This article will cover all the basics of Go Language in one place so we can use it as a cheat sheet too.
1. Fundamentals
1.1. Packages
Each Go program/app is made up of one or many packages (internal, standard, and external). By default, the program starts running in the package main and function main.
Remember – When we want to use a package in a program, by convention last element in the import paths is the package name.
Example: math/rand path last element rand is the name of the package too.
package main
import (
"fmt"
"math/rand"
)
func main() {
fmt.Println("My favorite number is", rand.Intn(10))
}1.2 Imports
General imports
import "fmt"
import "math"Equivalent Factored Import statement:
import (
"fmt"
"math"
)Good practice – It’s good practice to use factored import statements.
1.3 Exported Names
In Go, exported names begin with capital letters.
package main
import (
"fmt"
"math"
)
func main() {
// fmt.Println(math.pi) = Error message math.pi is not valid.
fmt.Println(math.Pi)
}1.4. Functions
1.4.1. Function can take one or more arguments
package main
import "fmt"
func add(x int, y int) int {
return x + y
}
func main() {
fmt.Println(add(42, 13))
}1.4.2. Omit type for shared args
When two or more consecutive named function parameters share a type, you can omit the type from all but the last.
package main
import "fmt"
func add(x, y int) int {
return x + y
}
func main() {
fmt.Println(add(42, 13))
}1.4.3. Function can return many results
package main
import "fmt"
func swap(x, y string) (string, string) {
return y, x
}
func main() {
a, b := swap("hello", "world")
fmt.Println(a, b)
}1.4.4 Function return values can be named
package main
import "fmt"
func split(sum int) (x, y int) {
x = sum * 4 / 9
y = sum - x
return
}
func main() {
fmt.Println(split(17))
}Naked Return – A return statement without arguments returns the named return values. This is known as a “naked” return.
Good Practice – Use naked return in short functions. It harms readability in longer functions.
1.5. Variables
1.5.1. var
The var statement declares one or more variables, as we did in function arguments and return, the type comes last.
The var can be at the package or function level.
package main
import "fmt"
var c, python, java bool
func main() {
var i int
fmt.Println(i, c, python, java)
}1.5.2. Variable with initializers
We can initialize variables while declaring.
Best Practice – If initialization is present explicit type is not necessary.
package main
import "fmt"
// Initialization with type
var i, j int = 1, 2
func main() {
// Initialization without type
var c, python, java = true, false, "no!"
fmt.Println(i, j, c, python, java)
}1.5.3. := short variable declaration inside the function
Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.
package main
import "fmt"
func main() {
// With var explicit type
var i, j int = 1, 2
// With := implicit type
k := 3
// Multiple variable := implicit type
c, python, java := true, false, "no!"
fmt.Println(i, j, k, c, python, java)
}Good practice – To use it when possible.
1.5.4. Variable declaration block
variable declarations may be “factored” into blocks, as with import statements
package main
import (
"fmt"
"math/cmplx"
)
var (
ToBe bool = false
MaxInt uint64 = 1<<64 - 1
z complex128 = cmplx.Sqrt(-5 + 12i)
)
func main() {
fmt.Printf("Type: %T Value: %v\n", ToBe, ToBe)
fmt.Printf("Type: %T Value: %v\n", MaxInt, MaxInt)
fmt.Printf("Type: %T Value: %v\n", z, z)
}1.6. Basic Types
1.6.1. List of Types
bool
string
int int8 int16 int32 int64
uint uint8 uint16 uint32 uint64 uintptr
byte // alias for uint8
rune // alias for int32
// represents a Unicode code point
float32 float64
complex64 complex128Best Practice – Use int (default – 32 bytes) always unless you have reasons to use unsigned and sized integers.
1.6.2. Default values
0for numeric types,falsefor the boolean type, and""(the empty string) for strings.
1.6.3. Type Conversion
The expression T(v) converts the value v to the type T.
i := 42
f := float64(i)
u := uint(f)Remember – Go assignment between items of different types requires an explicit conversion. Failing to do so causes error.
1.6.4. Type Detection / Inference
var i int
j := i // j is an int
k := 42 // int
l := 3.142 // float64
m := 0.867 + 0.5i // complex1281.6.5. Constants
Constants are declared like variables but with the const keyword. Constants can be character, string, boolean, or numeric values. Constants cannot be declared using the := syntax.
package main
import "fmt"
const Pi = 3.14
func main() {
const World = "World"
fmt.Println("Hello", World)
fmt.Println("Happy", Pi, "Day")
const Truth = true
fmt.Println("Go rules?", Truth)
}2. Flow Control
2.1. For
2.1.1 With init and post
Go has only one looping construct, the for loop.
package main
import "fmt"
func main() {
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
fmt.Println(sum)
}2.1.2. Without init and post
package main
import "fmt"
func main() {
sum := 1
for ; sum < 1000; {
sum += sum
}
fmt.Println(sum)
}2.1.2. For is while
package main
import "fmt"
func main() {
sum := 1
for sum < 1000 {
sum += sum
}
fmt.Println(sum)
}2.1.3. Infinite loop
package main
func main() {
for {
}
}2.2. If
2.2.1. If Basic
Go’s if statements are like its for loops; the expression need not be surrounded by parentheses ( ) but the braces { } are required.
package main
import (
"fmt"
"math"
)
func sqrt(x float64) string {
if x < 0 {
return sqrt(-x) + "i"
}
return fmt.Sprint(math.Sqrt(x))
}
func main() {
fmt.Println(sqrt(2), sqrt(-4))
}2.2.2. If Short Statement
package main
import (
"fmt"
"math"
)
func pow(x, n, lim float64) float64 {
if v := math.Pow(x, n); v < lim {
return v
}
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}Variables declared by the statement are only in scope until the end of the if.
2.2.4. If and Else
package main
import (
"fmt"
"math"
)
func pow(x, n, lim float64) float64 {
if v := math.Pow(x, n); v < lim {
return v
} else {
fmt.Printf("%g >= %g\n", v, lim)
}
// can't use v here, though
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}2.3. Switch
A switch statement is a shorter way to write a sequence of if - else statements. It runs the first case whose value is equal to the condition expression.
No break is needed it is implicit.
2.3.1. Switch basic
package main
import (
"fmt"
"runtime"
)
func main() {
fmt.Print("Go runs on ")
switch os := runtime.GOOS; os {
case "darwin":
fmt.Println("OS X.")
case "linux":
fmt.Println("Linux.")
default:
// freebsd, openbsd,
// plan9, windows...
fmt.Printf("%s.\n", os)
}
}
2.3.2. Switch case evaluation order
Switch cases evaluate cases from top to bottom, stopping when a case succeeds.
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println("When's Saturday?")
today := time.Now().Weekday()
switch time.Saturday {
case today + 0:
fmt.Println("Today.")
case today + 1:
fmt.Println("Tomorrow.")
case today + 2:
fmt.Println("In two days.")
default:
fmt.Println("Too far away.")
}
}2.3.3. Switch without condition
Switch without a condition is the same as switch true.
package main
import (
"fmt"
"time"
)
func main() {
t := time.Now()
switch {
case t.Hour() < 12:
fmt.Println("Good morning!")
case t.Hour() < 17:
fmt.Println("Good afternoon.")
default:
fmt.Println("Good evening.")
}
}
2.4. Defer
A defer statement defers the execution of a function until the surrounding function returns.
The deferred call’s arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.
package main
import "fmt"
func main() {
defer fmt.Println("world")
fmt.Println("hello")
}
Output:
hello
world2.4.1. Stacking Defer
Deferred function calls are pushed onto a stack. When a function returns, its deferred calls are executed in last-in-first-out order.
package main
import "fmt"
func main() {
fmt.Println("counting")
for i := 0; i < 10; i++ {
defer fmt.Println(i)
}
fmt.Println("done")
}Output
counting
done
9
8
7
6
5
4
3
2
1
03. Advance Data Structure
3.1. Pointers
Go has pointers. A pointer holds the memory address of a value.
The type *T is a pointer to a T value. Its zero value is nil.
var p *intThe & operator generates a pointer to its operand.
i := 42
p = &iThe * operator denotes the pointer’s underlying value.
fmt.Println(*p) // read i through the pointer p
*p = 21 // set i through the pointer pThis is known as “dereferencing” or “indirecting”.
package main
import "fmt"
func main() {
i, j := 42, 2701
p := &i // point to i
fmt.Println(*p) // read i through the pointer
*p = 21 // set i through the pointer
fmt.Println(i) // see the new value of i
p = &j // point to j
*p = *p / 37 // divide j through the pointer
fmt.Println(j) // see the new value of j
}Unlike C, Go has no pointer arithmetic.
3.2. Structs
A struct is a collection of fields.
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
fmt.Println(Vertex{1, 2})
}Note: Is it a kind of Java Record?
3.2.1. Accessing Struct field
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
v.X = 4
fmt.Println(v.X)
}Using . (Dot).
3.2.2. Pointer to Struct
Struct fields can be accessed through a struct pointer.
To access the field X of a struct when we have the struct pointer p we could write (*p).X. However, that notation is cumbersome, so the language permits us instead to write just p.X, without the explicit dereference.
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
p := &v
p.X = 1e9
fmt.Println(v)
}3.2.3. Struct Literals
A struct literal denotes a newly allocated struct value by listing the values of its fields.
You can list just a subset of fields by using the Name: syntax. (And the order of named fields is irrelevant.)
The special prefix & returns a pointer to the struct value.
package main
import "fmt"
type Vertex struct {
X, Y int
}
var (
v1 = Vertex{1, 2} // has type Vertex
v2 = Vertex{X: 1} // Y:0 is implicit
v3 = Vertex{} // X:0 and Y:0
p = &Vertex{1, 2} // has type *Vertex
)
func main() {
fmt.Println(v1, p, v2, v3)
}3.3. Arrays
he type [n]T is an array of n values of type T.
The expression
var a [10]int
declares a variable a as an array of ten integers.
An array’s length is part of its type, so arrays cannot be resized. This seems to limit, but don’t worry;
package main
import "fmt"
func main() {
var a [2]string
a[0] = "Hello"
a[1] = "World"
fmt.Println(a[0], a[1])
fmt.Println(a)
primes := [6]int{2, 3, 5, 7, 11, 13}
fmt.Println(primes)
}3.4. Slice
A slice is a dynamically sized, flexible view of the elements of an array. In practice, slices are much more common than arrays.
The type []T is a slice with elements of type T.
A slice is formed by specifying two indices, a low and high bound, separated by a colon:
a[low : high]
This selects a half-open range which includes the first element, but excludes the last one.
The following expression creates a slice which includes elements 1 through 3 of a:
a[1:4]
package main
import "fmt"
func main() {
primes := [6]int{2, 3, 5, 7, 11, 13}
var s []int = primes[1:4]
fmt.Println(s)
}Output:
[3 5 7]
3.4.1. Slice are like references to an array
A slice does not store any data, it just describes a section of an underlying array.
Changing the elements of a slice modifies the corresponding elements of its underlying array.
Other slices that share the same underlying array will see those changes.
package main
import "fmt"
func main() {
names := [4]string{
"John",
"Paul",
"George",
"Ringo",
}
fmt.Println(names)
a := names[0:2]
b := names[1:3]
fmt.Println(a, b)
b[0] = "XXX"
fmt.Println(a, b)
fmt.Println(names)
}Output:
[John Paul George Ringo]
[John Paul] [Paul George]
[John XXX] [XXX George]
[John XXX George Ringo]3.4.2. Slice Literals
A slice literal is like an array literal without the length.
This is an array literal:
[3]bool{true, true, false}
This creates the same array as above, and then builds a slice that references it:
[]bool{true, true, false}
package main
import "fmt"
func main() {
q := []int{2, 3, 5, 7, 11, 13}
fmt.Println(q)
r := []bool{true, false, true, true, false, true}
fmt.Println(r)
s := []struct {
i int
b bool
}{
{2, true},
{3, false},
{5, true},
{7, true},
{11, false},
{13, true},
}
fmt.Println(s)
}
Output:
[2 3 5 7 11 13]
[true false true true false true]
[{2 true} {3 false} {5 true} {7 true} {11 false} {13 true}]3.4.3. Slice defaults
For the array
var a [10]int
these slice expressions are equivalent:
a[0:10]
a[:10]
a[0:]
a[:]
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
s = s[1:4]
fmt.Println(s)
s = s[:2]
fmt.Println(s)
s = s[1:]
fmt.Println(s)
}Output:
[3 5 7]
[3 5]
[5]3.4.4. Slice length and capacity
A slice has both a length and a capacity.
The length of a slice is the number of elements it contains.
The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.
len(s) and cap(s) will return length and capacity.
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
printSlice(s)
// Slice the slice to give it zero length.
s = s[:0]
printSlice(s)
// Extend its length.
s = s[:4]
printSlice(s)
// Drop its first two values.
s = s[2:]
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}Output:
len=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=4 cap=6 [2 3 5 7]
len=2 cap=4 [5 7]3.4.5. Nil Slice
package main
import "fmt"
func main() {
var s []int
fmt.Println(s, len(s), cap(s))
if s == nil {
fmt.Println("nil!")
}
}Output:
[] 0 0
nil!3.4.6. Create Slice with Make
Slices can be created with the built-in make function; this is how you create dynamically sized arrays.
package main
import "fmt"
func main() {
a := make([]int, 5)
printSlice("a", a)
b := make([]int, 0, 5)
printSlice("b", b)
c := b[:2]
printSlice("c", c)
d := c[2:5]
printSlice("d", d)
}
func printSlice(s string, x []int) {
fmt.Printf("%s len=%d cap=%d %v\n",
s, len(x), cap(x), x)
}Output:
a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0]
d len=3 cap=3 [0 0 0]3.4.7. Slices of Slice
2D Array. We can have Slice value as Slice or any other Type.
package main
import (
"fmt"
"strings"
)
func main() {
// Create a tic-tac-toe board.
board := [][]string{
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
}
// The players take turns.
board[0][0] = "X"
board[2][2] = "O"
board[1][2] = "X"
board[1][0] = "O"
board[0][2] = "X"
for i := 0; i < len(board); i++ {
fmt.Printf("%s\n", strings.Join(board[i], " "))
}
}Output:
X _ X
O _ X
_ _ O3.4.8. Appending to Slice
package main
import "fmt"
func main() {
var s []int
printSlice(s)
// append works on nil slices.
s = append(s, 0)
printSlice(s)
// The slice grows as needed.
s = append(s, 1)
printSlice(s)
// We can add more than one element at a time.
s = append(s, 2, 3, 4)
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}Output:
len=0 cap=0 []
len=1 cap=1 [0]
len=2 cap=2 [0 1]
len=5 cap=6 [0 1 2 3 4]Go provides a built-in append function.
func append(s []T, vs ...T) []T
The first parameter s of append is a slice of type T, and the rest are T values to append to the slice.
The resulting value of append is a slice containing all the elements of the original slice plus the provided values.
If the backing array of s is too small to fit all the given values a bigger array will be allocated. The returned slice will point to the newly allocated array.
To deep dive into Slice, read this article.
3.4.9. Iterate on Slice
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
func main() {
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
}
The range form of the for the loop iterates over a slice or map.
We can skip the index or value by assigning it to _.
for i, _ := range pow
for _, value := range powIf you only want the index, you can omit the second variable.
for i := range pow
package main
import "fmt"
func main() {
pow := make([]int, 10)
for i := range pow {
pow[i] = 1 << uint(i) // == 2**i
}
for _, value := range pow {
fmt.Printf("%d\n", value)
}
}
3.5. Maps
A map maps keys to values.
The zero value of a map is nil. A nil map has no keys, nor can keys be added.
The make function returns a map of the given type, initialized and ready for use.
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m map[string]Vertex
func main() {
m = make(map[string]Vertex)
m["Bell Labs"] = Vertex{
40.68433, -74.39967,
}
fmt.Println(m["Bell Labs"])
}3.5.1. Map Literal
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m = map[string]Vertex{
"Bell Labs": Vertex{
40.68433, -74.39967,
},
"Google": Vertex{
37.42202, -122.08408,
},
}
func main() {
fmt.Println(m)
}If the top-level type is just a type name, you can omit it from the elements of the literal.
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m = map[string]Vertex{
"Bell Labs": {40.68433, -74.39967},
"Google": {37.42202, -122.08408},
}
func main() {
fmt.Println(m)
}3.5.2. Mutating Map
package main
import "fmt"
func main() {
m := make(map[string]int)
m["Answer"] = 42
fmt.Println("The value:", m["Answer"])
m["Answer"] = 48
fmt.Println("The value:", m["Answer"])
delete(m, "Answer")
fmt.Println("The value:", m["Answer"])
v, ok := m["Answer"]
fmt.Println("The value:", v, "Present?", ok)
}Output:
The value: 42
The value: 48
The value: 0
The value: 0 Present? false3.6. Function as value
Functions are values too. They can be passed around just like other values.
Function values may be used as function arguments and return values.
package main
import (
"fmt"
"math"
)
// Function passed as argument to a function
func compute(fn func(float64, float64) float64) float64 {
return fn(3, 4)
}
func main() {
hypot := func(x, y float64) float64 {
return math.Sqrt(x*x + y*y)
}
fmt.Println(hypot(5, 12))
fmt.Println(compute(hypot))
fmt.Println(compute(math.Pow))
}Output:
13
5
813.7. Function Closures
Go functions may be closures. A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense, the function is “bound” to the variables. Refer here for more.
4. Methods & Interfaces
4.1. Methods
There are no classes in Go. We can define methods on types.
A method is a function with a special receiver argument.
The receiver appears in its own argument list between the func keyword and the method name.
In this example, the Abs method has a receiver of type Vertex named v.
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(v.Abs())
}4.1.1. Methods are function
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func Abs(v Vertex) float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(Abs(v))
}
Here’s Abs written as a regular function with no change in functionality.
4.1.2. Methods on Non-Struct Type
package main
import (
"fmt"
"math"
)
type MyFloat float64
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
func main() {
f := MyFloat(-math.Sqrt2)
fmt.Println(f.Abs())
}4.1.3. Method Pointer & Value Receiver
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
v.Scale(10)
fmt.Println(v.Abs())
}Methods with pointer receivers can modify the value to which the receiver points (as Scale does here). Since methods often need to modify their receiver, pointer receivers are more common than value receivers.
With a value receiver, the Scale method operates on a copy of the original Vertex value. (This is the same behavior as for any other function argument.) The Scale method must have a pointer receiver to change the Vertex value declared in the main function.
It’s the same for function too.
4.1.4. Methods and pointer indirection
package main
import "fmt"
type Vertex struct {
X, Y float64
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func ScaleFunc(v *Vertex, f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
v.Scale(2)
ScaleFunc(&v, 10)
p := &Vertex{4, 3}
p.Scale(3)
ScaleFunc(p, 8)
fmt.Println(v, p)
} For the statement v.Scale(5), even though v is a value and not a pointer, the method with the pointer receiver is called automatically. That is, as a convenience, Go interprets the statement v.Scale(5) as (&v).Scale(5) since the Scale method has a pointer receiver.
4.1.5. Choosing Value or Pointer Receiver
There are two reasons to use a pointer receiver.
- The method can modify the value that its receiver points to.
- Avoid copying the value on each method call. This can be more efficient if the receiver is a large struct, for example.
4.2. Interface
An interface type is defined as a set of method signatures.
A value of interface type can hold any value that implements those methods.
package main
import (
"fmt"
"math"
)
type Abser interface {
Abs() float64
}
func main() {
var a Abser
f := MyFloat(-math.Sqrt2)
v := Vertex{3, 4}
a = f // a MyFloat implements Abser
a = &v // a *Vertex implements Abser
// In the following line, v is a Vertex (not *Vertex)
// and does NOT implement Abser.
// a = v
fmt.Println(a.Abs())
}
type MyFloat float64
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
type Vertex struct {
X, Y float64
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}4.2.1. Interfaces are implemented implicitly
A type implements an interface by implementing its methods. There is no explicit declaration of intent, no “implements” keyword.
Implicit interfaces decouple the definition of an interface from its implementation, which could then appear in any package without prearrangement.
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
// This method means type T implements the interface I,
// but we don't need to explicitly declare that it does so.
func (t T) M() {
fmt.Println(t.S)
}
func main() {
var i I = T{"hello"}
i.M()
}4.2.2. Interface Values
Under the hood, interface values can be thought of as a tuple of a value and a concrete type:(value, type)
An interface value holds a value of a specific underlying concrete type.
Calling a method on an interface value executes the method of the same name on its underlying type.
package main
import (
"fmt"
"math"
)
type I interface {
M()
}
type T struct {
S string
}
func (t *T) M() {
fmt.Println(t.S)
}
type F float64
func (f F) M() {
fmt.Println(f)
}
func main() {
var i I
i = &T{"Hello"}
describe(i)
i.M()
i = F(math.Pi)
describe(i)
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}Output:
(&{Hello}, *main.T)
Hello
(3.141592653589793, main.F)
3.1415926535897934.2.3. Interface values with nil underlying values
If the concrete value inside the interface itself is nil, the method will be called with a nil receiver.
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
func (t *T) M() {
if t == nil {
fmt.Println("<nil>")
return
}
fmt.Println(t.S)
}
func main() {
var i I
var t *T
i = t
describe(i)
i.M()
i = &T{"hello"}
describe(i)
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}Output:
(<nil>, *main.T)
<nil>
(&{hello}, *main.T)
hello4.2.4. Nil interface values
A nil interface value holds neither value nor concrete type.
Calling a method on a nil interface is a run-time error because there is no type inside the interface tuple to indicate which concrete method to call.
package main
import "fmt"
type I interface {
M()
}
func main() {
var i I
describe(i)
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}Output:
(<nil>, <nil>)
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x47c1b9]
goroutine 1 [running]:
main.main()
/tmp/sandbox2360631799/prog.go:12 +0x194.2.5. Empty Interface
The interface type that specifies zero methods is known as the empty interface:
interface{}
An empty interface may hold values of any type. (Every type implements at least zero methods.)
Empty interfaces are used by code that handles values of unknown type. For example, fmt.Print takes any number of arguments of type interface{}.
package main
import "fmt"
func main() {
var i interface{}
describe(i)
i = 42
describe(i)
i = "hello"
describe(i)
}
func describe(i interface{}) {
fmt.Printf("(%v, %T)\n", i, i)
}Output:
(<nil>, <nil>)
(42, int)
(hello, string)4.2.6. Type Assertions
A type assertion provides access to an interface value’s underlying concrete value.
t := i.(T)
package main
import "fmt"
func main() {
var i interface{} = "hello"
s := i.(string)
fmt.Println(s)
s, ok := i.(string)
fmt.Println(s, ok)
f, ok := i.(float64)
fmt.Println(f, ok)
f = i.(float64) // panic
fmt.Println(f)
}Output:
hello
hello true
0 false
panic: interface conversion: interface {} is string, not float64
goroutine 1 [running]:
main.main()
/tmp/sandbox4287718239/prog.go:17 +0x14f4.2.7. Type Switch
A type switch is a construct that permits several type assertions in series.
package main
import "fmt"
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)
}Output:
Twice 21 is 42
"hello" is 5 bytes long
I don't know about type bool!4.2.8. Stringers Interface
One of the most ubiquitous interfaces is Stringer defined by the fmt package.
type Stringer interface {
String() string
}A Stringer is a type that can describe itself as a string. The fmt package (and many others) look for this interface to print values.
package main
import "fmt"
type Person struct {
Name string
Age int
}
func (p Person) String() string {
return fmt.Sprintf("%v (%v years)", p.Name, p.Age)
}
func main() {
a := Person{"Arthur Dent", 42}
z := Person{"Zaphod Beeblebrox", 9001}
fmt.Println(a, z)
}Output:
Arthur Dent (42 years) Zaphod Beeblebrox (9001 years)
5. Generics
5.1. Type Parameters
Go functions can be written to work on multiple types using type parameters. The type parameters of a function appear between brackets, before the function’s arguments.
func Index[T comparable](s []T, x T) int
This declaration means that s is a slice of any type T that fulfills the built-in constraint comparable. x is also a value of the same type.
comparable is a useful constraint that makes it possible to use the == and != operators on values of the type. In this example, we use it to compare a value to all slice elements until a match is found. This Index function works for any type that supports comparison.
package main
import "fmt"
// Index returns the index of x in s, or -1 if not found.
func Index[T comparable](s []T, x T) int {
for i, v := range s {
// v and x are type T, which has the comparable
// constraint, so we can use == here.
if v == x {
return i
}
}
return -1
}
func main() {
// Index works on a slice of ints
si := []int{10, 20, 15, -10}
fmt.Println(Index(si, 15))
// Index also works on a slice of strings
ss := []string{"foo", "bar", "baz"}
fmt.Println(Index(ss, "hello"))
}5.2 Generic types
In addition to generic functions, Go also supports generic types. A type can be parameterized with a type parameter, which could be useful for implementing generic data structures.
This example demonstrates a simple type declaration for a singly-linked list holding any type of value.
package main
// List represents a singly-linked list that holds
// values of any type.
type List[T any] struct {
next *List[T]
val T
}
func main() {
}
6. Concurrency
6.1. Goroutine
A goroutine is a lightweight thread managed by the Go runtime.
go f(x, y, z)
starts a new goroutine running
f(x, y, z)
The evaluation of f, x, y, and z happens in the current goroutine and the execution of f happens in the new goroutine.
Goroutines run in the same address space, so access to shared memory must be synchronized. The sync package provides useful primitives, although you won’t need them much in Go as there are other primitives.
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
// Thread
go say("world")
say("hello")
}6.2. Channels
Channels are a typed conduit through which you can send and receive values with the channel operator, <-.
ch <- v // Send v to channel ch.
v := <-ch // Receive from ch, and
// assign value to v.(The data flows in the direction of the arrow.)
Like maps and slices, channels must be created before use:
ch := make(chan int)
By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.
The example code sums the numbers in a slice, distributing the work between two goroutines. Once both goroutines have completed their computation, it calculates the final result.
package main
import "fmt"
func sum(s []int, c chan int) {
sum := 0
for _, v := range s {
sum += v
}
c <- sum // send sum to c
}
func main() {
s := []int{7, 2, 8, -9, 4, 0}
c := make(chan int)
go sum(s[:len(s)/2], c)
go sum(s[len(s)/2:], c)
x, y := <-c, <-c // receive from c
fmt.Println(x, y, x+y)
}Result:
-5 17 12
6.3. Buffered Channel
Channels can be buffered. Provide the buffer length as the second argument to make to initialize a buffered channel:
ch := make(chan int, 100)
Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.
6.4. Range and Close
A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression: after
v, ok := <-ch
ok is false if there are no more values to receive and the channel is closed.
The loop for i := range c receives values from the channel repeatedly until it is closed.
Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
package main
import (
"fmt"
)
func fibonacci(n int, c chan int) {
x, y := 0, 1
for i := 0; i < n; i++ {
c <- x
x, y = y, x+y
}
close(c)
}
func main() {
c := make(chan int, 10)
go fibonacci(cap(c), c)
for i := range c {
fmt.Println(i)
}
}6.5. Select
The select statement lets a goroutine wait on multiple communication operations.
A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.
The default case in a select is run if no other case is ready.
6.6. sync.Mutex
We’ve seen how channels are great for communication among goroutines.
But what if we don’t need communication? What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts?
This concept is called mutual exclusion, and the conventional name for the data structure that provides it is mutex.
Go’s standard library provides mutual exclusion with sync.Mutex and its two methods:
LockUnlock
We can define a block of code to be executed in mutual exclusion by surrounding it with a call to Lock and Unlock as shown on the Inc method.
We can also use defer to ensure the mutex will be unlocked as in the Value method.
package main
import (
"fmt"
"sync"
"time"
)
// SafeCounter is safe to use concurrently.
type SafeCounter struct {
mu sync.Mutex
v map[string]int
}
// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
c.mu.Lock()
// Lock so only one goroutine at a time can access the map c.v.
c.v[key]++
c.mu.Unlock()
}
// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
c.mu.Lock()
// Lock so only one goroutine at a time can access the map c.v.
defer c.mu.Unlock()
return c.v[key]
}
func main() {
c := SafeCounter{v: make(map[string]int)}
for i := 0; i < 1000; i++ {
go c.Inc("somekey")
}
time.Sleep(time.Second)
fmt.Println(c.Value("somekey"))
}
