<!--
  This file was generated by stitchmd. DO NOT EDIT.
  To make changes, edit the files in the "src" directory.
-->

<!-- markdownlint-disable MD033 -->

# Uber Go Style Guide

- [Introduction](#introduction)
- [Guidelines](#guidelines)
  - [Pointers to Interfaces](#pointers-to-interfaces)
  - [Verify Interface Compliance](#verify-interface-compliance)
  - [Receivers and Interfaces](#receivers-and-interfaces)
  - [Zero-value Mutexes are Valid](#zero-value-mutexes-are-valid)
  - [Copy Slices and Maps at Boundaries](#copy-slices-and-maps-at-boundaries)
  - [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)
  - [Use `"time"` to handle time](#use-time-to-handle-time)
  - [Errors](#errors)
    - [Error Types](#error-types)
    - [Error Wrapping](#error-wrapping)
    - [Error Naming](#error-naming)
    - [Handle Errors Once](#handle-errors-once)
  - [Handle Type Assertion Failures](#handle-type-assertion-failures)
  - [Don't Panic](#dont-panic)
  - [Use go.uber.org/atomic](#use-gouberorgatomic)
  - [Avoid Mutable Globals](#avoid-mutable-globals)
  - [Avoid Embedding Types in Public Structs](#avoid-embedding-types-in-public-structs)
  - [Avoid Using Built-In Names](#avoid-using-built-in-names)
  - [Avoid `init()`](#avoid-init)
  - [Exit in Main](#exit-in-main)
    - [Exit Once](#exit-once)
  - [Use field tags in marshaled structs](#use-field-tags-in-marshaled-structs)
  - [Don't fire-and-forget goroutines](#dont-fire-and-forget-goroutines)
    - [Wait for goroutines to exit](#wait-for-goroutines-to-exit)
    - [No goroutines in `init()`](#no-goroutines-in-init)
- [Performance](#performance)
  - [Prefer strconv over fmt](#prefer-strconv-over-fmt)
  - [Avoid repeated string-to-byte conversions](#avoid-repeated-string-to-byte-conversions)
  - [Prefer Specifying Container Capacity](#prefer-specifying-container-capacity)
- [Style](#style)
  - [Avoid overly long lines](#avoid-overly-long-lines)
  - [Be Consistent](#be-consistent)
  - [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)
  - [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 Structs](#initializing-structs)
    - [Use Field Names to Initialize Structs](#use-field-names-to-initialize-structs)
    - [Omit Zero Value Fields in Structs](#omit-zero-value-fields-in-structs)
    - [Use `var` for Zero Value Structs](#use-var-for-zero-value-structs)
    - [Initializing Struct References](#initializing-struct-references)
  - [Initializing Maps](#initializing-maps)
  - [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)
- [Linting](#linting)

## Introduction

Styles are the conventions that govern our code. The term style is a bit of a
misnomer, since these conventions cover far more than just source file
formatting—gofmt handles that for us.

The goal of this guide is to manage this complexity by describing in detail the
Dos and Don'ts of writing Go code at Uber. These rules exist to keep the code
base manageable while still allowing engineers to use Go language features
productively.

This guide was originally created by [Prashant Varanasi](https://github.com/prashantv) and [Simon Newton](https://github.com/nomis52) as
a way to bring some colleagues up to speed with using Go. Over the years it has
been amended based on feedback from others.

This documents idiomatic conventions in Go code that we follow at Uber. A lot
of these are general guidelines for Go, while others extend upon external
resources:

1. [Effective Go](https://go.dev/doc/effective_go)
2. [Go Common Mistakes](https://go.dev/wiki/CommonMistakes)
3. [Go Code Review Comments](https://go.dev/wiki/CodeReviewComments)

We aim for the code samples to be accurate for the two most recent minor versions
of Go [releases](https://go.dev/doc/devel/release).

All code should be error-free when run through `golint` and `go vet`. We
recommend setting up your editor to:

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

You can find information in editor support for Go tools here:
https://go.dev/wiki/IDEsAndTextEditorPlugins

## Guidelines

### Pointers to Interfaces

You almost never need a pointer to an interface. You should be passing
interfaces as values—the underlying data can still be a pointer.

An interface is two fields:

1. A pointer to some type-specific information. You can think of this as
   "type."
2. Data pointer. If the data stored is a pointer, it’s stored directly. If
   the data stored is a value, then a pointer to the value is stored.

If you want interface methods to modify the underlying data, you must use a
pointer.

### Verify Interface Compliance

Verify interface compliance at compile time where appropriate. This includes:

- Exported types that are required to implement specific interfaces as part of
  their API contract
- Exported or unexported types that are part of a collection of types
  implementing the same interface
- Other cases where violating an interface would break users

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

```go
type Handler struct {
  // ...
}


func (h *Handler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  ...
}
```

</td><td>

```go
type Handler struct {
  // ...
}

var _ http.Handler = (*Handler)(nil)

func (h *Handler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  // ...
}
```

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

The statement `var _ http.Handler = (*Handler)(nil)` will fail to compile if
`*Handler` ever stops matching the `http.Handler` interface.

The right hand side of the assignment should be the zero value of the asserted
type. This is `nil` for pointer types (like `*Handler`), slices, and maps, and
an empty struct for struct types.

```go
type LogHandler struct {
  h   http.Handler
  log *zap.Logger
}

var _ http.Handler = LogHandler{}

func (h LogHandler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  // ...
}
```

### Receivers and Interfaces

Methods with value receivers can be called on pointers as well as values.
Methods with pointer receivers can only be called on pointers or [addressable values](https://go.dev/ref/spec#Method_values).

For example,

```go
type S struct {
  data string
}

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

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

// We cannot get pointers to values stored in maps, because they are not
// addressable values.
sVals := map[int]S{1: {"A"}}

// We can call Read on values stored in the map because Read
// has a value receiver, which does not require the value to
// be addressable.
sVals[1].Read()

// We cannot call Write on values stored in the map because Write
// has a pointer receiver, and it's not possible to get a pointer
// to a value stored in a map.
//
//  sVals[1].Write("test")

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

// You can call both Read and Write if the map stores pointers,
// because pointers are intrinsically addressable.
sPtrs[1].Read()
sPtrs[1].Write("test")
```

Similarly, an interface can be satisfied by a pointer, even if the method has a
value receiver.

```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

// The following doesn't compile, since s2Val is a value, and there is no value receiver for f.
//   i = s2Val
```

Effective Go has a good write up on [Pointers vs. Values](https://go.dev/doc/effective_go#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 should be a non-pointer field on
it. Do not embed the mutex on the struct, even if the struct is not exported.

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

```go
type SMap struct {
  sync.Mutex

  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><td>

The `Mutex` field, and the `Lock` and `Unlock` methods are unintentionally part
of the exported API of `SMap`.

</td><td>

The mutex and its methods are implementation details of `SMap` hidden from its
callers.

</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 subject to data races.
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 -->

### Use `"time"` to handle time

Time is complicated. Incorrect assumptions often made about time include the
following.

1. A day has 24 hours
2. An hour has 60 minutes
3. A week has 7 days
4. A year has 365 days
5. [And a lot more](https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time)

For example, *1* means that adding 24 hours to a time instant will not always
yield a new calendar day.

Therefore, always use the [`"time"`](https://pkg.go.dev/time) package when dealing with time because it
helps deal with these incorrect assumptions in a safer, more accurate manner.

#### Use `time.Time` for instants of time

Use [`time.Time`](https://pkg.go.dev/time#Time) when dealing with instants of time, and the methods on
`time.Time` when comparing, adding, or subtracting time.

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

```go
func isActive(now, start, stop int) bool {
  return start <= now && now < stop
}
```

</td><td>

```go
func isActive(now, start, stop time.Time) bool {
  return (start.Before(now) || start.Equal(now)) && now.Before(stop)
}
```

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

#### Use `time.Duration` for periods of time

Use [`time.Duration`](https://pkg.go.dev/time#Duration) when dealing with periods of time.

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

```go
func poll(delay int) {
  for {
    // ...
    time.Sleep(time.Duration(delay) * time.Millisecond)
  }
}

poll(10) // was it seconds or milliseconds?
```

</td><td>

```go
func poll(delay time.Duration) {
  for {
    // ...
    time.Sleep(delay)
  }
}

poll(10*time.Second)
```

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

Going back to the example of adding 24 hours to a time instant, the method we
use to add time depends on intent. If we want the same time of the day, but on
the next calendar day, we should use [`Time.AddDate`](https://pkg.go.dev/time#Time.AddDate). However, if we want an
instant of time guaranteed to be 24 hours after the previous time, we should
use [`Time.Add`](https://pkg.go.dev/time#Time.Add).

```go
newDay := t.AddDate(0 /* years */, 0 /* months */, 1 /* days */)
maybeNewDay := t.Add(24 * time.Hour)
```

#### Use `time.Time` and `time.Duration` with external systems

Use `time.Duration` and `time.Time` in interactions with external systems when
possible. For example:

- Command-line flags: [`flag`](https://pkg.go.dev/flag) supports `time.Duration` via
  [`time.ParseDuration`](https://pkg.go.dev/time#ParseDuration)
- JSON: [`encoding/json`](https://pkg.go.dev/encoding/json) supports encoding `time.Time` as an [RFC 3339](https://tools.ietf.org/html/rfc3339)
  string via its [`UnmarshalJSON` method](https://pkg.go.dev/time#Time.UnmarshalJSON)
- SQL: [`database/sql`](https://pkg.go.dev/database/sql) supports converting `DATETIME` or `TIMESTAMP` columns
  into `time.Time` and back if the underlying driver supports it
- YAML: [`gopkg.in/yaml.v2`](https://pkg.go.dev/gopkg.in/yaml.v2) supports `time.Time` as an [RFC 3339](https://tools.ietf.org/html/rfc3339) string, and
  `time.Duration` via [`time.ParseDuration`](https://pkg.go.dev/time#ParseDuration).

When it is not possible to use `time.Duration` in these interactions, use
`int` or `float64` and include the unit in the name of the field.

For example, since `encoding/json` does not support `time.Duration`, the unit
is included in the name of the field.

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

```go
// {"interval": 2}
type Config struct {
  Interval int `json:"interval"`
}
```

</td><td>

```go
// {"intervalMillis": 2000}
type Config struct {
  IntervalMillis int `json:"intervalMillis"`
}
```

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

When it is not possible to use `time.Time` in these interactions, unless an
alternative is agreed upon, use `string` and format timestamps as defined in
[RFC 3339](https://tools.ietf.org/html/rfc3339). This format is used by default by [`Time.UnmarshalText`](https://pkg.go.dev/time#Time.UnmarshalText) and is
available for use in `Time.Format` and `time.Parse` via [`time.RFC3339`](https://pkg.go.dev/time#RFC3339).

Although this tends to not be a problem in practice, keep in mind that the
`"time"` package does not support parsing timestamps with leap seconds
([8728](https://github.com/golang/go/issues/8728)), nor does it account for leap seconds in calculations ([15190](https://github.com/golang/go/issues/15190)). If
you compare two instants of time, the difference will not include the leap
seconds that may have occurred between those two instants.

### Errors

#### Error Types

There are few options for declaring errors.
Consider the following before picking the option best suited for your use case.

- Does the caller need to match the error so that they can handle it?
  If yes, we must support the [`errors.Is`](https://pkg.go.dev/errors#Is) or [`errors.As`](https://pkg.go.dev/errors#As) functions
  by declaring a top-level error variable or a custom type.
- Is the error message a static string,
  or is it a dynamic string that requires contextual information?
  For the former, we can use [`errors.New`](https://pkg.go.dev/errors#New), but for the latter we must
  use [`fmt.Errorf`](https://pkg.go.dev/fmt#Errorf) or a custom error type.
- Are we propagating a new error returned by a downstream function?
  If so, see the [section on error wrapping](#error-wrapping).

| Error matching? | Error Message | Guidance                                                           |
|-----------------|---------------|--------------------------------------------------------------------|
| No              | static        | [`errors.New`](https://pkg.go.dev/errors#New)                      |
| No              | dynamic       | [`fmt.Errorf`](https://pkg.go.dev/fmt#Errorf)                      |
| Yes             | static        | top-level `var` with [`errors.New`](https://pkg.go.dev/errors#New) |
| Yes             | dynamic       | custom `error` type                                                |

For example,
use [`errors.New`](https://pkg.go.dev/errors#New) for an error with a static string.
Export this error as a variable to support matching it with `errors.Is`
if the caller needs to match and handle this error.

<table>
<thead><tr><th>No error matching</th><th>Error matching</th></tr></thead>
<tbody>
<tr><td>

```go
// package foo

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

// package bar

if err := foo.Open(); err != nil {
  // Can't handle the error.
  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 errors.Is(err, foo.ErrCouldNotOpen) {
    // handle the error
  } else {
    panic("unknown error")
  }
}
```

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

For an error with a dynamic string,
use [`fmt.Errorf`](https://pkg.go.dev/fmt#Errorf) if the caller does not need to match it,
and a custom `error` if the caller does need to match it.

<table>
<thead><tr><th>No error matching</th><th>Error matching</th></tr></thead>
<tbody>
<tr><td>

```go
// package foo

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

// package bar

if err := foo.Open("testfile.txt"); err != nil {
  // Can't handle the error.
  panic("unknown error")
}
```

</td><td>

```go
// package foo

type NotFoundError struct {
  File string
}

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

func Open(file string) error {
  return &NotFoundError{File: file}
}


// package bar

if err := foo.Open("testfile.txt"); err != nil {
  var notFound *NotFoundError
  if errors.As(err, &notFound) {
    // handle the error
  } else {
    panic("unknown error")
  }
}
```

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

Note that if you export error variables or types from a package,
they will become part of the public API of the package.

#### Error Wrapping

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

- return the original error as-is
- add context with `fmt.Errorf` and the `%w` verb
- add context with `fmt.Errorf` and the `%v` verb

Return the original error as-is if there is no additional context to add.
This maintains the original error type and message.
This is well suited for cases when the underlying error message
has sufficient information to track down where it came from.

Otherwise, add context to the error message 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".

Use `fmt.Errorf` to add context to your errors,
picking between the `%w` or `%v` verbs
based on whether the caller should be able to
match and extract the underlying cause.

- Use `%w` if the caller should have access to the underlying error.
  This is a good default for most wrapped errors,
  but be aware that callers may begin to rely on this behavior.
  So for cases where the wrapped error is a known `var` or type,
  document and test it as part of your function's contract.
- Use `%v` to obfuscate the underlying error.
  Callers will be unable to match it,
  but you can switch to `%w` in the future if needed.

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: %w", err)
}
```

</td><td>

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

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

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

</td><td>

```plain
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](https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully).

#### Error Naming

For error values stored as global variables,
use the prefix `Err` or `err` depending on whether they're exported.
This guidance supersedes the [Prefix Unexported Globals with _](#prefix-unexported-globals-with-_).

```go
var (
  // The following two errors are exported
  // so that users of this package can match them
  // with errors.Is.

  ErrBrokenLink = errors.New("link is broken")
  ErrCouldNotOpen = errors.New("could not open")

  // This error is not exported because
  // we don't want to make it part of our public API.
  // We may still use it inside the package
  // with errors.Is.

  errNotFound = errors.New("not found")
)
```

For custom error types, use the suffix `Error` instead.

```go
// Similarly, this error is exported
// so that users of this package can match it
// with errors.As.

type NotFoundError struct {
  File string
}

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

// And this error is not exported because
// we don't want to make it part of the public API.
// We can still use it inside the package
// with errors.As.

type resolveError struct {
  Path string
}

func (e *resolveError) Error() string {
  return fmt.Sprintf("resolve %q", e.Path)
}
```

#### Handle Errors Once

When a caller receives an error from a callee,
it can handle it in a variety of different ways
depending on what it knows about the error.

These include, but not are limited to:

- if the callee contract defines specific errors,
  matching the error with `errors.Is` or `errors.As`
  and handling the branches differently
- if the error is recoverable,
  logging the error and degrading gracefully
- if the error represents a domain-specific failure condition,
  returning a well-defined error
- returning the error, either [wrapped](#error-wrapping) or verbatim

Regardless of how the caller handles the error,
it should typically handle each error only once.
The caller should not, for example, log the error and then return it,
because *its* callers may handle the error as well.

For example, consider the following cases:

<table>
<thead><tr><th>Description</th><th>Code</th></tr></thead>
<tbody>
<tr><td>

**Bad**: Log the error and return it

Callers further up the stack will likely take a similar action with the error.
Doing so causing a lot of noise in the application logs for little value.

</td><td>

```go
u, err := getUser(id)
if err != nil {
  // BAD: See description
  log.Printf("Could not get user %q: %v", id, err)
  return err
}
```

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

**Good**: Wrap the error and return it

Callers further up the stack will handle the error.
Use of `%w` ensures they can match the error with `errors.Is` or `errors.As`
if relevant.

</td><td>

```go
u, err := getUser(id)
if err != nil {
  return fmt.Errorf("get user %q: %w", id, err)
}
```

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

**Good**: Log the error and degrade gracefully

If the operation isn't strictly necessary,
we can provide a degraded but unbroken experience
by recovering from it.

</td><td>

```go
if err := emitMetrics(); err != nil {
  // Failure to write metrics should not
  // break the application.
  log.Printf("Could not emit metrics: %v", err)
}

```

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

**Good**: Match the error and degrade gracefully

If the callee defines a specific error in its contract,
and the failure is recoverable,
match on that error case and degrade gracefully.
For all other cases, wrap the error and return it.

Callers further up the stack will handle other errors.

</td><td>

```go
tz, err := getUserTimeZone(id)
if err != nil {
  if errors.Is(err, ErrUserNotFound) {
    // User doesn't exist. Use UTC.
    tz = time.UTC
  } else {
    return fmt.Errorf("get user %q: %w", id, err)
  }
}
```

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

### Handle Type Assertion Failures

The single return value form of a [type assertion](https://go.dev/ref/spec#Type_assertions) will panic on an incorrect
type. Therefore, always use the "comma ok" idiom.

<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](https://en.wikipedia.org/wiki/Cascading_failure). If an error occurs, the function must return an error and
allow the caller to decide how to handle it.

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

```go
func run(args []string) {
  if len(args) == 0 {
    panic("an argument is required")
  }
  // ...
}

func main() {
  run(os.Args[1:])
}
```

</td><td>

```go
func run(args []string) error {
  if len(args) == 0 {
    return errors.New("an argument is required")
  }
  // ...
  return nil
}

func main() {
  if err := run(os.Args[1:]); err != nil {
    fmt.Fprintln(os.Stderr, err)
    os.Exit(1)
  }
}
```

</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 := os.CreateTemp("", "test")
if err != nil {
  panic("failed to set up test")
}
```

</td><td>

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

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

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

### Use go.uber.org/atomic

Atomic operations with the [sync/atomic](https://pkg.go.dev/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](https://pkg.go.dev/go.uber.org/atomic) adds type safety to these operations by hiding the
underlying type. Additionally, it includes a convenient `atomic.Bool` type.

<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>

### Avoid Mutable Globals

Avoid mutating global variables, instead opting for dependency injection.
This applies to function pointers as well as other kinds of values.

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

```go
// sign.go

var _timeNow = time.Now

func sign(msg string) string {
  now := _timeNow()
  return signWithTime(msg, now)
}
```

</td><td>

```go
// sign.go

type signer struct {
  now func() time.Time
}

func newSigner() *signer {
  return &signer{
    now: time.Now,
  }
}

func (s *signer) Sign(msg string) string {
  now := s.now()
  return signWithTime(msg, now)
}
```

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

```go
// sign_test.go

func TestSign(t *testing.T) {
  oldTimeNow := _timeNow
  _timeNow = func() time.Time {
    return someFixedTime
  }
  defer func() { _timeNow = oldTimeNow }()

  assert.Equal(t, want, sign(give))
}
```

</td><td>

```go
// sign_test.go

func TestSigner(t *testing.T) {
  s := newSigner()
  s.now = func() time.Time {
    return someFixedTime
  }

  assert.Equal(t, want, s.Sign(give))
}
```

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

### Avoid Embedding Types in Public Structs

These embedded types leak implementation details, inhibit type evolution, and
obscure documentation.

Assuming you have implemented a variety of list types using a shared
`AbstractList`, avoid embedding the `AbstractList` in your concrete list
implementations.
Instead, hand-write only the methods to your concrete list that will delegate
to the abstract list.

```go
type AbstractList struct {}

// Add adds an entity to the list.
func (l *AbstractList) Add(e Entity) {
  // ...
}

// Remove removes an entity from the list.
func (l *AbstractList) Remove(e Entity) {
  // ...
}
```

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

```go
// ConcreteList is a list of entities.
type ConcreteList struct {
  *AbstractList
}
```

</td><td>

```go
// ConcreteList is a list of entities.
type ConcreteList struct {
  list *AbstractList
}

// Add adds an entity to the list.
func (l *ConcreteList) Add(e Entity) {
  l.list.Add(e)
}

// Remove removes an entity from the list.
func (l *ConcreteList) Remove(e Entity) {
  l.list.Remove(e)
}
```

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

Go allows [type embedding](https://go.dev/doc/effective_go#embedding) as a compromise between inheritance and composition.
The outer type gets implicit copies of the embedded type's methods.
These methods, by default, delegate to the same method of the embedded
instance.

The struct also gains a field by the same name as the type.
So, if the embedded type is public, the field is public.
To maintain backward compatibility, every future version of the outer type must
keep the embedded type.

An embedded type is rarely necessary.
It is a convenience that helps you avoid writing tedious delegate methods.

Even embedding a compatible AbstractList *interface*, instead of the struct,
would offer the developer more flexibility to change in the future, but still
leak the detail that the concrete lists use an abstract implementation.

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

```go
// AbstractList is a generalized implementation
// for various kinds of lists of entities.
type AbstractList interface {
  Add(Entity)
  Remove(Entity)
}

// ConcreteList is a list of entities.
type ConcreteList struct {
  AbstractList
}
```

</td><td>

```go
// ConcreteList is a list of entities.
type ConcreteList struct {
  list AbstractList
}

// Add adds an entity to the list.
func (l *ConcreteList) Add(e Entity) {
  l.list.Add(e)
}

// Remove removes an entity from the list.
func (l *ConcreteList) Remove(e Entity) {
  l.list.Remove(e)
}
```

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

Either with an embedded struct or an embedded interface, the embedded type
places limits on the evolution of the type.

- Adding methods to an embedded interface is a breaking change.
- Removing methods from an embedded struct is a breaking change.
- Removing the embedded type is a breaking change.
- Replacing the embedded type, even with an alternative that satisfies the same
  interface, is a breaking change.

Although writing these delegate methods is tedious, the additional effort hides
an implementation detail, leaves more opportunities for change, and also
eliminates indirection for discovering the full List interface in
documentation.

### Avoid Using Built-In Names

The Go [language specification](https://go.dev/ref/spec) outlines several built-in,
[predeclared identifiers](https://go.dev/ref/spec#Predeclared_identifiers) that should not be used as names within Go programs.

Depending on context, reusing these identifiers as names will either shadow
the original within the current lexical scope (and any nested scopes) or make
affected code confusing. In the best case, the compiler will complain; in the
worst case, such code may introduce latent, hard-to-grep bugs.

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

```go
var error string
// `error` shadows the builtin

// or

func handleErrorMessage(error string) {
    // `error` shadows the builtin
}
```

</td><td>

```go
var errorMessage string
// `error` refers to the builtin

// or

func handleErrorMessage(msg string) {
    // `error` refers to the builtin
}
```

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

```go
type Foo struct {
    // While these fields technically don't
    // constitute shadowing, grepping for
    // `error` or `string` strings is now
    // ambiguous.
    error  error
    string string
}

func (f Foo) Error() error {
    // `error` and `f.error` are
    // visually similar
    return f.error
}

func (f Foo) String() string {
    // `string` and `f.string` are
    // visually similar
    return f.string
}
```

</td><td>

```go
type Foo struct {
    // `error` and `string` strings are
    // now unambiguous.
    err error
    str string
}

func (f Foo) Error() error {
    return f.err
}

func (f Foo) String() string {
    return f.str
}
```

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

Note that the compiler will not generate errors when using predeclared
identifiers, but tools such as `go vet` should correctly point out these and
other cases of shadowing.

### Avoid `init()`

Avoid `init()` where possible. When `init()` is unavoidable or desirable, code
should attempt to:

1. Be completely deterministic, regardless of program environment or invocation.
2. Avoid depending on the ordering or side-effects of other `init()` functions.
   While `init()` ordering is well-known, code can change, and thus
   relationships between `init()` functions can make code brittle and
   error-prone.
3. Avoid accessing or manipulating global or environment state, such as machine
   information, environment variables, working directory, program
   arguments/inputs, etc.
4. Avoid I/O, including both filesystem, network, and system calls.

Code that cannot satisfy these requirements likely belongs as a helper to be
called as part of `main()` (or elsewhere in a program's lifecycle), or be
written as part of `main()` itself. In particular, libraries that are intended
to be used by other programs should take special care to be completely
deterministic and not perform "init magic".

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

```go
type Foo struct {
    // ...
}

var _defaultFoo Foo

func init() {
    _defaultFoo = Foo{
        // ...
    }
}
```

</td><td>

```go
var _defaultFoo = Foo{
    // ...
}

// or, better, for testability:

var _defaultFoo = defaultFoo()

func defaultFoo() Foo {
    return Foo{
        // ...
    }
}
```

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

```go
type Config struct {
    // ...
}

var _config Config

func init() {
    // Bad: based on current directory
    cwd, _ := os.Getwd()

    // Bad: I/O
    raw, _ := os.ReadFile(
        path.Join(cwd, "config", "config.yaml"),
    )

    yaml.Unmarshal(raw, &_config)
}
```

</td><td>

```go
type Config struct {
    // ...
}

func loadConfig() Config {
    cwd, err := os.Getwd()
    // handle err

    raw, err := os.ReadFile(
        path.Join(cwd, "config", "config.yaml"),
    )
    // handle err

    var config Config
    yaml.Unmarshal(raw, &config)

    return config
}
```

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

Considering the above, some situations in which `init()` may be preferable or
necessary might include:

- Complex expressions that cannot be represented as single assignments.
- Pluggable hooks, such as `database/sql` dialects, encoding type registries, etc.
- Optimizations to [Google Cloud Functions](https://cloud.google.com/functions/docs/bestpractices/tips#use_global_variables_to_reuse_objects_in_future_invocations) and other forms of deterministic
  precomputation.

### Exit in Main

Go programs use [`os.Exit`](https://pkg.go.dev/os#Exit) or [`log.Fatal*`](https://pkg.go.dev/log#Fatal) to exit immediately. (Panicking
is not a good way to exit programs, please [don't panic](#dont-panic).)

Call one of `os.Exit` or `log.Fatal*` **only in `main()`**. All other
functions should return errors to signal failure.

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

```go
func main() {
  body := readFile(path)
  fmt.Println(body)
}

func readFile(path string) string {
  f, err := os.Open(path)
  if err != nil {
    log.Fatal(err)
  }

  b, err := io.ReadAll(f)
  if err != nil {
    log.Fatal(err)
  }

  return string(b)
}
```

</td><td>

```go
func main() {
  body, err := readFile(path)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Println(body)
}

func readFile(path string) (string, error) {
  f, err := os.Open(path)
  if err != nil {
    return "", err
  }

  b, err := io.ReadAll(f)
  if err != nil {
    return "", err
  }

  return string(b), nil
}
```

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

Rationale: Programs with multiple functions that exit present a few issues:

- Non-obvious control flow: Any function can exit the program so it becomes
  difficult to reason about the control flow.
- Difficult to test: A function that exits the program will also exit the test
  calling it. This makes the function difficult to test and introduces risk of
  skipping other tests that have not yet been run by `go test`.
- Skipped cleanup: When a function exits the program, it skips function calls
  enqueued with `defer` statements. This adds risk of skipping important
  cleanup tasks.

#### Exit Once

If possible, prefer to call `os.Exit` or `log.Fatal` **at most once** in your
`main()`. If there are multiple error scenarios that halt program execution,
put that logic under a separate function and return errors from it.

This has the effect of shortening your `main()` function and putting all key
business logic into a separate, testable function.

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

```go
package main

func main() {
  args := os.Args[1:]
  if len(args) != 1 {
    log.Fatal("missing file")
  }
  name := args[0]

  f, err := os.Open(name)
  if err != nil {
    log.Fatal(err)
  }
  defer f.Close()

  // If we call log.Fatal after this line,
  // f.Close will not be called.

  b, err := io.ReadAll(f)
  if err != nil {
    log.Fatal(err)
  }

  // ...
}
```

</td><td>

```go
package main

func main() {
  if err := run(); err != nil {
    log.Fatal(err)
  }
}

func run() error {
  args := os.Args[1:]
  if len(args) != 1 {
    return errors.New("missing file")
  }
  name := args[0]

  f, err := os.Open(name)
  if err != nil {
    return err
  }
  defer f.Close()

  b, err := io.ReadAll(f)
  if err != nil {
    return err
  }

  // ...
}
```

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

The example above uses `log.Fatal`, but the guidance also applies to
`os.Exit` or any library code that calls `os.Exit`.

```go
func main() {
  if err := run(); err != nil {
    fmt.Fprintln(os.Stderr, err)
    os.Exit(1)
  }
}
```

You may alter the signature of `run()` to fit your needs.
For example, if your program must exit with specific exit codes for failures,
`run()` may return the exit code instead of an error.
This allows unit tests to verify this behavior directly as well.

```go
func main() {
  os.Exit(run(args))
}

func run() (exitCode int) {
  // ...
}
```

More generally, note that the `run()` function used in these examples
is not intended to be prescriptive.
There's flexibility in the name, signature, and setup of the `run()` function.
Among other things, you may:

- accept unparsed command line arguments (e.g., `run(os.Args[1:])`)
- parse command line arguments in `main()` and pass them onto `run`
- use a custom error type to carry the exit code back to `main()`
- put business logic in a different layer of abstraction from `package main`

This guidance only requires that there's a single place in your `main()`
responsible for actually exiting the process.

### Use field tags in marshaled structs

Any struct field that is marshaled into JSON, YAML,
or other formats that support tag-based field naming
should be annotated with the relevant tag.

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

```go
type Stock struct {
  Price int
  Name  string
}

bytes, err := json.Marshal(Stock{
  Price: 137,
  Name:  "UBER",
})
```

</td><td>

```go
type Stock struct {
  Price int    `json:"price"`
  Name  string `json:"name"`
  // Safe to rename Name to Symbol.
}

bytes, err := json.Marshal(Stock{
  Price: 137,
  Name:  "UBER",
})
```

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

Rationale:
The serialized form of the structure is a contract between different systems.
Changes to the structure of the serialized form--including field names--break
this contract. Specifying field names inside tags makes the contract explicit,
and it guards against accidentally breaking the contract by refactoring or
renaming fields.

### Don't fire-and-forget goroutines

Goroutines are lightweight, but they're not free:
at minimum, they cost memory for their stack and CPU to be scheduled.
While these costs are small for typical uses of goroutines,
they can cause significant performance issues
when spawned in large numbers without controlled lifetimes.
Goroutines with unmanaged lifetimes can also cause other issues
like preventing unused objects from being garbage collected
and holding onto resources that are otherwise no longer used.

Therefore, do not leak goroutines in production code.
Use [go.uber.org/goleak](https://pkg.go.dev/go.uber.org/goleak)
to test for goroutine leaks inside packages that may spawn goroutines.

In general, every goroutine:

- must have a predictable time at which it will stop running; or
- there must be a way to signal to the goroutine that it should stop

In both cases, there must be a way code to block and wait for the goroutine to
finish.

For example:

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

```go
go func() {
  for {
    flush()
    time.Sleep(delay)
  }
}()
```

</td><td>

```go
var (
  stop = make(chan struct{}) // tells the goroutine to stop
  done = make(chan struct{}) // tells us that the goroutine exited
)
go func() {
  defer close(done)

  ticker := time.NewTicker(delay)
  defer ticker.Stop()
  for {
    select {
    case <-ticker.C:
      flush()
    case <-stop:
      return
    }
  }
}()

// Elsewhere...
close(stop)  // signal the goroutine to stop
<-done       // and wait for it to exit
```

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

There's no way to stop this goroutine.
This will run until the application exits.

</td><td>

This goroutine can be stopped with `close(stop)`,
and we can wait for it to exit with `<-done`.

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

#### Wait for goroutines to exit

Given a goroutine spawned by the system,
there must be a way to wait for the goroutine to exit.
There are two popular ways to do this:

- Use a `sync.WaitGroup`.
  Do this if there are multiple goroutines that you want to wait for

  ```go
  var wg sync.WaitGroup
  for i := 0; i < N; i++ {
    wg.Add(1)
    go func() {
      defer wg.Done()
      // ...
    }()
  }

  // To wait for all to finish:
  wg.Wait()
  ```

- Add another `chan struct{}` that the goroutine closes when it's done.
  Do this if there's only one goroutine.

  ```go
  done := make(chan struct{})
  go func() {
    defer close(done)
    // ...
  }()

  // To wait for the goroutine to finish:
  <-done
  ```

#### No goroutines in `init()`

`init()` functions should not spawn goroutines.
See also [Avoid init()](#avoid-init).

If a package has need of a background goroutine,
it must expose an object that is responsible for managing a goroutine's
lifetime.
The object must provide a method (`Close`, `Stop`, `Shutdown`, etc)
that signals the background goroutine to stop, and waits for it to exit.

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

```go
func init() {
  go doWork()
}

func doWork() {
  for {
    // ...
  }
}
```

</td><td>

```go
type Worker struct{ /* ... */ }

func NewWorker(...) *Worker {
  w := &Worker{
    stop: make(chan struct{}),
    done: make(chan struct{}),
    // ...
  }
  go w.doWork()
}

func (w *Worker) doWork() {
  defer close(w.done)
  for {
    // ...
    case <-w.stop:
      return
  }
}

// Shutdown tells the worker to stop
// and waits until it has finished.
func (w *Worker) Shutdown() {
  close(w.stop)
  <-w.done
}
```

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

Spawns a background goroutine unconditionally when the user exports this package.
The user has no control over the goroutine or a means of stopping it.

</td><td>

Spawns the worker only if the user requests it.
Provides a means of shutting down the worker so that the user can free up
resources used by the worker.

Note that you should use `WaitGroup`s if the worker manages multiple
goroutines.
See [Wait for goroutines to exit](#wait-for-goroutines-to-exit).

</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>

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

</td><td>

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

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

### Avoid repeated string-to-byte conversions

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)
}
```

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

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

</td><td>

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

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

### Prefer Specifying Container Capacity

Specify container capacity where possible in order to allocate memory for the
container up front. This minimizes subsequent allocations (by copying and
resizing of the container) as elements are added.

#### Specifying Map Capacity Hints

Where possible, provide capacity hints when initializing
maps with `make()`.

```go
make(map[T1]T2, hint)
```

Providing a capacity hint to `make()` tries to right-size the
map at initialization time, which reduces the need for growing
the map and allocations as elements are added to the map.

Note that, unlike slices, map capacity hints do not guarantee complete,
preemptive allocation, but are used to approximate the number of hashmap buckets
required. Consequently, allocations may still occur when adding elements to the
map, even up to the specified capacity.

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

```go
m := make(map[string]os.FileInfo)

files, _ := os.ReadDir("./files")
for _, f := range files {
    m[f.Name()] = f
}
```

</td><td>

```go

files, _ := os.ReadDir("./files")

m := make(map[string]os.DirEntry, len(files))
for _, f := range files {
    m[f.Name()] = f
}
```

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

`m` is created without a size hint; there may be more
allocations at assignment time.

</td><td>

`m` is created with a size hint; there may be fewer
allocations at assignment time.

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

#### Specifying Slice Capacity

Where possible, provide capacity hints when initializing slices with `make()`,
particularly when appending.

```go
make([]T, length, capacity)
```

Unlike maps, slice capacity is not a hint: the compiler will allocate enough
memory for the capacity of the slice as provided to `make()`, which means that
subsequent `append()` operations will incur zero allocations (until the length
of the slice matches the capacity, after which any appends will require a resize
to hold additional elements).

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

```go
for n := 0; n < b.N; n++ {
  data := make([]int, 0)
  for k := 0; k < size; k++{
    data = append(data, k)
  }
}
```

</td><td>

```go
for n := 0; n < b.N; n++ {
  data := make([]int, 0, size)
  for k := 0; k < size; k++{
    data = append(data, k)
  }
}
```

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

```plain
BenchmarkBad-4    100000000    2.48s
```

</td><td>

```plain
BenchmarkGood-4   100000000    0.21s
```

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

## Style

### Avoid overly long lines

Avoid lines of code that require readers to scroll horizontally
or turn their heads too much.

We recommend a soft line length limit of **99 characters**.
Authors should aim to wrap lines before hitting this limit,
but it is not a hard limit.
Code is allowed to exceed this limit.

### Be Consistent

Some of the guidelines outlined in this document can be evaluated objectively;
others are situational, contextual, or subjective.

Above all else, **be consistent**.

Consistent code is easier to maintain, is easier to rationalize, requires less
cognitive overhead, and is easier to migrate or update as new conventions emerge
or classes of bugs are fixed.

Conversely, having multiple disparate or conflicting styles within a single
codebase causes maintenance overhead, uncertainty, and cognitive dissonance,
all of which can directly contribute to lower velocity, painful code reviews,
and bugs.

When applying these guidelines to a codebase, it is recommended that changes
are made at a package (or larger) level: application at a sub-package level
violates the above concern by introducing multiple styles into the same code.

### 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
  EnvVar = "MY_ENV"
)
```

</td><td>

```go
type Operation int

const (
  Add Operation = iota + 1
  Subtract
  Multiply
)

const EnvVar = "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 {
  red := color.New(0xff0000)
  green := color.New(0x00ff00)
  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>

Exception: Variable declarations, particularly inside functions, should be
grouped together if declared adjacent to other variables. Do this for variables
declared together even if they are unrelated.

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

```go
func (c *client) request() {
  caller := c.name
  format := "json"
  timeout := 5*time.Second
  var err error

  // ...
}
```

</td><td>

```go
func (c *client) request() {
  var (
    caller  = c.name
    format  = "json"
    timeout = 5*time.Second
    err error
  )

  // ...
}
```

</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](https://go.dev/blog/package-names) and [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](https://go.dev/doc/effective_go#mixed-caps). An exception is made for test functions, which may contain underscores
for the purpose of grouping related test cases, e.g.,
`TestMyFunction_WhatIsBeingTested`.

### 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.

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>

**Exception**: Unexported error values may use the prefix `err` without the underscore.
See [Error Naming](#error-naming).

### Embedding in Structs

Embedded types 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>

Embedding should provide tangible benefit, like adding or augmenting
functionality in a semantically-appropriate way. It should do this with zero
adverse user-facing effects (see also: [Avoid Embedding Types in Public Structs](#avoid-embedding-types-in-public-structs)).

Exception: Mutexes should not be embedded, even on unexported types. See also: [Zero-value Mutexes are Valid](#zero-value-mutexes-are-valid).

Embedding **should not**:

- Be purely cosmetic or convenience-oriented.
- Make outer types more difficult to construct or use.
- Affect outer types' zero values. If the outer type has a useful zero value, it
  should still have a useful zero value after embedding the inner type.
- Expose unrelated functions or fields from the outer type as a side-effect of
  embedding the inner type.
- Expose unexported types.
- Affect outer types' copy semantics.
- Change the outer type's API or type semantics.
- Embed a non-canonical form of the inner type.
- Expose implementation details of the outer type.
- Allow users to observe or control type internals.
- Change the general behavior of inner functions through wrapping in a way that
  would reasonably surprise users.

Simply put, embed consciously and intentionally. A good litmus test is, "would
all of these exported inner methods/fields be added directly to the outer type";
if the answer is "some" or "no", don't embed the inner type - use a field
instead.

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

```go
type A struct {
    // Bad: A.Lock() and A.Unlock() are
    //      now available, provide no
    //      functional benefit, and allow
    //      users to control details about
    //      the internals of A.
    sync.Mutex
}
```

</td><td>

```go
type countingWriteCloser struct {
    // Good: Write() is provided at this
    //       outer layer for a specific
    //       purpose, and delegates work
    //       to the inner type's Write().
    io.WriteCloser

    count int
}

func (w *countingWriteCloser) Write(bs []byte) (int, error) {
    w.count += len(bs)
    return w.WriteCloser.Write(bs)
}
```

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

```go
type Book struct {
    // Bad: pointer changes zero value usefulness
    io.ReadWriter

    // other fields
}

// later

var b Book
b.Read(...)  // panic: nil pointer
b.String()   // panic: nil pointer
b.Write(...) // panic: nil pointer
```

</td><td>

```go
type Book struct {
    // Good: has useful zero value
    bytes.Buffer

    // other fields
}

// later

var b Book
b.Read(...)  // ok
b.String()   // ok
b.Write(...) // ok
```

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

```go
type Client struct {
    sync.Mutex
    sync.WaitGroup
    bytes.Buffer
    url.URL
}
```

</td><td>

```go
type Client struct {
    mtx sync.Mutex
    wg  sync.WaitGroup
    buf bytes.Buffer
    url url.URL
}
```

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

### 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 used. [Declaring Empty Slices](https://go.dev/wiki/CodeReviewComments#declaring-empty-slices), for example.

<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>

Remember that, while it is a valid slice, a nil slice is not equivalent to an
allocated slice of length 0 - one is nil and the other is not - and the two may
be treated differently in different situations (such as serialization).

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

</td><td>

```go
if err := os.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 := os.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 := os.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 Status = 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://go.dev/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 Structs

#### Use Field Names to Initialize Structs

You should almost always specify field names when initializing structs. This is
now enforced by [`go vet`](https://pkg.go.dev/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"},
}
```

#### Omit Zero Value Fields in Structs

When initializing structs with field names, omit fields that have zero values
unless they provide meaningful context. Otherwise, let Go set these to zero
values automatically.

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

```go
user := User{
  FirstName: "John",
  LastName: "Doe",
  MiddleName: "",
  Admin: false,
}
```

</td><td>

```go
user := User{
  FirstName: "John",
  LastName: "Doe",
}
```

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

This helps reduce noise for readers by omitting values that are default in
that context. Only meaningful values are specified.

Include zero values where field names provide meaningful context. For example,
test cases in [Test Tables](#test-tables) can benefit from names of fields
even when they are zero-valued.

```go
tests := []struct{
  give string
  want int
}{
  {give: "0", want: 0},
  // ...
}
```

#### Use `var` for Zero Value Structs

When all the fields of a struct are omitted in a declaration, use the `var`
form to declare the struct.

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

```go
user := User{}
```

</td><td>

```go
var user User
```

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

This differentiates zero valued structs from those with non-zero fields
similar to the distinction created for [map initialization](#initializing-maps), and matches how
we prefer to [declare empty slices](https://go.dev/wiki/CodeReviewComments#declaring-empty-slices).

#### 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>

### Initializing Maps

Prefer `make(..)` for empty maps, and maps populated
programmatically. This makes map initialization visually
distinct from declaration, and it makes it easy to add size
hints later if available.

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

```go
var (
  // m1 is safe to read and write;
  // m2 will panic on writes.
  m1 = map[T1]T2{}
  m2 map[T1]T2
)
```

</td><td>

```go
var (
  // m1 is safe to read and write;
  // m2 will panic on writes.
  m1 = make(map[T1]T2)
  m2 map[T1]T2
)
```

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

Declaration and initialization are visually similar.

</td><td>

Declaration and initialization are visually distinct.

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

Where possible, provide capacity hints when initializing
maps with `make()`. See
[Specifying Map Capacity Hints](#specifying-map-capacity-hints)
for more information.

On the other hand, if the map holds a fixed list of elements,
use map literals to initialize the map.

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

```go
m := make(map[T1]T2, 3)
m[k1] = v1
m[k2] = v2
m[k3] = v3
```

</td><td>

```go
m := map[T1]T2{
  k1: v1,
  k2: v2,
  k3: v3,
}
```

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

The basic rule of thumb is to use map literals when adding a fixed set of
elements at initialization time, otherwise use `make` (and specify a size hint
if available).

### 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 predefined `Printf`-style function
names if possible. `go vet` will check these by default. See [Printf family](https://pkg.go.dev/cmd/vet#hdr-Printf_family)
for more information.

If using the predefined 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](https://kuzminva.wordpress.com/2017/11/07/go-vet-printf-family-check/).

## Patterns

### Test Tables

Table-driven tests with [subtests](https://go.dev/blog/subtests) can be a helpful pattern for writing tests
to avoid duplicating code when the core test logic is repetitive.

If a system under test needs to be tested against *multiple conditions* where
certain parts of the the inputs and outputs change, a table-driven test should
be used to reduce redundancy and improve readability.

<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 {
  // ...
}
```

#### Avoid Unnecessary Complexity in Table Tests

Table tests can be difficult to read and maintain if the subtests contain conditional
assertions or other branching logic. Table tests should **NOT** be used whenever
there needs to be complex or conditional logic inside subtests (i.e. complex logic inside the `for` loop).

Large, complex table tests harm readability and maintainability because test readers may
have difficulty debugging test failures that occur.

Table tests like this should be split into either multiple test tables or multiple
individual `Test...` functions.

Some ideals to aim for are:

* Focus on the narrowest unit of behavior
* Minimize "test depth", and avoid conditional assertions (see below)
* Ensure that all table fields are used in all tests
* Ensure that all test logic runs for all table cases

In this context, "test depth" means "within a given test, the number of
successive assertions that require previous assertions to hold" (similar
to cyclomatic complexity).
Having "shallower" tests means that there are fewer relationships between
assertions and, more importantly, that those assertions are less likely
to be conditional by default.

Concretely, table tests can become confusing and difficult to read if they use multiple branching
pathways (e.g. `shouldError`, `expectCall`, etc.), use many `if` statements for
specific mock expectations (e.g. `shouldCallFoo`), or place functions inside the
table (e.g. `setupMocks func(*FooMock)`).

However, when testing behavior that only
changes based on changed input, it may be preferable to group similar cases
together in a table test to better illustrate how behavior changes across all inputs,
rather than splitting otherwise comparable units into separate tests
and making them harder to compare and contrast.

If the test body is short and straightforward,
it's acceptable to have a single branching pathway for success versus failure cases
with a table field like `shouldErr` to specify error expectations.

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

```go
func TestComplicatedTable(t *testing.T) {
  tests := []struct {
    give          string
    want          string
    wantErr       error
    shouldCallX   bool
    shouldCallY   bool
    giveXResponse string
    giveXErr      error
    giveYResponse string
    giveYErr      error
  }{
    // ...
  }

  for _, tt := range tests {
    t.Run(tt.give, func(t *testing.T) {
      // setup mocks
      ctrl := gomock.NewController(t)
      xMock := xmock.NewMockX(ctrl)
      if tt.shouldCallX {
        xMock.EXPECT().Call().Return(
          tt.giveXResponse, tt.giveXErr,
        )
      }
      yMock := ymock.NewMockY(ctrl)
      if tt.shouldCallY {
        yMock.EXPECT().Call().Return(
          tt.giveYResponse, tt.giveYErr,
        )
      }

      got, err := DoComplexThing(tt.give, xMock, yMock)

      // verify results
      if tt.wantErr != nil {
        require.EqualError(t, err, tt.wantErr)
        return
      }
      require.NoError(t, err)
      assert.Equal(t, want, got)
    })
  }
}
```

</td><td>

```go
func TestShouldCallX(t *testing.T) {
  // setup mocks
  ctrl := gomock.NewController(t)
  xMock := xmock.NewMockX(ctrl)
  xMock.EXPECT().Call().Return("XResponse", nil)

  yMock := ymock.NewMockY(ctrl)

  got, err := DoComplexThing("inputX", xMock, yMock)

  require.NoError(t, err)
  assert.Equal(t, "want", got)
}

func TestShouldCallYAndFail(t *testing.T) {
  // setup mocks
  ctrl := gomock.NewController(t)
  xMock := xmock.NewMockX(ctrl)

  yMock := ymock.NewMockY(ctrl)
  yMock.EXPECT().Call().Return("YResponse", nil)

  _, err := DoComplexThing("inputY", xMock, yMock)
  assert.EqualError(t, err, "Y failed")
}
```
</td></tr>
</tbody></table>

This complexity makes it more difficult to change, understand, and prove the
correctness of the test.

While there are no strict guidelines, readability and maintainability should
always be top-of-mind when deciding between Table Tests versus separate tests
for multiple inputs/outputs to a system.

#### Parallel Tests

Parallel tests, like some specialized loops (for example, those that spawn
goroutines or capture references as part of the loop body),
must take care to explicitly assign loop variables within the loop's scope to
ensure that they hold the expected values.

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

for _, tt := range tests {
  tt := tt // for t.Parallel
  t.Run(tt.give, func(t *testing.T) {
    t.Parallel()
    // ...
  })
}
```

In the example above, we must declare a `tt` variable scoped to the loop
iteration because of the use of `t.Parallel()` below.
If we do not do that, most or all tests will receive an unexpected value for
`tt`, or a value that changes as they're running.

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

### 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 Open(
  addr string,
  cache bool,
  logger *zap.Logger
) (*Connection, error) {
  // ...
}
```

</td><td>

```go
// package db

type Option interface {
  // ...
}

func WithCache(c bool) Option {
  // ...
}

func WithLogger(log *zap.Logger) Option {
  // ...
}

// Open creates a connection.
func Open(
  addr string,
  opts ...Option,
) (*Connection, error) {
  // ...
}
```

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

The cache and logger parameters must always be provided, even if the user
wants to use the default.

```go
db.Open(addr, db.DefaultCache, zap.NewNop())
db.Open(addr, db.DefaultCache, log)
db.Open(addr, false /* cache */, zap.NewNop())
db.Open(addr, false /* cache */, log)
```

</td><td>

Options are provided only if needed.

```go
db.Open(addr)
db.Open(addr, db.WithLogger(log))
db.Open(addr, db.WithCache(false))
db.Open(
  addr,
  db.WithCache(false),
  db.WithLogger(log),
)
```

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

Our suggested way of implementing this pattern is with an `Option` interface
that holds an unexported method, recording options on an unexported `options`
struct.

```go
type options struct {
  cache  bool
  logger *zap.Logger
}

type Option interface {
  apply(*options)
}

type cacheOption bool

func (c cacheOption) apply(opts *options) {
  opts.cache = bool(c)
}

func WithCache(c bool) Option {
  return cacheOption(c)
}

type loggerOption struct {
  Log *zap.Logger
}

func (l loggerOption) apply(opts *options) {
  opts.logger = l.Log
}

func WithLogger(log *zap.Logger) Option {
  return loggerOption{Log: log}
}

// Open creates a connection.
func Open(
  addr string,
  opts ...Option,
) (*Connection, error) {
  options := options{
    cache:  defaultCache,
    logger: zap.NewNop(),
  }

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

  // ...
}
```

Note that there's a method of implementing this pattern with closures but we
believe that the pattern above provides more flexibility for authors and is
easier to debug and test for users. In particular, it allows options to be
compared against each other in tests and mocks, versus closures where this is
impossible. Further, it lets options implement other interfaces, including
`fmt.Stringer` which allows for user-readable string representations of the
options.

See also,

- [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 -->

## Linting

More importantly than any "blessed" set of linters, lint consistently across a
codebase.

We recommend using the following linters at a minimum, because we feel that they
help to catch the most common issues and also establish a high bar for code
quality without being unnecessarily prescriptive:

- [errcheck](https://github.com/kisielk/errcheck) to ensure that errors are handled
- [goimports](https://pkg.go.dev/golang.org/x/tools/cmd/goimports) to format code and manage imports
- [golint](https://github.com/golang/lint) to point out common style mistakes
- [govet](https://pkg.go.dev/cmd/vet) to analyze code for common mistakes
- [staticcheck](https://staticcheck.dev) to do various static analysis checks

### Lint Runners

We recommend [golangci-lint](https://github.com/golangci/golangci-lint) as the go-to lint runner for Go code, largely due
to its performance in larger codebases and ability to configure and use many
canonical linters at once. This repo has an example [.golangci.yml](https://github.com/uber-go/guide/blob/master/.golangci.yml) config file
with recommended linters and settings.

golangci-lint has [various linters](https://golangci-lint.run/usage/linters/) available for use. The above linters are
recommended as a base set, and we encourage teams to add any additional linters
that make sense for their projects.