Go lang Cheat Sheet

File hello.go:

package main

import "fmt"

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

$ go run hello.go

Type goes after identifier!

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"

// iota can be used for incrementing numbers, starting from 0
const (
    _ = iota
    a
    b
    c = 1 << iota
    d
)
    fmt.Println(a, b) // 1 2 (0 is skipped)
    fmt.Println(c, d) // 8 16 (2^3, 2^4)

package main

import "fmt"
import "math"

// `const` declares a constant value.
const s string = "constant"

func main() {
    fmt.Println(s)

    // A `const` statement can appear anywhere a `var`
    // statement can.
    const n = 500000000

    fmt.Println(n)

    // Constant expressions perform arithmetic with
    // arbitrary precision.
    const d = 3e20 / n
    fmt.Println(d)
}

// this does *nothing*
_ := 42

// this calls `getThing` but throws away the 
// two return values
_, _ := getThing();

Importing

import _ "log"

package main

import "fmt"

func main() {

    // Strings, which can be added together with `+`.
    fmt.Println("go" + "lang")

    // Integers and floats.
    fmt.Println("1+1 =", 1+1)
    fmt.Println("7.0/3.0 =", 7.0/3.0)

    // Booleans, with boolean operators as you'd expect.
    fmt.Println(true && false)
    fmt.Println(true || false)
    fmt.Println(!true)
}

// 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()

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
func outer() (func() int, int) {
    outer_var := 2
    inner := func() int {
        outer_var += 99 // outer_var from outer scope is mutated.
        return outer_var
    }
    inner()
    return inner, outer_var // return inner func and mutated outer_var 101
}

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
}

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

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

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

• 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)

func main() {
    // Basic one
    if x > 10 {
        return x
    } else if x == 10 {
        return 10
    } 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)
    }
}

    // 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)
    }

    // use break/continue on current loop
    // use break/continue with label on outer loop
here:
    for i := 0; i < 2; i++ {
        for j := i + 1; j < 3; j++ {
            if i == 0 {
                continue here
            }
            fmt.Println(j)
            if j == 2 {
                break
            }
        }
    }

there:
    for i := 0; i < 2; i++ {
        for j := i + 1; j < 3; j++ {
            if j == 1 {
                continue
            }
            fmt.Println(j)
            if j == 2 {
                break there
            }
        }
    }

    // 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")
    }

    // cases can be presented in comma-separated lists
    var char byte = '?'
    switch char {
        case ' ', '?', '&', '=', '#', '+', '%':
            fmt.Println("Should escape")
    }

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

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

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

// 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
}

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},
}

// iterate over map content
for key, value := range m {
}

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

// 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{}.

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

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

// 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!"
}

Another example

package main

import (
    "fmt"
)

type Circle interface{
    GetRadius() int64
}

type CircleImpl struct{}

func NewCircle() *CircleImpl {
    return &(CircleImpl{})
}


func (*CircleImpl) GetRadius() int64{
    return int64(100)
}
func main() {
    circle := NewCircle()
    fmt.Println(circle.GetRadius())
}

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

// 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

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:

type error interface {
    Error() string
}

A function that might return an error:

func doStuff() (int, error) {
}

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

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).

// 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)
}

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 until there is a 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")
    }
}

• A send to a nil channel blocks forever

var c chan string
c <- "Hello, World!"
// fatal error: all goroutines are asleep - deadlock!

• A receive from a nil channel blocks forever

var c chan string
fmt.Println(<-c)
// fatal error: all goroutines are asleep - deadlock!

• A send to a closed channel panics

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

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

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

A type switch is like a regular switch statement, but the cases in a type switch specify types (not values), and those values are compared against the type of the value held by the given interface value.

func do(i interface{}) {
    switch v := i.(type) {
    case int:
        fmt.Printf("Twice %v is %v\n", v, v*2)
    case string:
        fmt.Printf("%q is %v bytes long\n", v, len(v))
    default:
        fmt.Printf("I don't know about type %T!\n", v)
    }
}

func main() {
    do(21)
    do("hello")
    do(true)
}

package main

import (
    "fmt"
    "io/ioutil"
)

func main() {
    data, err := ioutil.ReadFile("text.txt")
    if err != nil {
        return
    }
    fmt.Println(string(data))
}

package main

import "os"

func main() {
    file, err := os.Create("text.txt")
    if err != nil {
        return
    }
    defer file.Close()

    file.WriteString("test\nhello")
}

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)
// }

Without dependency injection

https://elliotchance.medium.com/a-new-simpler-way-to-do-dependency-injection-in-go-9e191bef50d5

type SendEmail struct {
    From string
}
func (sender *SendEmail) Send(to, subject, body string) error {
    // It sends an email here, and perhaps returns an error.
}

type CustomerWelcome struct{}
func (welcomer *CustomerWelcome) Welcome(name, email string) error {
    body := fmt.Sprintf("Hi, %s!", name)
    subject := "Welcome"
    emailer := &SendEmail{
        From: "hi@welcome.com",
    }
    return emailer.Send(email, subject, body)
}

// Usage
welcomer := &CustomerWelcome{}
err := welcomer.Welcome("Bob", "bob@smith.com")
// check error...

With dependency injection

// EmailSender provides an interface so we can swap out the
// implementation of SendEmail under tests.
type EmailSender interface {
    Send(to, subject, body string) error
}
type CustomerWelcome struct{
    Emailer EmailSender
}
func (welcomer *CustomerWelcome) Welcome(name, email string) error {
    body := fmt.Sprintf("Hi, %s!", name)
    subject := "Welcome"

    return welcomer.Emailer.Send(email, subject, body)
}

// Usage
emailer := &SendEmail{
    From: "hi@welcome.com",
}
welcomer := &CustomerWelcome{
    Emailer: emailer,
}
err := welcomer.Welcome("Bob", "bob@smith.com")
// check error...