hajjboy95 / go-learn

Short snippets for learning Go language quickly

Learning Go

This library gives a quick overview of Go for programmers already familiar with C/C++.

Table of Contents

• General
• Loops
• Switch
• Basic Types
  • Pointers
  • Arrays
• Concurrency
  • Channels
  • Select
• Object-Oriented
  • Polymorphism
  • Type Assertion
• Reflection

General

Composite types are array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals.

func swap(x, y string) (string, string) {
	return y, x
}

func main() {
	var i, j = 1, 2
	var k, str, isFinished = 1, "foo", false
	a, b := swap("hello", "world")
	fmt.Println(i, j, k, str, a, b)
}

A defer statement defers the execution of a function until the surrounding function returns:

func main() {
	defer fmt.Println("world")
	defer fmt.Println("my")
	fmt.Println("hello")
}

Deferred function calls are pushed onto a stack. This will output: “Hello my world”

Loops

Go has only one looping construct, the for loop:

for i := 0; i < 10; i++ {
	sum += i
}

While is similar to for loop:

for sum < 1000 {
	sum += sum
}

Infinite loop:

for {
}

Local variable in if block:

if v := math.Pow(x, n); v < lim {
	return v
}
return v	// ERROR: v is undefined!

Switch

switch os {
case "darwin":
	fmt.Println("OS X.")
case "linux":
	fmt.Println("Linux.")
default:
	// freebsd, openbsd,
	// plan9, windows...
	fmt.Printf("%s.", os)
}

Switch with no condition is good to write long if-then-else chains:

switch {
case t.Hour() < 12:
	fmt.Println("Good morning!")
case t.Hour() < 17:
	fmt.Println("Good afternoon.")
default:
	fmt.Println("Good evening.")
}

Basic Types

bool, string, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr
byte (alias for uint8), rune (alias for int32), float32, float64, complex64, complex128

var i int = 42

var f float64 = float64(i)	// cast: T(v) converts the value v to the type T

NOTE: Unlike C, in Go assignment between items of different type requires an explicit conversion.

const Pi = 3.14

Pointers

Go has pointers similar to the behaviors of * and & in C. Unlike C, Go has no pointer arithmetic.


var i = 56

var p *int = &i

fmt.Println(*p)

Arrays

var a [3]string		// to define an array
primes := []int{2, 3, 5}	// to define and init at the same time
a[0] = "Hello"			// to access and set
var i = len(a)			// to get array length

Concurrency

A "go" statement starts the execution of a function call as an independent concurrent thread of control, or goroutine, within the same address space:

// nameless goroutine
go func(x, y) {
	panic("Signal: ")
}()

// named goroutine
func findSum(x int, y int) {
	...
}


go findSum(x, y)

Channels

Channels are the pipes that connect concurrent goroutines. You can send values into channels from one goroutine and receive those values into another goroutine.

channel := make(chan string)

go func() {
	time.Sleep(2 * time.Second)
	channel <- "ping"
}()

fmt.Println("Goroutine started...")

// Wait until something is written on the channel
x := <-channel

fmt.Println("Goroutine finished!")

Select

select allows waiting on multiple channels:

c1 := make(chan int)
c2 := make(chan int)

go func() {
	delay := rand.Intn(5)
	time.Sleep(time.Duration(delay) * time.Second)
	c1 <- delay
}()

go func() {
	delay := rand.Intn(5)
	time.Sleep(time.Duration(delay) * time.Second)
	c2 <- delay
}()

fmt.Println("Goroutines started...")

// Wait until something is written on the channel
for i := 0; i < 2; i++ {
	select {
	case delay := <-c1:
		fmt.Println("First goroutine finished after " + strconv.Itoa(delay) + " sec.")
	case delay := <-c2:
		fmt.Println("Second goroutine finished after " + strconv.Itoa(delay) + " sec.")
	}
}

Object-Oriented

Go does not support all object-oriented mechanisms common in Java and C#, and it is for a reason: keeping objects lightweight. Go does not support type inheritance, and its subclassing and polymorphism is limited.

A struct is a collection of fields:


type Vertex struct {
	x int
	y int
}

func (v *Vertex) addVertex(v2 Vertex) {
	v.x += v2.x;
	v.y += v2.y;
}

var v Vertex		// to create a Vertex
v := Vertex{56, 31}	// to create and init at the same time
v := Vertex{x: 56}	// to create and set specific fields
fmt.Println(v.x)		// to access fields of a struct

The (v *Vertex) is called a method receiver. If the method receiver is not defined as a pointer, then the object will be copied by value, i.e., changes to the object will not remain after the method returns. For example, the following code will print {1,2} while it would print {4,6} if we had func (v *Vertex):

func (v Vertex) addVertex(v2 Vertex) {
	v.x += v2.x;
	v.y += v2.y;
}

v1 := Vertex{x: 1, y: 2}
v2 := Vertex{x: 3, y: 4}
v1.addVertex(v2)
fmt.Println(v1)

Inheritance is done using embedding which is to embed a struct name inside another struct:

type animal struct {
}

type duck struct {
	animal
	featherCount int
}

Go does not support type inheritance. So, something like this won't compile:

var a animal
d := duck{}
a = d		// COMPILE ERROR: cannot use d (type duck) as type animal in assignment 

Instead, we can use interfaces. They are satisfied implicitly, unlike Java or C#, so interfaces can be defined for code we don’t own.

type bird interface {
	Fly() string
}

func (d *duck) Fly() {
	return "Duck flying..."
}

Now, we can write:

var b bird
d := duck{}
b = d

Polymorphism

Polymorphism in Go is limited and can only be achieved through interfaces. Method Run() in the following code has a polymorphic behavior:

type ClientProtocol struct {
	Id int
}

type ServerProtocol struct {
	IsUp bool
}

type Protocol interface {
	Run() error
}

func (p *ClientProtocol) Run() error {
	fmt.Println("I'm a client.")
	return nil
}

func (p *ServerProtocol) Run() error {
	fmt.Println("I'm a server.")
	return nil
}

func main() {
	client := ClientProtocol{Id: 76}
	server := ServerProtocol{}

	protocols := [2]Protocol{&client, &server}
	protocols[0].Run()
	protocols[1].Run()
}

Type Assertion

Type assertions are similar to dynamic type casting in Java and C#. The notation x.(T) is called type assertion. In the Protocol example above, we can write:

cp, ok := protocols[0].(*ClientProtocol)		// Type assertion
if ok {
	fmt.Println(cp.Id)
} else {
	fmt.Println("Not a client protocol!")
}

This is especially useful when no common interface method applies to the sub types. In that case, we can use an empty interface (i.e., interface{}) that essentially applies to all types. For example, in the above Protocol example, we can write:

var anything [2]interface{}
anything[0] = client
anything[1] = server
cp, ok := anything[0].(ClientProtocol) 	// Type assertion
if ok {
    fmt.Println(cp.Id)
} else {
    fmt.Println("Not a client protocol!")
}

While the use of interface{} type can be considered as a deviation from the strongly-typed nature of Go, type assertions allow safe type checking at runtime.

Reflection

Using reflection, a program can inspect its own structure (e.g., types and functions). In Go, the reflect package allows reflecting program structure, for example:

var x int = 65
fmt.Println("type:", reflect.TypeOf(x))

will print type: int. To enumerate the fields of a struct type, we can write:

type Person struct {
	Age int
	Name string
	Height float64
}

func main() {
	var x int = 56
	fmt.Println("type:", reflect.TypeOf(x))

	t := Person{26, "Mahdi", 5.7}
	s := reflect.ValueOf(&t).Elem()
	typeOfT := s.Type()
	for i := 0; i < s.NumField(); i++ {
		f := s.Field(i)
		fmt.Printf("%s %s = %v\n", typeOfT.Field(i).Name, f.Type(), f.Interface())
	}
}