Uber Go Style

AFS-Golang

Go语言入门/开源/标准

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.

~~~

BAD CODE GOES HERE

GOOD CODE GOES HERE

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"}}
// You can only call Read using a value
sVals[1].Read()
// This will not compile:
//  sVals[1].Write("test")
sPtrs := map[int]*S{1: {"A"}}
// You can call both Read and Write using a pointer
sPtrs[1].Read()
sPtrs[1].Write("test")

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

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

var mu sync.Mutex
mu.Lock()

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]
}

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]
}

func (d *Driver) SetTrips(trips []Trip) {
  d.trips = trips
}
trips := ...
d1.SetTrips(trips)
// Did you mean to modify d1.trips?
trips[0] = ...

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] = ...

Returning Slices and Maps

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

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()

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()

Defer to Clean Up

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

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

p.Lock()
defer p.Unlock()
if p.count < 10 {
  return p.count
}
p.count++
return p.count
// more readable

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.

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

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

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.

type Operation int
const (
  Add Operation = iota
  Subtract
  Multiply
)
// Add=0, Subtract=1, Multiply=2

type Operation int
const (
  Add Operation = iota + 1
  Subtract
  Multiply
)
// Add=1, Subtract=2, Multiply=3

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

type LogOutput int
const (
  LogToStdout LogOutput = iota
  LogToFile
  LogToRemote
)
// LogToStdout=0, LogToFile=1, LogToRemote=2

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

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"] 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] when dealing with instants of time, and the methods on
time.Time when comparing, adding, or subtracting time.

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

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

Use time.Duration for periods of time

Use [time.Duration] when dealing with periods of time.

func poll(delay int) {
  for {
    // ...
    time.Sleep(time.Duration(delay) * time.Millisecond)
  }
}
poll(10) // was it seconds or milliseconds?

func poll(delay time.Duration) {
  for {
    // ...
    time.Sleep(delay)
  }
}
poll(10*time.Second)

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]. However, if we want an
instant of time guaranteed to be 24 hours after the previous time, we should
use [Time.Add].

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] supports time.Duration via
  [time.ParseDuration]
• JSON: [encoding/json] supports encoding time.Time as an RFC 3339
  string via its [UnmarshalJSON method]
• SQL: [database/sql] supports converting DATETIME or TIMESTAMP columns
  into time.Time and back if the underlying driver supports it
• YAML: [gopkg.in/yaml.v2] supports time.Time as an RFC 3339 string, and
  time.Duration via [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.

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

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

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. This format is used by default by [Time.UnmarshalText] and is
available for use in Time.Format and time.Parse via [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), nor does it account for leap seconds in calculations (15190). If
you compare two instants of time, the difference will not include the leap
seconds that may have occurred between those two instants.

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.
• Otherwise, [fmt.Errorf] is okay.

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

// 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")
    }
  }
}

// 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")
  }
}

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.

func open(file string) error {
  return fmt.Errorf("file %q not found", file)
}
func use() {
  if err := open("testfile.txt"); err != nil {
    if strings.Contains(err.Error(), "not found") {
      // handle
    } else {
      panic("unknown error")
    }
  }
}

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("testfile.txt"); err != nil {
    if _, ok := err.(errNotFound); ok {
      // handle
    } else {
      panic("unknown error")
    }
  }
}

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.

// 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")
  }
}

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:

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

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

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

x: y: new store: the error

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.

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.

t := i.(string)

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

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.

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])
}

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)
  }
}

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.

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.

// func TestFoo(t *testing.T)
f, err := ioutil.TempFile("", "test")
if err != nil {
  panic("failed to set up test")
}

// func TestFoo(t *testing.T)
f, err := ioutil.TempFile("", "test")
if err != nil {
  t.Fatal("failed to set up test")
}

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.

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!
}

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()
}

Avoid Mutable Globals

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

// sign.go
var _timeNow = time.Now
func sign(msg string) string {
  now := _timeNow()
  return signWithTime(msg, now)
}

// 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)
}

// 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))
}

// sign_test.go
func TestSigner(t *testing.T) {
  s := newSigner()
  s.now = func() time.Time {
    return someFixedTime
  }
  assert.Equal(t, want, s.Sign(give))
}

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.

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) {
  // ...
}

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

// ConcreteList is a list of entities.
type ConcreteList struct {
  list *AbstractList
}
// Add adds an entity to the list.
func (l *ConcreteList) Add(e Entity) {
  return l.list.Add(e)
}
// Remove removes an entity from the list.
func (l *ConcreteList) Remove(e Entity) {
  return l.list.Remove(e)
}

Go allows type 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.

// 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
}

// ConcreteList is a list of entities.
type ConcreteList struct {
  list *AbstractList
}
// Add adds an entity to the list.
func (l *ConcreteList) Add(e Entity) {
  return l.list.Add(e)
}
// Remove removes an entity from the list.
func (l *ConcreteList) Remove(e Entity) {
  return l.list.Remove(e)
}

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.

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.

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

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

BenchmarkFmtSprint-4    143 ns/op    2 allocs/op

BenchmarkStrconv-4    64.2 ns/op    1 allocs/op

Avoid string-to-byte conversion

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

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

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

BenchmarkBad-4   50000000   22.2 ns/op

BenchmarkGood-4  500000000   3.25 ns/op

Prefer Specifying Map Capacity Hints

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

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 the capacity hint is not guaranteed for maps, so adding
elements may still allocate even if a capacity hint is provided.

m := make(map[string]os.FileInfo)
files, _ := ioutil.ReadDir("./files")
for _, f := range files {
    m[f.Name()] = f
}

files, _ := ioutil.ReadDir("./files")
m := make(map[string]os.FileInfo, len(files))
for _, f := range files {
    m[f.Name()] = f
}

Style

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.

import "a"
import "b"

import (
  "a"
  "b"
)

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

const a = 1
const b = 2
var a = 1
var b = 2
type Area float64
type Volume float64

const (
  a = 1
  b = 2
)
var (
  a = 1
  b = 2
)
type (
  Area float64
  Volume float64
)

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

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

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

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

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

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

Import Group Ordering

There should be two import groups:

• Standard library
• Everything else

This is the grouping applied by goimports by default.

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

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

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.

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.

Import Aliasing

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

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.

import (
  "fmt"
  "os"
  nettrace "golang.net/x/trace"
)

import (
  "fmt"
  "os"
  "runtime/trace"
  nettrace "golang.net/x/trace"
)

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.

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{}
}

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 {...}

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.

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)
  }
}

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()
}

Unnecessary Else

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

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

a := 10
if b {
  a = 100
}

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.

var _s string = F()
func F() string { return "A" }

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" }

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

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 vars and consts 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.

// 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.
}

// foo.go
const (
  _defaultPort = 8080
  _defaultUser = "user"
)

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.

type Client struct {
  version int
  http.Client
}

type Client struct {
  http.Client
  version int
}

Use Field Names to Initialize Structs

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

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

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

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

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.

var s = "foo"

s := "foo"

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

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

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

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.

  Bad	Good
  if x == "" { return []int{} }	if x == "" { return nil }

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

  Bad	Good
  func isEmpty(s []string) bool { return s == nil }	func isEmpty(s []string) bool { return len(s) == 0 }

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

  Bad	Good
  nums := []int{} // or, nums := make([]int) if add1 { nums = append(nums, 1) } if add2 { nums = append(nums, 2) }	var nums []int if add1 { nums = append(nums, 1) } if add2 { nums = append(nums, 2) }

Reduce Scope of Variables

Where possible, reduce scope of variables. Do not reduce the scope if it
conflicts with Reduce Nesting.

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

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

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

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
}

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

Avoid Naked Parameters

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

// func printInfo(name string, isLocal, done bool)
printInfo("foo", true, true)

// func printInfo(name string, isLocal, done bool)
printInfo("foo", true /* isLocal */, true /* done */)

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.

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,
which can span multiple lines and include quotes. Use these to avoid
hand-escaped strings which are much harder to read.

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

wantError := `unknown error:"test"`

Initializing Struct References

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

sval := T{Name: "foo"}
// inconsistent
sptr := new(T)
sptr.Name = "bar"

sval := T{Name: "foo"}
sptr := &T{Name: "bar"}

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.

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

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

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

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

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

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.

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

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

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
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.

$ go vet -printfuncs=wrapf,statusf

See also 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.

// 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)

// 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)
  })
}

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.

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.

// 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) {
  // ...
}

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)

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

// package db
func Open(
  addr string,
  cache bool,
  logger *zap.Logger
) (*Connection, error) {
  // ...
}

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

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)
  }
  // ...
}