d%bc%ec%9d%b8-guidelines)
    - [인터페이스에 대한 포인터 (Pointers to Interfaces)](#%ec%9d%b8%ed%84%b0%ed%8e%98%ec%9d%b4%ec%8a%a4%ec%97%90-%eb%8c%80%ed%95%9c-%ed%8f%ac%ec%9d%b8%ed%84%b0-pointers-to-interfaces)
    - [수신자(Receivers)와 인터페이스(Interfaces)](#%ec%88%98%ec%8b%a0%ec%9e%90receivers%ec%99%80-%ec%9d%b8%ed%84%b0%ed%8e%98%ec%9d%b4%ec%8a%a4interfaces)
    - [Zero-value Mutexes are Valid](#zero-value-mutexes-are-valid)
    - [Copy Slices and Maps at Boundaries](#copy-slices-and-maps-at-boundaries)
      - [Receiving Slices and Maps](#receiving-slices-and-maps)
      - [Returning Slices and Maps](#returning-slices-and-maps)
    - [Defer to Clean Up](#defer-to-clean-up)
    - [Channel Size is One or None](#channel-size-is-one-or-none)
    - [Start Enums at One](#start-enums-at-one)
    - [Error Types](#error-types)
    - [Error Wrapping](#error-wrapping)
    - [Handle Type Assertion Failures](#handle-type-assertion-failures)
    - [Don't Panic](#dont-panic)
    - [Use go.uber.org/atomic](#use-gouberorgatomic)
  - [Performance](#performance)
    - [Prefer strconv over fmt](#prefer-strconv-over-fmt)
    - [Avoid string-to-byte conversion](#avoid-string-to-byte-conversion)
  - [Style](#style)
    - [Group Similar Declarations](#group-similar-declarations)
    - [Import Group Ordering](#import-group-ordering)
    - [Package Names](#package-names)
    - [Function Names](#function-names)
    - [Import Aliasing](#import-aliasing)
    - [Function Grouping and Ordering](#function-grouping-and-ordering)
    - [Reduce Nesting](#reduce-nesting)
    - [Unnecessary Else](#unnecessary-else)
    - [Top-level Variable Declarations](#top-level-variable-declarations)
    - [Prefix Unexported Globals with _](#prefix-unexported-globals-with)
    - [Embedding in Structs](#embedding-in-structs)
    - [Use Field Names to initialize Structs](#use-field-names-to-initialize-structs)
    - [Local Variable Declarations](#local-variable-declarations)
    - [nil is a valid slice](#nil-is-a-valid-slice)
    - [Reduce Scope of Variables](#reduce-scope-of-variables)
    - [Avoid Naked Parameters](#avoid-naked-parameters)
    - [Use Raw String Literals to Avoid Escaping](#use-raw-string-literals-to-avoid-escaping)
    - [Initializing Struct References](#initializing-struct-references)
    - [Format Strings outside Printf](#format-strings-outside-printf)
    - [Naming Printf-style Functions](#naming-printf-style-functions)
  - [Patterns](#patterns)
    - [Test Tables](#test-tables)
    - [Functional Options](#functional-options)

## 소개 (Introduction)

스타일은 코드를 통제하는(govern) 관습이다. 이러한 관습(convention)은 소스파일 포맷팅 (e.g. gofmt)보다 더 많은 영역을 다루기(cover) 때문에, "스타일" 이라는 단어 자체가 약간 부적절 할 수 있다.

본 가이드의 목표는 Uber에서 Go 코드를 작성할 때 해야 할 것과 하지 말아야 할 것 (Dos and Don'ts)에 대하여 자세하게 설명하여 이러한 복잡성을 관리하는 것이다. 이런 규칙들은 엔지니어들이 Go 언어의 특성을(feature) 생산적으로개계속 사용할 수 있도록 코드 베이스를 관리가능하게 유지하기위해 존재한다.

이 가이드는 원래 [Prashant Varanasi]와 [Simon Newton]이 동료들에게 Go를 사용하면서 개발속도 향상을 도모하기 위해 소개되었다. 또한, 수 년에 거쳐서 다른 사람들로부터의 피드백을 통해서 개정되 오고 있다.

  [Prashant Varanasi]: https://github.com/prashantv
  [Simon Newton]: https://github.com/nomis52 

이 문서는 Uber에서의 엔지니어들이 지향하는 Go언어 코드의 관용적 규칙을 설명한다. 상당 수의 규칙들은 Go언어에 대한 일반적인 가이드라인이며, 다른 부분에 대해서는 외부 레퍼런스에 의해 확장된다 (아래 참고)

1. [Effective Go](https://golang.org/doc/effective_go.html)
2. [The Go common mistakes guide](https://github.com/golang/go/wiki/CodeReviewComments)

모든 코드는 `golint` 와 `go vet`를 실행할 때 에러가 없어야 한다. 또한 우리는 여러분들의 에디터를 아래와 같이 설정하기를 권고한다:

- Run `goimports` on save
- Run `golint` and `go vet` to check for errors

아래의 링크를 통해서 Go 툴을 지원하는 에디터에 대한 정보를 얻을 수 있다:
<https://github.com/golang/go/wiki/IDEsAndTextEditorPlugins>

## 가이드라인 (Guidelines)

### 인터페이스에 대한 포인터 (Pointers to Interfaces)

일반적으로 인터페이스에 대한 포인터는 거의 필요하지 않을 것이다. 여러분들은 인터페이스를 값(value)으로서 전달(passing)해야 할 것이며, 인터페이스에 대한 기본 데이터(underlying data)는 여전히 포인터가 될 수 있다.

한 인터페이스는 두 가지 필드이다:

1. 타입-특정 정보(type-specific information)에 대한 포인터. 여러분들을 이것을 "타입"으로 간주할 수 있다.
2. 데이터 포인터. 저장된 데이터가 포인터일 경우, 이것은 직접적으로 저장될 수 있다. 만약, 저장된 데이터가 값(value)인 경우, 값에 대한 포인터가 저장된다.

만약 여러분들이 기본 데이터(underlying data) 수정하기 위한 인터페이스 메서드 (interface methods)를 원한다면, 여러분들은 반드시 포인터를 사용해야 한다.

### 수신자(Receivers)와 인터페이스(Interfaces)

값 수신자 (value receivers)와 메서드(Methods)는 포인터 혹은 값에 의해서 호출 될 수 있다.

예를 들면,

```go
type S struct {
  data string
}

func (s S) Read() string {
  return s.data
}

func (s *S) Write(str string) {
  s.data = str
}

sVals := map[int]S{1: {"A"}}

// 오직 값만 사용하여 Read를 호출 할 수 있다.
sVals[1].Read()

// 아래 코드는 컴파일 되지 않을 것:
//  sVals[1].Write("test")

sPtrs := map[int]*S{1: {"A"}}

// 포인터를 사용하여 Read와 Write 모두 호출 할 수 있다.
sPtrs[1].Read()
sPtrs[1].Write("test")
```

마찬가지로, 메서드가 값 수신자(value receiver)를 가지고 있다고 하더라도 포인터가 인터페이스를 충족시킬 수 있다. 

```go
type F interface {
  f()
}

type S1 struct{}

func (s S1) f() {}

type S2 struct{}

func (s *S2) f() {}

s1Val := S1{}
s1Ptr := &S1{}
s2Val := S2{}
s2Ptr := &S2{}

var i F
i = s1Val
i = s1Ptr
i = s2Ptr

// s2Val이 값이고 f에 대한 수신자가 없기 때문에, 아래의 코드는 컴파일 되지 않는다.
//   i = s2Val
```

Effective Go에 [Pointers vs. Values]에 대한 좋은 글이 있으니 참고하기 바란다.

  [Pointers vs. Values]: https://golang.org/doc/effective_go.html#pointers_vs_values

### Zero-value Mutexes are Valid

The zero-value of `sync.Mutex` and `sync.RWMutex` is valid, so you almost
never need a pointer to a mutex.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
mu := new(sync.Mutex)
mu.Lock()
```

</td><td>

```go
var mu sync.Mutex
mu.Lock()
```

</td></tr>
</tbody></table>

If you use a struct by pointer, then the mutex can be a non-pointer field.

Unexported structs that use a mutex to protect fields of the struct may embed
the mutex.

<table>
<tbody>
<tr><td>

```go
type smap struct {
  sync.Mutex // only for unexported types

  data map[string]string
}

func newSMap() *smap {
  return &smap{
    data: make(map[string]string),
  }
}

func (m *smap) Get(k string) string {
  m.Lock()
  defer m.Unlock()

  return m.data[k]
}
```

</td><td>

```go
type SMap struct {
  mu sync.Mutex

  data map[string]string
}

func NewSMap() *SMap {
  return &SMap{
    data: make(map[string]string),
  }
}

func (m *SMap) Get(k string) string {
  m.mu.Lock()
  defer m.mu.Unlock()

  return m.data[k]
}
```

</td></tr>

</tr>
<tr>
<td>Embed for private types or types that need to implement the Mutex interface.</td>
<td>For exported types, use a private field.</td>
</tr>

</tbody></table>

### Copy Slices and Maps at Boundaries

Slices and maps contain pointers to the underlying data so be wary of scenarios
when they need to be copied.

#### Receiving Slices and Maps

Keep in mind that users can modify a map or slice you received as an argument
if you store a reference to it.

<table>
<thead><tr><th>Bad</th> <th>Good</th></tr></thead>
<tbody>
<tr>
<td>

```go
func (d *Driver) SetTrips(trips []Trip) {
  d.trips = trips
}

trips := ...
d1.SetTrips(trips)

// Did you mean to modify d1.trips?
trips[0] = ...
```

</td>
<td>

```go
func (d *Driver) SetTrips(trips []Trip) {
  d.trips = make([]Trip, len(trips))
  copy(d.trips, trips)
}

trips := ...
d1.SetTrips(trips)

// We can now modify trips[0] without affecting d1.trips.
trips[0] = ...
```

</td>
</tr>

</tbody>
</table>

#### Returning Slices and Maps

Similarly, be wary of user modifications to maps or slices exposing internal
state.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

// Snapshot returns the current stats.
func (s *Stats) Snapshot() map[string]int {
  s.mu.Lock()
  defer s.mu.Unlock()

  return s.counters
}

// snapshot is no longer protected by the mutex, so any
// access to the snapshot is racy.
snapshot := stats.Snapshot()
```

</td><td>

```go
type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

func (s *Stats) Snapshot() map[string]int {
  s.mu.Lock()
  defer s.mu.Unlock()

  result := make(map[string]int, len(s.counters))
  for k, v := range s.counters {
    result[k] = v
  }
  return result
}

// Snapshot is now a copy.
snapshot := stats.Snapshot()
```

</td></tr>
</tbody></table>

### Defer to Clean Up

Use defer to clean up resources such as files and locks.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
p.Lock()
if p.count < 10 {
  p.Unlock()
  return p.count
}

p.count++
newCount := p.count
p.Unlock()

return newCount

// easy to miss unlocks due to multiple returns
```

</td><td>

```go
p.Lock()
defer p.Unlock()

if p.count < 10 {
  return p.count
}

p.count++
return p.count

// more readable
```

</td></tr>
</tbody></table>

Defer has an extremely small overhead and should be avoided only if you can
prove that your function execution time is in the order of nanoseconds. The
readability win of using defers is worth the miniscule cost of using them. This
is especially true for larger methods that have more than simple memory
accesses, where the other computations are more significant than the `defer`.

### Channel Size is One or None

Channels should usually have a size of one or be unbuffered. By default,
channels are unbuffered and have a size of zero. Any other size
must be subject to a high level of scrutiny. Consider how the size is
determined, what prevents the channel from filling up under load and blocking
writers, and what happens when this occurs.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// Ought to be enough for anybody!
c := make(chan int, 64)
```

</td><td>

```go
// Size of one
c := make(chan int, 1) // or
// Unbuffered channel, size of zero
c := make(chan int)
```

</td></tr>
</tbody></table>

### Start Enums at One

The standard way of introducing enumerations in Go is to declare a custom type
and a `const` group with `iota`. Since variables have a 0 default value, you
should usually start your enums on a non-zero value.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
type Operation int

const (
  Add Operation = iota
  Subtract
  Multiply
)

// Add=0, Subtract=1, Multiply=2
```

</td><td>

```go
type Operation int

const (
  Add Operation = iota + 1
  Subtract
  Multiply
)

// Add=1, Subtract=2, Multiply=3
```

</td></tr>
</tbody></table>

There are cases where using the zero value makes sense, for example when the
zero value case is the desirable default behavior.

```go
type LogOutput int

const (
  LogToStdout LogOutput = iota
  LogToFile
  LogToRemote
)

// LogToStdout=0, LogToFile=1, LogToRemote=2
```

<!-- TODO: section on String methods for enums -->

### Error Types

There are various options for declaring errors:

- [`errors.New`] for errors with simple static strings
- [`fmt.Errorf`] for formatted error strings
- Custom types that implement an `Error()` method
- Wrapped errors using [`"pkg/errors".Wrap`]

When returning errors, consider the following to determine the best choice:

- Is this a simple error that needs no extra information? If so, [`errors.New`]
  should suffice.
- Do the clients need to detect and handle this error? If so, you should use a
  custom type, and implement the `Error()` method.
- Are you propagating an error returned by a downstream function? If so, check
  the [section on error wrapping](#error-wrapping).
- Otherwise, [`fmt.Errorf`] is okay.

  [`errors.New`]: https://golang.org/pkg/errors/#New
  [`fmt.Errorf`]: https://golang.org/pkg/fmt/#Errorf
  [`"pkg/errors".Wrap`]: https://godoc.org/github.com/pkg/errors#Wrap

If the client needs to detect the error, and you have created a simple error
using [`errors.New`], use a var for the error.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// package foo

func Open() error {
  return errors.New("could not open")
}

// package bar

func use() {
  if err := foo.Open(); err != nil {
    if err.Error() == "could not open" {
      // handle
    } else {
      panic("unknown error")
    }
  }
}
```

</td><td>

```go
// package foo

var ErrCouldNotOpen = errors.New("could not open")

func Open() error {
  return ErrCouldNotOpen
}

// package bar

if err := foo.Open(); err != nil {
  if err == foo.ErrCouldNotOpen {
    // handle
  } else {
    panic("unknown error")
  }
}
```

</td></tr>
</tbody></table>

If you have an error that clients may need to detect, and you would like to add
more information to it (e.g., it is not a static string), then you should use a
custom type.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
func open(file string) error {
  return fmt.Errorf("file %q not found", file)
}

func use() {
  if err := open(); err != nil {
    if strings.Contains(err.Error(), "not found") {
      // handle
    } else {
      panic("unknown error")
    }
  }
}
```

</td><td>

```go
type errNotFound struct {
  file string
}

func (e errNotFound) Error() string {
  return fmt.Sprintf("file %q not found", e.file)
}

func open(file string) error {
  return errNotFound{file: file}
}

func use() {
  if err := open(); err != nil {
    if _, ok := err.(errNotFound); ok {
      // handle
    } else {
      panic("unknown error")
    }
  }
}
```

</td></tr>
</tbody></table>

Be careful with exporting custom error types directly since they become part of
the public API of the package. It is preferable to expose matcher functions to
check the error instead.

```go
// package foo

type errNotFound struct {
  file string
}

func (e errNotFound) Error() string {
  return fmt.Sprintf("file %q not found", e.file)
}

func IsNotFoundError(err error) bool {
  _, ok := err.(errNotFound)
  return ok
}

func Open(file string) error {
  return errNotFound{file: file}
}

// package bar

if err := foo.Open("foo"); err != nil {
  if foo.IsNotFoundError(err) {
    // handle
  } else {
    panic("unknown error")
  }
}
```

<!-- TODO: Exposing the information to callers with accessor functions. -->

### Error Wrapping

There are three main options for propagating errors if a call fails:

- Return the original error if there is no additional context to add and you
  want to maintain the original error type.
- Add context using [`"pkg/errors".Wrap`] so that the error message provides
  more context and [`"pkg/errors".Cause`] can be used to extract the original
  error.
- Use [`fmt.Errorf`] if the callers do not need to detect or handle that
  specific error case.

It is recommended to add context where possible so that instead of a vague
error such as "connection refused", you get more useful errors such as
"call service foo: connection refused".

When adding context to returned errors, keep the context succinct by avoiding
phrases like "failed to", which state the obvious and pile up as the error
percolates up through the stack:

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
s, err := store.New()
if err != nil {
    return fmt.Errorf(
        "failed to create new store: %s", err)
}
```

</td><td>

```go
s, err := store.New()
if err != nil {
    return fmt.Errorf(
        "new store: %s", err)
}
```

<tr><td>

```
failed to x: failed to y: failed to create new store: the error
```

</td><td>

```
x: y: new store: the error
```

</td></tr>
</tbody></table>

However once the error is sent to another system, it should be clear the
message is an error (e.g. an `err` tag or "Failed" prefix in logs).

See also [Don't just check errors, handle them gracefully].

  [`"pkg/errors".Cause`]: https://godoc.org/github.com/pkg/errors#Cause
  [Don't just check errors, handle them gracefully]: https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully

### Handle Type Assertion Failures

The single return value form of a [type assertion] will panic on an incorrect
type. Therefore, always use the "comma ok" idiom.

  [type assertion]: https://golang.org/ref/spec#Type_assertions

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
t := i.(string)
```

</td><td>

```go
t, ok := i.(string)
if !ok {
  // handle the error gracefully
}
```

</td></tr>
</tbody></table>

<!-- TODO: There are a few situations where the single assignment form is
fine. -->

### Don't Panic

Code running in production must avoid panics. Panics are a major source of
[cascading failures]. If an error occurs, the function must return an error and
allow the caller to decide how to handle it.

  [cascading failures]: https://en.wikipedia.org/wiki/Cascading_failure

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
func foo(bar string) {
  if len(bar) == 0 {
    panic("bar must not be empty")
  }
  // ...
}

func main() {
  if len(os.Args) != 2 {
    fmt.Println("USAGE: foo <bar>")
    os.Exit(1)
  }
  foo(os.Args[1])
}
```

</td><td>

```go
func foo(bar string) error {
  if len(bar) == 0 {
    return errors.New("bar must not be empty")
  }
  // ...
  return nil
}

func main() {
  if len(os.Args) != 2 {
    fmt.Println("USAGE: foo <bar>")
    os.Exit(1)
  }
  if err := foo(os.Args[1]); err != nil {
    panic(err)
  }
}
```

</td></tr>
</tbody></table>

Panic/recover is not an error handling strategy. A program must panic only when
something irrecoverable happens such as a nil dereference. An exception to this is
program initialization: bad things at program startup that should abort the
program may cause panic.

```go
var _statusTemplate = template.Must(template.New("name").Parse("_statusHTML"))
```

Even in tests, prefer `t.Fatal` or `t.FailNow` over panics to ensure that the
test is marked as failed.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// func TestFoo(t *testing.T)

f, err := ioutil.TempFile("", "test")
if err != nil {
  panic("failed to set up test")
}
```

</td><td>

```go
// func TestFoo(t *testing.T)

f, err := ioutil.TempFile("", "test")
if err != nil {
  t.Fatal("failed to set up test")
}
```

</td></tr>
</tbody></table>

<!-- TODO: Explain how to use _test packages. -->

### Use go.uber.org/atomic

Atomic operations with the [sync/atomic] package operate on the raw types
(`int32`, `int64`, etc.) so it is easy to forget to use the atomic operation to
read or modify the variables.

[go.uber.org/atomic] adds type safety to these operations by hiding the
underlying type. Additionally, it includes a convenient `atomic.Bool` type.

  [go.uber.org/atomic]: https://godoc.org/go.uber.org/atomic
  [sync/atomic]: https://golang.org/pkg/sync/atomic/

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
type foo struct {
  running int32  // atomic
}

func (f* foo) start() {
  if atomic.SwapInt32(&f.running, 1) == 1 {
     // already running…
     return
  }
  // start the Foo
}

func (f *foo) isRunning() bool {
  return f.running == 1  // race!
}
```

</td><td>

```go
type foo struct {
  running atomic.Bool
}

func (f *foo) start() {
  if f.running.Swap(true) {
     // already running…
     return
  }
  // start the Foo
}

func (f *foo) isRunning() bool {
  return f.running.Load()
}
```

</td></tr>
</tbody></table>

## Performance

Performance-specific guidelines apply only to the hot path.

### Prefer strconv over fmt

When converting primitives to/from strings, `strconv` is faster than
`fmt`.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
for i := 0; i < b.N; i++ {
  s := fmt.Sprint(rand.Int())
}
```

</td><td>

```go
for i := 0; i < b.N; i++ {
  s := strconv.Itoa(rand.Int())
}
```

</td></tr>
<tr><td>

```
BenchmarkFmtSprint-4    143 ns/op    2 allocs/op
```

</td><td>

```
BenchmarkStrconv-4    64.2 ns/op    1 allocs/op
```

</td></tr>
</tbody></table>

### Avoid string-to-byte conversion

Do not create byte slices from a fixed string repeatedly. Instead, perform the
conversion once and capture the result.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
for i := 0; i < b.N; i++ {
  w.Write([]byte("Hello world"))
}
```

</td><td>

```go
data := []byte("Hello world")
for i := 0; i < b.N; i++ {
  w.Write(data)
}
```

</tr>
<tr><td>

```
BenchmarkBad-4   50000000   22.2 ns/op
```

</td><td>

```
BenchmarkGood-4  500000000   3.25 ns/op
```

</td></tr>
</tbody></table>

## Style

### Group Similar Declarations

Go supports grouping similar declarations.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
import "a"
import "b"
```

</td><td>

```go
import (
  "a"
  "b"
)
```

</td></tr>
</tbody></table>

This also applies to constants, variables, and type declarations.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go

const a = 1
const b = 2



var a = 1
var b = 2



type Area float64
type Volume float64
```

</td><td>

```go
const (
  a = 1
  b = 2
)

var (
  a = 1
  b = 2
)

type (
  Area float64
  Volume float64
)
```

</td></tr>
</tbody></table>

Only group related declarations. Do not group declarations that are unrelated.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
type Operation int

const (
  Add Operation = iota + 1
  Subtract
  Multiply
  ENV_VAR = "MY_ENV"
)
```

</td><td>

```go
type Operation int

const (
  Add Operation = iota + 1
  Subtract
  Multiply
)

const ENV_VAR = "MY_ENV"
```

</td></tr>
</tbody></table>

Groups are not limited in where they can be used. For example, you can use them
inside of functions.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
func f() string {
  var red = color.New(0xff0000)
  var green = color.New(0x00ff00)
  var blue = color.New(0x0000ff)

  ...
}
```

</td><td>

```go
func f() string {
  var (
    red   = color.New(0xff0000)
    green = color.New(0x00ff00)
    blue  = color.New(0x0000ff)
  )

  ...
}
```

</td></tr>
</tbody></table>

### Import Group Ordering

There should be two import groups:

- Standard library
- Everything else

This is the grouping applied by goimports by default.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
import (
  "fmt"
  "os"
  "go.uber.org/atomic"
  "golang.org/x/sync/errgroup"
)
```

</td><td>

```go
import (
  "fmt"
  "os"

  "go.uber.org/atomic"
  "golang.org/x/sync/errgroup"
)
```

</td></tr>
</tbody></table>

### Package Names

When naming packages, choose a name that is:

- All lower-case. No capitals or underscores.
- Does not need to be renamed using named imports at most call sites.
- Short and succinct. Remember that the name is identified in full at every call
  site.
- Not plural. For example, `net/url`, not `net/urls`.
- Not "common", "util", "shared", or "lib". These are bad, uninformative names.

See also [Package Names] and [Style guideline for Go packages].

  [Package Names]: https://blog.golang.org/package-names
  [Style guideline for Go packages]: https://rakyll.org/style-packages/

### Function Names

We follow the Go community's convention of using [MixedCaps for function
names]. An exception is made for test functions, which may contain underscores
for the purpose of grouping related test cases, e.g.,
`TestMyFunction_WhatIsBeingTested`.

  [MixedCaps for function names]: https://golang.org/doc/effective_go.html#mixed-caps

### Import Aliasing

Import aliasing must be used if the package name does not match the last
element of the import path.

```go
import (
  "net/http"

  client "example.com/client-go"
  trace "example.com/trace/v2"
)
```

In all other scenarios, import aliases should be avoided unless there is a
direct conflict between imports.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
import (
  "fmt"
  "os"


  nettrace "golang.net/x/trace"
)
```

</td><td>

```go
import (
  "fmt"
  "os"
  "runtime/trace"

  nettrace "golang.net/x/trace"
)
```

</td></tr>
</tbody></table>

### Function Grouping and Ordering

- Functions should be sorted in rough call order.
- Functions in a file should be grouped by receiver.

Therefore, exported functions should appear first in a file, after
`struct`, `const`, `var` definitions.

A `newXYZ()`/`NewXYZ()` may appear after the type is defined, but before the
rest of the methods on the receiver.

Since functions are grouped by receiver, plain utility functions should appear
towards the end of the file.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
func (s *something) Cost() {
  return calcCost(s.weights)
}

type something struct{ ... }

func calcCost(n []int) int {...}

func (s *something) Stop() {...}

func newSomething() *something {
    return &something{}
}
```

</td><td>

```go
type something struct{ ... }

func newSomething() *something {
    return &something{}
}

func (s *something) Cost() {
  return calcCost(s.weights)
}

func (s *something) Stop() {...}

func calcCost(n []int) int {...}
```

</td></tr>
</tbody></table>

### Reduce Nesting

Code should reduce nesting where possible by handling error cases/special
conditions first and returning early or continuing the loop. Reduce the amount
of code that is nested multiple levels.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
for _, v := range data {
  if v.F1 == 1 {
    v = process(v)
    if err := v.Call(); err == nil {
      v.Send()
    } else {
      return err
    }
  } else {
    log.Printf("Invalid v: %v", v)
  }
}
```

</td><td>

```go
for _, v := range data {
  if v.F1 != 1 {
    log.Printf("Invalid v: %v", v)
    continue
  }

  v = process(v)
  if err := v.Call(); err != nil {
    return err
  }
  v.Send()
}
```

</td></tr>
</tbody></table>

### Unnecessary Else

If a variable is set in both branches of an if, it can be replaced with a
single if.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
var a int
if b {
  a = 100
} else {
  a = 10
}
```

</td><td>

```go
a := 10
if b {
  a = 100
}
```

</td></tr>
</tbody></table>

### Top-level Variable Declarations

At the top level, use the standard `var` keyword. Do not specify the type,
unless it is not the same type as the expression.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
var _s string = F()

func F() string { return "A" }
```

</td><td>

```go
var _s = F()
// Since F already states that it returns a string, we don't need to specify
// the type again.

func F() string { return "A" }
```

</td></tr>
</tbody></table>

Specify the type if the type of the expression does not match the desired type
exactly.

```go
type myError struct{}

func (myError) Error() string { return "error" }

func F() myError { return myError{} }

var _e error = F()
// F returns an object of type myError but we want error.
```

### Prefix Unexported Globals with _

Prefix unexported top-level `var`s and `const`s with `_` to make it clear when
they are used that they are global symbols.

Exception: Unexported error values, which should be prefixed with `err`.

Rationale: Top-level variables and constants have a package scope. Using a
generic name makes it easy to accidentally use the wrong value in a different
file.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// foo.go

const (
  defaultPort = 8080
  defaultUser = "user"
)

// bar.go

func Bar() {
  defaultPort := 9090
  ...
  fmt.Println("Default port", defaultPort)

  // We will not see a compile error if the first line of
  // Bar() is deleted.
}
```

</td><td>

```go
// foo.go

const (
  _defaultPort = 8080
  _defaultUser = "user"
)
```

</td></tr>
</tbody></table>

### Embedding in Structs

Embedded types (such as mutexes) should be at the top of the field list of a
struct, and there must be an empty line separating embedded fields from regular
fields.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
type Client struct {
  version int
  http.Client
}
```

</td><td>

```go
type Client struct {
  http.Client

  version int
}
```

</td></tr>
</tbody></table>

### Use Field Names to initialize Structs

You should almost always specify field names when initializing structs. This is
now enforced by [`go vet`].

  [`go vet`]: https://golang.org/cmd/vet/

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
k := User{"John", "Doe", true}
```

</td><td>

```go
k := User{
    FirstName: "John",
    LastName: "Doe",
    Admin: true,
}
```

</td></tr>
</tbody></table>

Exception: Field names *may* be omitted in test tables when there are 3 or
fewer fields.

```go
tests := []struct{
  op Operation
  want string
}{
  {Add, "add"},
  {Subtract, "subtract"},
}
```

### Local Variable Declarations

Short variable declarations (`:=`) should be used if a variable is being set to
some value explicitly.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
var s = "foo"
```

</td><td>

```go
s := "foo"
```

</td></tr>
</tbody></table>

However, there are cases where the default value is clearer when the `var`
keyword is use. [Declaring Empty Slices], for example.

  [Declaring Empty Slices]: https://github.com/golang/go/wiki/CodeReviewComments#declaring-empty-slices

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
func f(list []int) {
  filtered := []int{}
  for _, v := range list {
    if v > 10 {
      filtered = append(filtered, v)
    }
  }
}
```

</td><td>

```go
func f(list []int) {
  var filtered []int
  for _, v := range list {
    if v > 10 {
      filtered = append(filtered, v)
    }
  }
}
```

</td></tr>
</tbody></table>

### nil is a valid slice

`nil` is a valid slice of length 0. This means that,

- You should not return a slice of length zero explicitly. Return `nil`
  instead.

  <table>
  <thead><tr><th>Bad</th><th>Good</th></tr></thead>
  <tbody>
  <tr><td>

  ```go
  if x == "" {
    return []int{}
  }
  ```

  </td><td>

  ```go
  if x == "" {
    return nil
  }
  ```

  </td></tr>
  </tbody></table>

- To check if a slice is empty, always use `len(s) == 0`. Do not check for
  `nil`.

  <table>
  <thead><tr><th>Bad</th><th>Good</th></tr></thead>
  <tbody>
  <tr><td>

  ```go
  func isEmpty(s []string) bool {
    return s == nil
  }
  ```

  </td><td>

  ```go
  func isEmpty(s []string) bool {
    return len(s) == 0
  }
  ```

  </td></tr>
  </tbody></table>

- The zero value (a slice declared with `var`) is usable immediately without
  `make()`.

  <table>
  <thead><tr><th>Bad</th><th>Good</th></tr></thead>
  <tbody>
  <tr><td>

  ```go
  nums := []int{}
  // or, nums := make([]int)

  if add1 {
    nums = append(nums, 1)
  }

  if add2 {
    nums = append(nums, 2)
  }
  ```

  </td><td>

  ```go
  var nums []int

  if add1 {
    nums = append(nums, 1)
  }

  if add2 {
    nums = append(nums, 2)
  }
  ```

  </td></tr>
  </tbody></table>

### Reduce Scope of Variables

Where possible, reduce scope of variables. Do not reduce the scope if it
conflicts with [Reduce Nesting](#reduce-nesting).

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
err := ioutil.WriteFile(name, data, 0644)
if err != nil {
 return err
}
```

</td><td>

```go
if err := ioutil.WriteFile(name, data, 0644); err != nil {
 return err
}
```

</td></tr>
</tbody></table>

If you need a result of a function call outside of the if, then you should not
try to reduce the scope.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
if data, err := ioutil.ReadFile(name); err == nil {
  err = cfg.Decode(data)
  if err != nil {
    return err
  }

  fmt.Println(cfg)
  return nil
} else {
  return err
}
```

</td><td>

```go
data, err := ioutil.ReadFile(name)
if err != nil {
   return err
}

if err := cfg.Decode(data); err != nil {
  return err
}

fmt.Println(cfg)
return nil
```

</td></tr>
</tbody></table>

### Avoid Naked Parameters

Naked parameters in function calls can hurt readability. Add C-style comments
(`/* ... */`) for parameter names when their meaning is not obvious.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// func printInfo(name string, isLocal, done bool)

printInfo("foo", true, true)
```

</td><td>

```go
// func printInfo(name string, isLocal, done bool)

printInfo("foo", true /* isLocal */, true /* done */)
```

</td></tr>
</tbody></table>

Better yet, replace naked `bool` types with custom types for more readable and
type-safe code. This allows more than just two states (true/false) for that
parameter in the future.

```go
type Region int

const (
  UnknownRegion Region = iota
  Local
)

type Status int

const (
  StatusReady = iota + 1
  StatusDone
  // Maybe we will have a StatusInProgress in the future.
)

func printInfo(name string, region Region, status Status)
```

### Use Raw String Literals to Avoid Escaping

Go supports [raw string literals](https://golang.org/ref/spec#raw_string_lit),
which can span multiple lines and include quotes. Use these to avoid
hand-escaped strings which are much harder to read.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
wantError := "unknown name:\"test\""
```

</td><td>

```go
wantError := `unknown error:"test"`
```

</td></tr>
</tbody></table>

### Initializing Struct References

Use `&T{}` instead of `new(T)` when initializing struct references so that it
is consistent with the struct initialization.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
sval := T{Name: "foo"}

// inconsistent
sptr := new(T)
sptr.Name = "bar"
```

</td><td>

```go
sval := T{Name: "foo"}

sptr := &T{Name: "bar"}
```

</td></tr>
</tbody></table>

### Format Strings outside Printf

If you declare format strings for `Printf`-style functions outside a string
literal, make them `const` values.

This helps `go vet` perform static analysis of the format string.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
msg := "unexpected values %v, %v\n"
fmt.Printf(msg, 1, 2)
```

</td><td>

```go
const msg = "unexpected values %v, %v\n"
fmt.Printf(msg, 1, 2)
```

</td></tr>
</tbody></table>

### Naming Printf-style Functions

When you declare a `Printf`-style function, make sure that `go vet` can detect
it and check the format string.

This means that you should use pre-defined `Printf`-style function
names if possible. `go vet` will check these by default. See [Printf family]
for more information.

  [Printf family]: https://golang.org/cmd/vet/#hdr-Printf_family

If using the pre-defined names is not an option, end the name you choose with
f: `Wrapf`, not `Wrap`. `go vet` can be asked to check specific `Printf`-style
names but they must end with f.

```shell
$ go vet -printfuncs=wrapf,statusf
```

See also [go vet: Printf family check].

  [go vet: Printf family check]: https://kuzminva.wordpress.com/2017/11/07/go-vet-printf-family-check/

## Patterns

### Test Tables

Use table-driven tests with [subtests] to avoid duplicating code when the core
test logic is repetitive.

  [subtests]: https://blog.golang.org/subtests

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// func TestSplitHostPort(t *testing.T)

host, port, err := net.SplitHostPort("192.0.2.0:8000")
require.NoError(t, err)
assert.Equal(t, "192.0.2.0", host)
assert.Equal(t, "8000", port)

host, port, err = net.SplitHostPort("192.0.2.0:http")
require.NoError(t, err)
assert.Equal(t, "192.0.2.0", host)
assert.Equal(t, "http", port)

host, port, err = net.SplitHostPort(":8000")
require.NoError(t, err)
assert.Equal(t, "", host)
assert.Equal(t, "8000", port)

host, port, err = net.SplitHostPort("1:8")
require.NoError(t, err)
assert.Equal(t, "1", host)
assert.Equal(t, "8", port)
```

</td><td>

```go
// func TestSplitHostPort(t *testing.T)

tests := []struct{
  give     string
  wantHost string
  wantPort string
}{
  {
    give:     "192.0.2.0:8000",
    wantHost: "192.0.2.0",
    wantPort: "8000",
  },
  {
    give:     "192.0.2.0:http",
    wantHost: "192.0.2.0",
    wantPort: "http",
  },
  {
    give:     ":8000",
    wantHost: "",
    wantPort: "8000",
  },
  {
    give:     "1:8",
    wantHost: "1",
    wantPort: "8",
  },
}

for _, tt := range tests {
  t.Run(tt.give, func(t *testing.T) {
    host, port, err := net.SplitHostPort(tt.give)
    require.NoError(t, err)
    assert.Equal(t, tt.wantHost, host)
    assert.Equal(t, tt.wantPort, port)
  })
}
```

</td></tr>
</tbody></table>

Test tables make it easier to add context to error messages, reduce duplicate
logic, and add new test cases.

We follow the convention that the slice of structs is referred to as `tests`
and each test case `tt`. Further, we encourage explicating the input and output
values for each test case with `give` and `want` prefixes.

```go
tests := []struct{
  give     string
  wantHost string
  wantPort string
}{
  // ...
}

for _, tt := range tests {
  // ...
}
```

### Functional Options

Functional options is a pattern in which you declare an opaque `Option` type
that records information in some internal struct. You accept a variadic number
of these options and act upon the full information recorded by the options on
the internal struct.

Use this pattern for optional arguments in constructors and other public APIs
that you foresee needing to expand, especially if you already have three or
more arguments on those functions.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

```go
// package db

func Connect(
  addr string,
  timeout time.Duration,
  caching bool,
) (*Connection, error) {
  // ...
}

// Timeout and caching must always be provided,
// even if the user wants to use the default.

db.Connect(addr, db.DefaultTimeout, db.DefaultCaching)
db.Connect(addr, newTimeout, db.DefaultCaching)
db.Connect(addr, db.DefaultTimeout, false /* caching */)
db.Connect(addr, newTimeout, false /* caching */)
```

</td><td>

```go
type options struct {
  timeout time.Duration
  caching bool
}

// Option overrides behavior of Connect.
type Option interface {
  apply(*options)
}

type optionFunc func(*options)

func (f optionFunc) apply(o *options) {
  f(o)
}

func WithTimeout(t time.Duration) Option {
  return optionFunc(func(o *options) {
    o.timeout = t
  })
}

func WithCaching(cache bool) Option {
  return optionFunc(func(o *options) {
    o.caching = cache
  })
}

// Connect creates a connection.
func Connect(
  addr string,
  opts ...Option,
) (*Connection, error) {
  options := options{
    timeout: defaultTimeout,
    caching: defaultCaching,
  }

  for _, o := range opts {
    o.apply(&options)
  }

  // ...
}

// Options must be provided only if needed.

db.Connect(addr)
db.Connect(addr, db.WithTimeout(newTimeout))
db.Connect(addr, db.WithCaching(false))
db.Connect(
  addr,
  db.WithCaching(false),
  db.WithTimeout(newTimeout),
)
```

</td></tr>
</tbody></table>

See also,

- [Self-referential functions and the design of options]
- [Functional options for friendly APIs]

  [Self-referential functions and the design of options]: https://commandcenter.blogspot.com/2014/01/self-referential-functions-and-design.html
  [Functional options for friendly APIs]: https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

<!-- TODO: replace this with parameter structs and functional options, when to
use one vs other -->