uber-go-style-guide-kr

Translated in Korean Currently WIP, but translation will done by 20th of Oct, 2019

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Uber의 Go언어 스타일 가이드

목차

• uber-go-style-guide-kr
• Uber의 Go언어 스타일 가이드
  • 목차
  • Introduction
  • Guidelines
    • Pointers to Interfaces
    • Receivers and Interfaces
    • Zero-value Mutexes are Valid
    • Copy Slices and Maps at Boundaries
      • Receiving Slices and Maps
      • Returning Slices and Maps
    • Defer to Clean Up
    • Channel Size is One or None
    • Start Enums at One
    • Error Types
    • Error Wrapping
    • Handle Type Assertion Failures
    • Don't Panic
    • Use go.uber.org/atomic
  • Performance
    • Prefer strconv over fmt
    • Avoid string-to-byte conversion
  • Style
    • Group Similar Declarations
    • Import Group Ordering
    • Package Names
    • Function Names
    • Import Aliasing
    • Function Grouping and Ordering
    • Reduce Nesting
    • Unnecessary Else
    • Top-level Variable Declarations
    • Prefix Unexported Globals with _
    • Embedding in Structs
    • Use Field Names to initialize Structs
    • Local Variable Declarations
    • nil is a valid slice
    • Reduce Scope of Variables
    • Avoid Naked Parameters
    • Use Raw String Literals to Avoid Escaping
    • Initializing Struct References
    • Format Strings outside Printf
    • Naming Printf-style Functions
  • Patterns
    • Test Tables
    • Functional Options

Introduction

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

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

이 가이드는 원래 Prashant Varanasi Simon Newtonas a way to bring some colleagues up to speed with using Go. Over the years it has been amended based on feedback from others.

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

1. Effective Go
2. The Go common mistakes guide

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

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

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

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.

Receivers and Interfaces

Methods with value receivers can be called on pointers as well as values.

For example,

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")

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

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.

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.

Bad	Good
mu := new(sync.Mutex) mu.Lock()	var mu sync.Mutex mu.Lock()

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

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

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] }
Embed for private types or types that need to implement the Mutex interface.	For exported types, use a private field.

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.

Bad	Good
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.

Bad	Good
type Stats struct { mu sync.Mutex counters map[string]int } // Snapshot returns the current stats. func (s *Stats) Snapshot() map[string]int { s.mu.Lock() defer s.mu.Unlock() return s.counters } // snapshot is no longer protected by the mutex, so any // access to the snapshot is racy. snapshot := stats.Snapshot()	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.

Bad	Good
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.

Bad	Good
// 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.

Bad	Good
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

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.

Bad	Good
// 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.

Bad	Good
func open(file string) error { return fmt.Errorf("file %q not found", file) } func use() { if err := open(); err != nil { if strings.Contains(err.Error(), "not found") { // handle } else { panic("unknown error") } } }	type errNotFound struct { file string } func (e errNotFound) Error() string { return fmt.Sprintf("file %q not found", e.file) } func open(file string) error { return errNotFound{file: file} } func use() { if err := open(); err != nil { if _, ok := err.(errNotFound); ok { // handle } else { panic("unknown error") } } }

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:

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
// 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.

Bad	Good
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() }

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.

Bad	Good
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.

Bad	Good
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

Style

Group Similar Declarations

Go supports grouping similar declarations.

Bad	Good
import "a" import "b"	import ( "a" "b" )

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

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
// 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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
var s = "foo"	s := "foo"

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

Bad	Good
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.

Bad	Good
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.

Bad	Good
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.

Bad	Good
// 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.

Bad	Good
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.

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

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.

Bad	Good
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 pre-defined Printf-style function names if possible. go vet will check these by default. See Printf family for more information.

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

$ 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.

Bad

Good

// 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.

Bad

Good

// package db func Connect( addr string, timeout time.Duration, caching bool, ) (*Connection, error) { // ... } // Timeout and caching must always be provided, // even if the user wants to use the default. db.Connect(addr, db.DefaultTimeout, db.DefaultCaching) db.Connect(addr, newTimeout, db.DefaultCaching) db.Connect(addr, db.DefaultTimeout, false /* caching */) db.Connect(addr, newTimeout, false /* caching */)

type options struct { timeout time.Duration caching bool } // Option overrides behavior of Connect. type Option interface { apply(*options) } type optionFunc func(*options) func (f optionFunc) apply(o *options) { f(o) } func WithTimeout(t time.Duration) Option { return optionFunc(func(o *options) { o.timeout = t }) } func WithCaching(cache bool) Option { return optionFunc(func(o *options) { o.caching = cache }) } // Connect creates a connection. func Connect( addr string, opts ...Option, ) (*Connection, error) { options := options{ timeout: defaultTimeout, caching: defaultCaching, } for _, o := range opts { o.apply(&options) } // ... } // Options must be provided only if needed. db.Connect(addr) db.Connect(addr, db.WithTimeout(newTimeout)) db.Connect(addr, db.WithCaching(false)) db.Connect( addr, db.WithCaching(false), db.WithTimeout(newTimeout), )

See also,

• Self-referential functions and the design of options

• Functional options for friendly APIs