[转] Go Cheat Sheet

Go Cheat Sheet

Credits

Most example code taken from A Tour of Go, which is an excellent introduction to Go.
If you’re new to Go, do that tour. Seriously.

Go in a Nutshell

  • Imperative language
  • Statically typed
  • Syntax tokens similar to C (but less parentheses and no semicolons) and the structure to Oberon-2
  • Compiles to native code (no JVM)
  • No classes, but structs with methods
  • Interfaces
  • No implementation inheritance. There’s type embedding, though.
  • Functions are first class citizens
  • Functions can return multiple values
  • Has closures
  • Pointers, but not pointer arithmetic
  • Built-in concurrency primitives: Goroutines and Channels

Basic Syntax

Hello World

File hello.go:

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package main

import "fmt"

func main() {
fmt.Println("Hello Go")
}

$ go run hello.go

Operators

Arithmetic

OperatorDescription
+addition
-subtraction
*multiplication
/quotient
%remainder
&bitwise and
?bitwise or
^bitwise xor
&^bit clear (and not)
<<left shift
>>right shift

? -> |

Comparison

OperatorDescription
==equal
!=not equal
<less than
<=less than or equal
>greater than
>=greater than or equal

Logical

OperatorDescription
&&logical and
??logical or
!logical not

?? -> ||

Other

OperatorDescription
&address of / create pointer
*dereference pointer
<-send / receive operator (see ‘Channels’ below)

Declarations

Type goes after identifier!

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var foo int // declaration without initialization
var foo int = 42 // declaration with initialization
var foo, bar int = 42, 1302 // declare and init multiple vars at once
var foo = 42 // type omitted, will be inferred
foo := 42 // shorthand, only in func bodies, omit var keyword, type is always implicit
const constant = "This is a constant"

Functions

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// a simple function
func functionName() {}

// function with parameters (again, types go after identifiers)
func functionName(param1 string, param2 int) {}

// multiple parameters of the same type
func functionName(param1, param2 int) {}

// return type declaration
func functionName() int {
return 42
}

// Can return multiple values at once
func returnMulti() (int, string) {
return 42, "foobar"
}
var x, str = returnMulti()

// Return multiple named results simply by return
func returnMulti2() (n int, s string) {
n = 42
s = "foobar"
// n and s will be returned
return
}
var x, str = returnMulti2()

Functions As Values And Closures

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func main() {
// assign a function to a name
add := func(a, b int) int {
return a + b
}
// use the name to call the function
fmt.Println(add(3, 4))
}

// Closures, lexically scoped: Functions can access values that were
// in scope when defining the function
func scope() func() int{
outer_var := 2
foo := func() int { return outer_var}
return foo
}

func another_scope() func() int{
// won't compile because outer_var and foo not defined in this scope
outer_var = 444
return foo
}


// Closures: don't mutate outer vars, instead redefine them!
func outer() (func() int, int) {
outer_var := 2
inner := func() int {
outer_var += 99 // attempt to mutate outer_var from outer scope
return outer_var // => 101 (but outer_var is a newly redefined
// variable visible only inside inner)
}
return inner, outer_var // => 101, 2 (outer_var is still 2, not mutated by foo!)
}

Variadic Functions

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func main() {
fmt.Println(adder(1, 2, 3)) // 6
fmt.Println(adder(9, 9)) // 18

nums := []int{10, 20, 30}
fmt.Println(adder(nums...)) // 60
}

// By using ... before the type name of the last parameter you can indicate that it takes zero or more of those parameters.
// The function is invoked like any other function except we can pass as many arguments as we want.
func adder(args ...int) int {
total := 0
for _, v := range args { // Iterates over the arguments whatever the number.
total += v
}
return total
}

Built-in Types

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bool

string

int int8 int16 int32 int64
uint uint8 uint16 uint32 uint64 uintptr

byte // alias for uint8

rune // alias for int32 ~= a character (Unicode code point) - very Viking

float32 float64

complex64 complex128

Type Conversions

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var i int = 42
var f float64 = float64(i)
var u uint = uint(f)

// alternative syntax
i := 42
f := float64(i)
u := uint(f)

Packages

  • Package declaration at top of every source file
  • Executables are in package main
  • Convention: package name == last name of import path (import path math/rand => package rand)
  • Upper case identifier: exported (visible from other packages)
  • Lower case identifier: private (not visible from other packages)

Control structures

If

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func main() {
// Basic one
if x > 0 {
return x
} else {
return -x
}

// You can put one statement before the condition
if a := b + c; a < 42 {
return a
} else {
return a - 42
}

// Type assertion inside if
var val interface{}
val = "foo"
if str, ok := val.(string); ok {
fmt.Println(str)
}
}

Loops

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// There's only `for`, no `while`, no `until`
for i := 1; i < 10; i++ {
}
for ; i < 10; { // while - loop
}
for i < 10 { // you can omit semicolons if there is only a condition
}
for { // you can omit the condition ~ while (true)
}

Switch

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// switch statement
switch operatingSystem {
case "darwin":
fmt.Println("Mac OS Hipster")
// cases break automatically, no fallthrough by default
case "linux":
fmt.Println("Linux Geek")
default:
// Windows, BSD, ...
fmt.Println("Other")
}

// as with for and if, you can have an assignment statement before the switch value
switch os := runtime.GOOS; os {
case "darwin": ...
}

// you can also make comparisons in switch cases
number := 42
switch {
case number < 42:
fmt.Println("Smaller")
case number == 42:
fmt.Println("Equal")
case number > 42:
fmt.Println("Greater")
}

Arrays, Slices, Ranges

Arrays

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var a [10]int // declare an int array with length 10. Array length is part of the type!
a[3] = 42 // set elements
i := a[3] // read elements

// declare and initialize
var a = [2]int{1, 2}
a := [2]int{1, 2} //shorthand
a := [...]int{1, 2} // elipsis -> Compiler figures out array length

Slices

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var a []int                              // declare a slice - similar to an array, but length is unspecified
var a = []int {1, 2, 3, 4} // declare and initialize a slice (backed by the array given implicitly)
a := []int{1, 2, 3, 4} // shorthand
chars := []string{0:"a", 2:"c", 1: "b"} // ["a", "b", "c"]

var b = a[lo:hi] // creates a slice (view of the array) from index lo to hi-1
var b = a[1:4] // slice from index 1 to 3
var b = a[:3] // missing low index implies 0
var b = a[3:] // missing high index implies len(a)
a = append(a,17,3) // append items to slice a
c := append(a,b...) // concatenate slices a and b

// create a slice with make
a = make([]byte, 5, 5) // first arg length, second capacity
a = make([]byte, 5) // capacity is optional

// create a slice from an array
x := [3]string{"Лайка", "Белка", "Стрелка"}
s := x[:] // a slice referencing the storage of x

Operations on Arrays and Slices

len(a) gives you the length of an array/a slice. It’s a built-in function, not a attribute/method on the array.

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// loop over an array/a slice
for i, e := range a {
// i is the index, e the element
}

// if you only need e:
for _, e := range a {
// e is the element
}

// ...and if you only need the index
for i := range a {
}

// In Go pre-1.4, you'll get a compiler error if you're not using i and e.
// Go 1.4 introduced a variable-free form, so that you can do this
for range time.Tick(time.Second) {
// do it once a sec
}

Maps

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var m map[string]int
m = make(map[string]int)
m["key"] = 42
fmt.Println(m["key"])

delete(m, "key")

elem, ok := m["key"] // test if key "key" is present and retrieve it, if so

// map literal
var m = map[string]Vertex{
"Bell Labs": {40.68433, -74.39967},
"Google": {37.42202, -122.08408},
}

Structs

There are no classes, only structs. Structs can have methods.

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// A struct is a type. It's also a collection of fields

// Declaration
type Vertex struct {
X, Y int
}

// Creating
var v = Vertex{1, 2}
var v = Vertex{X: 1, Y: 2} // Creates a struct by defining values with keys
var v = []Vertex{{1,2},{5,2},{5,5}} // Initialize a slice of structs

// Accessing members
v.X = 4

// You can declare methods on structs. The struct you want to declare the
// method on (the receiving type) comes between the the func keyword and
// the method name. The struct is copied on each method call(!)
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

// Call method
v.Abs()

// For mutating methods, you need to use a pointer (see below) to the Struct
// as the type. With this, the struct value is not copied for the method call.
func (v *Vertex) add(n float64) {
v.X += n
v.Y += n
}

Anonymous structs:
Cheaper and safer than using map[string]interface{}.

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point := struct {
X, Y int
}{1, 2}

Pointers

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p := Vertex{1, 2}  // p is a Vertex
q := &p // q is a pointer to a Vertex
r := &Vertex{1, 2} // r is also a pointer to a Vertex

// The type of a pointer to a Vertex is *Vertex

var s *Vertex = new(Vertex) // new creates a pointer to a new struct instance

Interfaces

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// interface declaration
type Awesomizer interface {
Awesomize() string
}

// types do *not* declare to implement interfaces
type Foo struct {}

// instead, types implicitly satisfy an interface if they implement all required methods
func (foo Foo) Awesomize() string {
return "Awesome!"
}

Embedding

There is no subclassing in Go. Instead, there is interface and struct embedding.

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// ReadWriter implementations must satisfy both Reader and Writer
type ReadWriter interface {
Reader
Writer
}

// Server exposes all the methods that Logger has
type Server struct {
Host string
Port int
*log.Logger
}

// initialize the embedded type the usual way
server := &Server{"localhost", 80, log.New(...)}

// methods implemented on the embedded struct are passed through
server.Log(...) // calls server.Logger.Log(...)

// the field name of the embedded type is its type name (in this case Logger)
var logger *log.Logger = server.Logger

Errors

There is no exception handling. Functions that might produce an error just declare an additional return value of type Error. This is the Error interface:

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type error interface {
Error() string
}

A function that might return an error:

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func doStuff() (int, error) {
}

func main() {
result, error := doStuff()
if (error != nil) {
// handle error
} else {
// all is good, use result
}
}

Concurrency

Goroutines

Goroutines are lightweight threads (managed by Go, not OS threads). go f(a, b) starts a new goroutine which runs f (given f is a function).

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// just a function (which can be later started as a goroutine)
func doStuff(s string) {
}

func main() {
// using a named function in a goroutine
go doStuff("foobar")

// using an anonymous inner function in a goroutine
go func (x int) {
// function body goes here
}(42)
}

Channels

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ch := make(chan int) // create a channel of type int
ch <- 42 // Send a value to the channel ch.
v := <-ch // Receive a value from ch

// Non-buffered channels block. Read blocks when no value is available, write blocks if a value already has been written but not read.

// Create a buffered channel. Writing to a buffered channels does not block if less than <buffer size> unread values have been written.
ch := make(chan int, 100)

close(ch) // closes the channel (only sender should close)

// read from channel and test if it has been closed
v, ok := <-ch

// if ok is false, channel has been closed

// Read from channel until it is closed
for i := range ch {
fmt.Println(i)
}

// select blocks on multiple channel operations, if one unblocks, the corresponding case is executed
func doStuff(channelOut, channelIn chan int) {
select {
case channelOut <- 42:
fmt.Println("We could write to channelOut!")
case x := <- channelIn:
fmt.Println("We could read from channelIn")
case <-time.After(time.Second * 1):
fmt.Println("timeout")
}
}

Channel Axioms

  • A send to a nil channel blocks forever

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    var c chan string
    c <- "Hello, World!"
    // fatal error: all goroutines are asleep - deadlock!
  • A receive from a nil channel blocks forever

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    var c chan string
    fmt.Println(<-c)
    // fatal error: all goroutines are asleep - deadlock!
  • A send to a closed channel panics

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    var c = make(chan string, 1)
    c <- "Hello, World!"
    close(c)
    c <- "Hello, Panic!"
    // panic: send on closed channel
  • A receive from a closed channel returns the zero value immediately

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    var c = make(chan int, 2)
    c <- 1
    c <- 2
    close(c)
    for i := 0; i < 3; i++ {
    fmt.Printf("%d ", <-c)
    }
    // 1 2 0

Printing

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fmt.Println("Hello, 你好, नमस्ते, Привет, ᎣᏏᏲ") // basic print, plus newline
p := struct { X, Y int }{ 17, 2 }
fmt.Println( "My point:", p, "x coord=", p.X ) // print structs, ints, etc
s := fmt.Sprintln( "My point:", p, "x coord=", p.X ) // print to string variable

fmt.Printf("%d hex:%x bin:%b fp:%f sci:%e",17,17,17,17.0,17.0) // c-ish format
s2 := fmt.Sprintf( "%d %f", 17, 17.0 ) // formatted print to string variable

hellomsg := `
"Hello" in Chinese is 你好 ('Ni Hao')
"Hello" in Hindi is नमस्ते ('Namaste')
` // multi-line string literal, using back-tick at beginning and end

Snippets

HTTP Server

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package main

import (
"fmt"
"net/http"
)

// define a type for the response
type Hello struct{}

// let that type implement the ServeHTTP method (defined in interface http.Handler)
func (h Hello) ServeHTTP(w http.ResponseWriter, r *http.Request) {
fmt.Fprint(w, "Hello!")
}

func main() {
var h Hello
http.ListenAndServe("localhost:4000", h)
}

// Here's the method signature of http.ServeHTTP:
// type Handler interface {
// ServeHTTP(w http.ResponseWriter, r *http.Request)
// }

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