lets go
1.0.0
Table of Contents
unique packageFinding Golang documentation isn't a big deal. There are many good resources, just choose one & start your learning journey. I mainly follow Learning Go - Miek Gieben.
NOTE: Every examples in this documentation are stored in directories named by section. I assume that every commands in section X will be executed in example/X directory, so I don't write a full path to Go script file.
/* hello_world.go */
package main
import "fmt" // Implements formatted I/O
/* Say Hello-World */
func main() {
fmt.Printf("Hello World")
}go build helloworld.go # Return an executable called helloworld./helloworldgo run helloworld.go/* When you declare a variable it is assigned the "natural" null value for the type */
var a int // a has a value of 0
var s string // s is assigned the zero string, which is ""
a = 26
s = "hello"
/* Declaring & assigning in Go is a two step process, but they may be combined */
a := 26 // In this case the variable type is deduced from the value. A value of 26 indicates an int for example.
b := "hello" // The type should be string
/* Multiple var declarations may also be grouped (import & const also allow this) */
var (
a int
b string
)
/* Multiple variables of the same type ca also be declared on a single line */
var a, b int
a, b := 26, 9
/* A special name for a variable is _, any value assigned to it is discarded. */
_, b := 26, 9Boolean: bool
Numerical:
int - it has the appropriate length for your machine (32-bit machine - 32 bits, 64-bit machine - 64 bits)int8, int16, int32, int64 & byte (an alias for uint8), uint8, uint16, uint32, uint64.float32, float64, /* numerical_types.go */
package main
func main() {
var a int
var b int32
b = a + a // Give an error: cannot use a + a (type int) as type int32 in assignment.
b = b + 5
}Constants: Constants are created at compile time, & can only be numbers, strings, or booleans. You can use iota to enumerate values.
const (
a = iota // First use of iota will yield 0. Whenever iota is used again on a new line its value is incremented with 1, so b has a vaue of 1.
b
)Strings:
string in Go. Note that! In Python (my favourite programming language), I can use both of them for string assignment.s1 := "Hello"
c := []rune(s) // Convert s1 to an array of runes
c[0] := 'M'
s2 := string(c) // Create a new string s2 with the alteration
fmt.Printf("%sn", s2)Rune: Rune is an alias for int32, (use when you're iterating over characters in a string).
Complex Numbers: complex128 (64 bit real & imaginary parts) or complex32.
Errors: Go has a builtin type specially for errors, called error.var e.
Go 1.18 brings support for Generic types. The generics implementation provided by Go 1.18 follows the type parameter proposal and allows developers to add optional type parameters to type and function declarations. Checkout Golang's generics tutorial.
package main
import "fmt"
type Number interface {
int64 | float64
}
func main() {
// Initialize a map for the integer values
ints := map[string]int64{
"first": 34,
"second": 12,
}
// Initialize a map for the float values
floats := map[string]float64{
"first": 35.98,
"second": 26.99,
}
fmt.Printf("Non-Generic Sums: %v and %vn",
SumInts(ints),
SumFloats(floats))
fmt.Printf("Generic Sums: %v and %vn",
SumIntsOrFloats[string, int64](ints),
SumIntsOrFloats[string, float64](floats))
fmt.Printf("Generic Sums, type parameters inferred: %v and %vn",
SumIntsOrFloats(ints),
SumIntsOrFloats(floats))
fmt.Printf("Generic Sums with Constraint: %v and %vn",
SumNumbers(ints),
SumNumbers(floats))
}
// SumInts adds together the values of m.
func SumInts(m map[string]int64) int64 {
var s int64
for _, v := range m {
s += v
}
return s
}
// SumFloats adds together the values of m.
func SumFloats(m map[string]float64) float64 {
var s float64
for _, v := range m {
s += v
}
return s
}
// SumIntsOrFloats sums the values of map m. It supports both floats and integers
// as map values.
func SumIntsOrFloats[K comparable, V int64 | float64](m map[K]V) V {
var s V
for _, v := range m {
s += v
}
return s
}
// SumNumbers sums the values of map m. Its supports both integers
// and floats as map values.
func SumNumbers[K comparable, V Number](m map[K]V) V {
var s V
for _, v := range m {
s += v
}
return s
}Precedence Operator(s)
Highest * / % << >> & &^
`+ -
== != < <= > >=
<-
&&
Lowest ||
& bitwise and, | bitwise or, ^ bitwise xor, &^ bit clear respectively.break default func interface select
case defer go map struct
chan else goto package switch
const fallthrough if range type
continue for import return var
var, const, package, import are used in the previous sections.func is used to declare functions & methods.return is used to return from functions.go is used for concurrency.select is used to choose from different types of communication.interface.struct is used for abstract data types.type.if x > 0 {
return y
} esle {
return x
}
if err := MagicFunction(); err != nil {
return err
}
// do somethinggoto you jump to a label which must be defined within the current function./* goto_test */
/* Create a loop */
func gototestfunc() {
i := 0
Here:
fmt.Println()
i++
goto Here
}for loop has three forms, only one of which has semicolons:for init; condition; post { } // aloop using the syntax borrowed from C
for condition { } // a while loop
for { } // a endless loop
sum := 0
for i := 0; i < 10; i++ {
sum = sum + i
}for i := 0; i < 10; i++ {
if i > 5 {
break
}
fmt.Println(i)
}
/* With loops within loop you can specify a label after `break` to identify which loop to stop */
J: for j := 0; j < 5; j++ {
for i := 0; i < 10; i++ {
if i > 5 {
break J
}
fmt.Println(i)
}
}Range:
range can be used for loops. It can loop over slices, arrays, strings, maps & channels.range is an iterator that, when called, returns the next key-value pair from the "thing" it loops over.list := []string{"a", "b", "c", "d", "e", "f"}
for k, v := range list {
// do some fancy thing with k & v
}Switch:
switch has no expression it switches on true.if-else-if-else chain as a switch./* Convert hexadecimal character to an int value */
switch { // switch without condition = switch true
case '0' <= c && c <= '9':
return c - '0'
case 'a' <= c && c <= 'f':
return c - 'a' + 10
case 'A' <= c && c <= 'F':
return c - 'A' + 10
}
return 0
/* Automatic fall through */
switch i {
case 0: fallthrough
case 1:
f()
default:
g()
}close new panic complex
delete make recover real
len append print imag
cap copy println
len is used to return the lengths of strrings, maps, slices & arrays.copy is for copying slices. And append is for concatenating slices.Brief: list -> arrays, slices. dict -> map
Arrays:
[n]<type>.var arr [10]int // The size is part of the type, fixed size
arr[0] = 42
arr[1] = 13
fmt.Printf("The first element is %sn", arr[0])
// Initialize an array to something other than zero, using composite literal
a := [3]int{1, 2, 3}
a := [...]int{1, 2, 3}Slices:
// Init array primes
primes := [6]int{2, 3, 5, 7, 11, 13}
// Init slice s
var s []int = primes[1:4]
fmt.Println(s) // Return [3, 5, 7]
/* slice_length_capacity.go */
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 %vn", len(s), cap(s), s)
}s := make([]byte, 5)len is the number of elements referred to by the slice.
cap is the number of elements in the underlying array (beginning at the element referred to by the slice pointer).
s = s[2:4]Slicing does not copy the slice's data. It creates a new slice that points to the original array. This makes slice operations as efficient as manipulating array indicies. Therefore, modifying the elements (not the slice itself) of a re-slice modifies the elements of the original slice:
d := []byte{'r', 'o', 'a', 'd'}
e := d[2:]
// e = []byte{'a', 'd'}
e[1] = 'm'
// e = []byte{'a', 'm'}
// d = []byte{'r', 'o', 'a', 'm'}s to a length shorter than its capacity. We can grow s to its capacity by slicing it again.s = s[:cap(s)]A slice cannot be grown beyond its capacity.
// Another example
var array [m]int
slice := array[:n]
// len(slice) == n
// cap(slice) == m
// len(array) == cap(array) == mappend & copy.s0 := []int{0, 0}
s1 := append(s0, 2) // same type as s0 - int.
// If the original slice isn't big enough to fit the added values,
// append will allocate a new slice that is big enough. So the slice
// returned by append may refer to a different underlaying array than
// the original slices does.
s2 := append(s1, 3, 5, 7)
s3 := append(s2, s0...) // []int{0, 0, 2, 3, 5, 7, 0, 0} - three dots used after s0 is needed make it clear explicit that you're appending another slice, instead of a single value
var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
var s = make([]int, 6)
// copy function copies slice elements from source to a destination
// returns the number of elements it copied
n1 := copy(s, a[0:]) // n1 = 6; s := []int{0, 1, 2, 3, 4, 5}
n2 := copy(s, s[2:]) // n2 = 4; s := []int{2, 3, 4, 5, 4, 5}Maps:
map type.monthdays := map[string]int{
"Jan": 31, "Feb": 28, "Mar": 31,
"Apr": 30, "May": 31, "Jun": 30,
"Jul": 31, "Aug": 31, "Sep": 30,
"Oct": 31, "Nov": 30, "Dec": 31, // A trailing comma is required
}
value, key := monthdays["Jan"]make when only declaring a map. A map is reference type. A map is not reference variable, its value is a pointer to a runtime.hmap structure.// A struct is a type. It's also a collection of fields
// Declaration
type Vertex struct {
X, Y float64
}
// 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
}map[string]intefaces.// 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// General Function
type mytype int
func (p mytype) funcname(q, int) (r, s int) { return 0,0 }
// p (optional) bind to a specific type called receiver (a function with a receiver is usually called a method)
// q - input parameter
// r,s - return parametersimport "fmt"
func main() {
a := func() { // a is defined as an anonymous (nameless) function,
fmt.Println("Hello")
}
a()
}func printit(x int) {
fmt.Println("%vn", x)
}
func callback(y int, f func(int)) {
f(y)
}/* Open a file & perform various writes & reads on it. */
func ReadWrite() bool {
file.Open("file")
// Do your thing
if failureX {
file.Close()
return false
}
// Repeat a lot of code.
if failureY {
file.Close()
return false
}
file.Close()
return true
}
/* Same situation but using defer */
func ReadWrite() bool {
file.Open("file")
defer file.Close() // add file.Close() to the defer list
// Do your thing
if failureX {
return false
}
if failureY {
return false
}
return true
}Defer functions are executed in LIFO order.for i := 0; i < 5; i++ {
defer fmt.Printf("%d ", i) // 4 3 2 1 0
}defer you can even change return values, provided that you are using named result parameters & a function literal (def func(x int) {/*....*/}(5)).func f() (ret int)
defer func() { // Initialized with zero
ret++
}()
return 0 // This will not be the returned value, because of defer. Ths function f will return 1
}func func1(arg... int) { // the variadic parameter is just a slice.
for _, n := range arg {
fmt.Printf("And the number is: %dn", n)
}
}pacnic, execution of F stops, any deferred functions in F are executed normally, & then F returns to its caller. To the caller, F then behaves like a call to panic. The process continues up the stack until all functions in the current goroutine have returned, at which point the program crashes. Panics can be initiated by invoking panic directly. They can also be caused by runtime errors, such as out-of-bounds array accesses./* defer_panic_recover.go */
package main
import "fmt"
func main() {
f()
fmt.Println("Returned normally from f.")
}
func f() {
defer func() {
if r := recover(); r != nil {
fmt.Println("Recovered in f", r)
}
}()
fmt.Println("Calling g.")
g(0)
fmt.Println("Returned normally from g.")
}
func g(i int) {
if i > 3 {
fmt.Println("Panicking!")
panic(fmt.Sprintf("%v", i))
}
defer fmt.Println("Defer in g", i)
fmt.Println("Printing in g", i)
g(i + 1)
}
/* Result */
// Calling g.
// Printing in g 0
// Printing in g 1
// Printing in g 2
// Printing in g 3
// Panicking!
// Defer in g 3
// Defer in g 2
// Defer in g 1
// Defer in g 0
// Recovered in f 4
// Returned normally from f.package even
func Even(i int) bool { // starts with capital -> exported
return i%2 == 0
}
func odd(i int) bool { // start with lower-case -> private
return i%2 == 1
}mkdir $GOPATH/src/even
cp even.go $GOPATH/src/even
go build
go installimport "even".import bar "bytes".src/compress/gzip is imported as compress/gzip but has name gzip, not compress/gzip.ring.Ring package (package container/ring), would normally be called NewRing, but since Ring is the only type exported by the package, since the package is called ring, it's called just New. Clients of the package see that as ring.New.doc.go that only holds the package comment./*
The regexp package implements a simple library for
regular expressions.
The syntax of the regular expressions accepted is:
regexp:
concatenation { '|' concatenation }
*/
package regexpmain function is entry file for execution.main function, init function is called by Go when a package is initialized. It does not take any arguments & doesn’t return any value. initfunction is implicitly declared by Go. You can have multiple init functions in a file or a package. Order of the execution of init function in a file will be according to the order of their appearances.null container.go run *.go
├── Main package is executed
├── All imported packages are initialized
| ├── All imported packages are initialized (recursive definition)
| ├── All global variables are initialized
| └── init functions are called in lexical file name order
└── Main package is initialized
├── All global variables are initialized
└── init functions are called in lexical file name orderInstalling a 3rd party package is nothing but cloning the remote code into local src/<package> directory. Unfortunately, Go does not support package version or provide package manager but a proposal is waiting here.
testing package & the program go test.package main
func Sum(x int, y int) int {
return x + y
}
func main() {
Sum(5, 5)
}
// Testcase
package main
import "testing"
func TestSum(t *testing.T) {
total := Sum(5, 5)
if total != 10 {
t.Errorf("Sum was incorrect, got: %d, want: %d.", total, 10)
}
}package main
import "testing"
func TestSum(t *testing.T) {
tables := []struct {
x int
y int
n int
}{
{1, 1, 2},
{1, 2, 3},
{2, 2, 4},
{5, 2, 7},
}
for _, table := range tables {
total := Sum(table.x, table.y)
if total != table.n {
t.Errorf("Sum of (%d+%d) was incorrect, got: %d, want: %d.", table.x, table.y, total, table.n)
}
}
}Launching tests:
go testgo test github.com/username/packageHTTP testing:
net/http/httptest sub-package facilitates the testing automation of both HTTP server and client code.httptest.ResponseRecorder is designed specifically to provide unit testing capabilities for exercising the HTTP handler methods by inspecting state changes to the http.ResponseWriter in the tested function.httptest provides type httptest.Server to programmatically create servers to test client requests and send back mock responses to the client.Statement coverage: The go test tool has built-in code-coverage for statements.
$ go test -cover
PASS
coverage: 50.0% of statements
ok github.com/alexellis/golangbasics1 0.009s
# Generate a HTML coverage report.
$ go test -cover -coverprofile=c.out
$ go tool cover -html=c.out -o coverage.htmlCode benchmark: The purpose of benchmarking is to measure a code's performance. The go test command-line tool comes with support for the automated generation and measurement of benchmark metrics. Similar to unit tests, the test tool uses benchmark functions to specify what portion of the code to measure.
Running the benchmark
$> go test -bench=.
PASS
BenchmarkVectorAdd-2 2000000 761 ns/op
BenchmarkVectorSub-2 2000000 788 ns/op
BenchmarkVectorScale-2 5000000 269 ns/op
BenchmarkVectorMag-2 5000000 243 ns/op
BenchmarkVectorUnit-2 3000000 507 ns/op
BenchmarkVectorDotProd-2 3000000 549 ns/op
BenchmarkVectorAngle-2 2000000 659 ns/op
ok github.com/vladimirvivien/learning-go/ch12/vector 14.123sSkipping test functions
> go test -bench=. -run=NONE -v
PASS
BenchmarkVectorAdd-2 2000000 791 ns/op
BenchmarkVectorSub-2 2000000 777 ns/op
Code Testing
[ 314 ]
...
BenchmarkVectorAngle-2 2000000 653 ns/op
ok github.com/vladimirvivien/learning-go/ch12/vector 14.069sComparative benchmarks: to compare the performance of different algorithms that implement similar functionalities. Exercising the algorithms using performance benchmarks will indicate which of the implementations may be more compute and memory efficient.
Isolating dependencies: The Key factor that defines a unit test is isolation from runtime dependencies or collaborators. Check out Dependency Injection.
fmt: Package fmt implements formatted I/O with functions analogous to C'sprintf & scanf. The format verbs are derived from C's but are simpler. Some verbs (%-sequences) that can be used:
io: The package provides basic interfaces to I/O primitives. Its primary job is to wrap existing implementation of such primitives, such as those in package os, into shared public interfaces that abstract the functionality, plus some other related primitives.
bufio: This package implements buffered I/O. It wraps an io.Reader or io.Writer object, creating another object (Reader or Writer) that also implements the interface but provides buffering & some help for textual I/O.
sort: The sort package provides primitives for sorting arrays & user-defined collections.
strconv: The strconv package implements conversions to & from string representations of basic data types.
os: The os package provides a platform-independent interface to operating system functionality. The design is Unix-like.
sync: The package sync provides basic synchronization primitives such as mutual exclusion locks.
flag: The flag package implements command-line flag parsing.
encoding/json: The encoding/json package implements encoding & decoding of JSON objects as defined in RFC 4627.
html/template: Data-driven templates for generating textual output such as HTML.
net/http: The net/http package implements parsing of HTTP requests, replies, & URLs & provides an extensible HTTP server & a basic HTTP client.
unsafe: The unsafe package contains operations that step around the type safety of Go programs. Normally you don’t need this package, but it is worth mentioning that unsafe Go programs are possible.
reflect: The reflect package implements run-time reflection, allowing a program to manipulate objects with arbitrary types. The typical use is to take a value with static type interface{} & extract its dynamic type information by calling TypeOf, which returns an object with interface type Type.
os/exec: The os/exec package runs external commands.
Go has pointers but not pointer arthmetic, so they act more like references than pointers that you may know from C.
var p *int
p++this.Pointers are useful. Remember that when you call a function in Go, the variables are pass-by-value. So, for efficiency & the possibility to modify a passed value in functions we have pointers.
Pointer type (* type) & address-of (&) operators *: If a variable is declared var x int, the expression &x ("address of x") yields a pointer to an integer variable (a value of type * int). If this value is called p, we say "p points to to x", or equivalently "p contains the address of x". The variable to which p points is written *p. The expression *p yields the value of that variable, an int, but since *p denotes a variable, it may also appear on the left-hand side of an assignment, in which case the assignment updates the variable. Reference here
x := 1
p := &x // p, of type *int, points to x
fmt.Println(*p) // "1"
*p = 2 // equivalent to x = 2
fmt.Println(x) // "2"All newly declared variables are assigned their zero value & pointers are no different. A newly declared pointer, or just a pointer that points to nothing, has a nil-value.
var p *int // declare a pointer
fmt.Printf("%v", p)
var i int
p = &i // Make p point to i
fmt.Printf("%v", p) // Print somthing like 0x7ff96b81c000a// Go program to illustrate the
// concept of the Pointer to Pointer
package main
import "fmt"
// Main Function
func main() {
// taking a variable
// of integer type
var V int = 100
// taking a pointer
// of integer type
var pt1 *int = &V
// taking pointer to
// pointer to pt1
// storing the address
// of pt1 into pt2
var pt2 **int = &pt1
fmt.Println("The Value of Variable V is = ", V)
fmt.Println("Address of variable V is = ", &V)
fmt.Println("The Value of pt1 is = ", pt1)
fmt.Println("Address of pt1 is = ", &pt1)
fmt.Println("The value of pt2 is = ", pt2)
// Dereferencing the
// pointer to pointer
fmt.Println("Value at the address of pt2 is or *pt2 = ", *pt2)
// double pointer will give the value of variable V
fmt.Println("*(Value at the address of pt2 is) or **pt2 = ", **pt2)
}
// The Value of Variable V is = 100
// Address of variable V is = 0x414020
// The Value of pt1 is = 0x414020
// Address of pt1 is = 0x40c128
// The value of pt2 is = 0x40c128
// Value at the address of pt2 is or *pt2 = 0x414020
// *(Value at the address of pt2 is) or **pt2 = 100Go also has garbage collection.
To allocate memory Go has 2 primitives, new & make.
new allocates; make initializes.
Constructors & compiste literals
// A lot of boiler plate
func NewFile(fd int, name string) *File {
if fd < 0 {
return nil
}
f := new(File)
f.fd = fd
f.name = name
f.dirinfo = nil
f.nepipe = 0
return f
}
// Using a composite literal
func NewFile(fd int, name string) *File {
if fd < 0 {
return nil
}
f := File{fd, name, nil. 0}
return &f // Return the address of a local variable. The storage associated with the variable survives after the function returns.
// return &File{fd, name, nil, 0}
// return &File{fd: fd, name: name}
}As a limiting case, if a composite literal contains no fields at all, it creates a zero value for the type. The expression new(File) & &File{} are equivalent.
Composite literal can also be created for arrays, slices, & maps, with the field labels being indices or map keys as appropriate.
ar := [...]string{Enone: "no error", Einval: "invalid argument"}
sl := []string{Enone: "no error", Einval: "invalid argument"}
ma := map[int]string {Enone: "no error", Einval: "invalid argument"}/* defining_own_type.go */
package main
import "fmt"
type NameAge struct {
name string // both non exported fiedls
age int
}
func main() {
a := new(NameAge)
a.name = "Kien"
a.age = 25
fmt.Printf("%vn", a) // &{Kien, 25}
}struct {
x, y int
A *[]int
F func()
}Methods:
func doSomething1(n1 *NameAge, n2 int) {/* */}
// method call
var n *NameAge
n.doSomething1(2)func (n1 *NameAge) doSomething2(n2 int) {/* */}NOTE: If x is addressable & &x's method set contains m, x.m() is shorthand for (&x).m().
// A mutex is a data type with two methods, Lock & Unlock
type Mutex struct {/* Mutex fields */}
func (m *Mutex) Lock() {/* Lock impl */}
func (m *Mutext) Unlock {/* Unlock impl */}
// NewMutex is equal to Mutex, but it does not have any of the methods of Mutex.
type NewMutex Mutex
// PrintableMutex hash inherited the method set from Mutex, contains the methods
// Lock & Unlock bound to its anonymous field Mutex
type PrintableMutex struct{Mutex}FROM b []byte i []int r []rune s string f float32 i int
TO
[]byte . []byte(s)
[]int . []int(s)
[]rune []rune(s)
string string(b) string(i) string(r) .
float32 . float32(i)
int int(f) .
string to a slice of bytes or runesmystring := "hello this is string"
byteslice := []byte(mystring)
runeslice := []rune(string)b := []byte{'h', 'e', 'l', 'l', 'o'} // Composite literal
s := string(b)
i := []rune(26, 9, 1994)
r := string(i)For numeric values:
uint8(int).int(float32). This discards the fraction part from the floating point value.float32(int)User defined types & conversions
type foo struct { int } // Anonymous struct field
type bar foo // bar is an alias for foo
var b bar = bar{1} // Declare `b` to be a `bar`
var f foo = b // Assign `b` to `f` --> Cannot use b (type bar) as type foo in assignment
var f foo = foo(b) // OK!/* a struct type S with 1 field, 2 methods */
type S struct { i int }
func (p *S) Get() int { return p.i }
func (p *S) Put(v int) { p.i = v }
/* an interface type */
type I interface {
Get() int
Put() int
}
/* S is a valid implementation for interface I */func f(p I) {
fmt.Println(p.Get())
p.Put(1)
}
var s S
/* Because S implements I, we can call the
function f passing in a pointer to a value
of type S */
/* The reason we need to take the address of s,
rather than a value of type S, is because
we defined the methods on s to operae on pointers */
f(&s)The fact that you do not need to declare whether or not a type implements an interface means that Go implements a form of duck typing. This is not pure duck typing, because when possible the Go complier will statically check whether the type implements the inerface. However, Go does have a purely dynamic aspect, in that you can convert from one interface to another. In the general case, that conversion is checked at run time. If the conversion is invalid - if the type of the value stored in the existing interface value does not satisfy the interface to which it is being converted - the program will fail with a run time error.
package main
import "fmt"
type Duck interface {
Quack()
}
type Donald struct {
}
func (d Donald) Quack() {
fmt.Println("quack quack!")
}
type Daisy struct {
}
func (d Daisy) Quack() {
fmt.Println("-quack -quack")
}
func sayQuack(duck Duck) {
duck.Quack()
}
type Dog struct {
}
func (d Dog) Bark() {
fmt.Println("go go")
}
func main() {
donald := Donald{}
sayQuack(donald) // quack
daisy := Daisy{}
sayQuack(daisy) // --quack
dog := Dog()
sayQuack(dog) // compile error - cannot use dog (type Dog) as type Duck
}Go's interfaces let you use duck typing like you would in a purely dynamic language like Python but still have the compiler catch obvious mistakes like passing an int where an object with a Read method was expected, or like calling the Read method with the wrong number of arguments.
type R struct { i int }
func (p * R) Get() int { return p.i }
func (p *R) Put(v int) { p.i = v }
func f(p I) {
switch t := p.(type) {
case *S:
case *R:
default:
}
}func g(something interface{}) int {
return something.(I).Get()
}.(I) is a type assertion which converts something to an interface of type I. If we have the type we can invoke the Get() function.s = new(S)
fmt.Println(g(s))Methods are functions that have a receiver.
You can definen methods on any type (except on non-local types, this includes built-in types: the type int can not have methods).
Methods on interface types
By convention, one-method interfaces are named by the method name plus the -er suffix: Reader, Writer, Formatter,...
Pointer & Non-pointer method receivers.
func (s *MyStruct) pointerMethod() {} // method on pointer
func (s MyStruct) valueMethod() {} // method on valuepackage main
import "fmt"
type Mutatable struct {
a int
b int
}
func (m Mutatable) StayTheSame() {
m.a = 5
m.b = 7
}
func (m *Mutatable) Mutate() {
m.a = 5
m.b = 7
}
func main() {
m := &Mutatable{0, 0}
fmt.Println(m)
m.StayTheSame()
fmt.Println(m)
m.Mutate()
fmt.Println(m)
}struct for instance, it will be much cheaper to use a pointer receiver.struct, a value receiver is very cheap so unless the semantics of the methods requires a pointer, a value receiver is effient & clear.package main
import "fmt"
func do(i interface{}) {
switch v := i.(type) {
case int:
fmt.Printf("Twice %v is %vn", v, v*2)
case string:
fmt.Printf("%q is %v bytes longn", v, len(v))
default:
fmt.Printf("I don't know about type %T!n", v)
}
}
func main() {
do(21)
do("hello")
do(true)
}
// Twice 21 is 42
// "hello" is 5 bytes long
// I don't know about type bool!ready("Tea", 2) // Normal function call
go ready("Tea", 2) // .. Bum! Here is goroutine
/* X ready example */
package main
import (
"fmt"
"time"
)
func ready(w string, sec int) {
time.Sleep(time.Duration(sec) * time.Second)
fmt.Println(w, "is ready!")
}
func main() {
go ready("Tea", 2) // Tea is ready - After 2 second (3)
go ready("Coffee", 1) // Coffee is ready - After 1 second (2)
fmt.Println("I'm waiting") // Right away (1)
// If did not wait for the goroutines, the program would be terminated
// immediately & any running goroutines would die with it!
time.Sleep(5 * time.Second)
}/* Define a channel, we must also define the type of
the values we can send on the channel */
ci := make(chan int)
cs := make(chan string)
cf := make(chan interface{})
ci <- 1 // Send the integer 1 to the channel ci
<-ci // Receive an integer from the channel ci
i := <-ci // Receive from the channel ci & store it in ipackage main
import (
"fmt"
"time"
)
var c chan int
func ready(w string, sec int) {
time.Sleep(time.Duration(sec) * time.Second)
fmt.Println(w, "is ready!")
c <- 1
}
func main() {
c = make(chan int)
go ready("Tea", 2)
go ready("Coffee", 1)
fmt.Println("I'm waiting")
<-c // Wait until we receive a value from the channel
<-c
}Buffered channel:
Buffered channel has capacity.
Buffered channel is used to perform asynchronous communcation.
A buffered channel has no such guarantee.
A receive will block only if there's no value in the channel to receive.
A send will block only if there's no available buffer to place the value being sent.
Unbuffered channel:
Unbuffered channel has no capacity and therefore require both goroutines to be ready to make any exchange.
Unbuffered channel is used to perform synchronous communication between goroutines. Unbuffered channel provides a guarantee that an exchange between 2 goroutines is performed at the instant the send and receive take place.
Synchronization is fundamental in the interaction between the send and receive on the channel.
unbuffered := make(chan int) // Unbuffered channel of integer type
buffered := make(chan int, 10) // Buffered channel of integer typeselect.L: for {
select {
case <-c:
i++
if i > 1 {
break L
}
}
}package main
import "fmt"
func fibonacci(c, quit chan int) {
x, y := 0, 1
for {
select {
case c <- x:
x, y = y, x+y
case <-quit:
fmt.Println("quit")
return
}
}
}
func main() {
c := make(chan int)
quit := make(chan int)
go func() {
for i := 0; i < 10; i++ {
fmt.Println(<-c)
}
quit <- 0
}()
fibonacci(c, quit)
}
// 2
// 3
// 5
// 8
// 13
// 21
// 34
// quit// Default selection
// The default case in a select is run if no other case is ready.
// Use a default case to try a send or receive without blocking:
// select {
// case i := <-c:
// // use i
// default:
// // receiving from c would block
// }
package main
import (
"fmt"
"time"
)
func main() {
tick := time.Tick(100 * time.Millisecond)
boom := time.After(500 * time.Millisecond)
for {
select {
case <-tick:
fmt.Println("tick.")
case <-boom:
fmt.Println("BOOM!")
return
default:
fmt.Println(" .")
time.Sleep(50 * time.Millisecond)
}
}
}While our goroutines were running concurrently, they were not running in parallel! (Once more time, make sure you know that Concurrency is not Parralel!)
With runtime.GOMAXPROCS(n) or set an environment variable GOMAXPROCS you can set the number of goroutines that can run in parallel.
From version 1.5 & above, GOMAXPROCS defaults to the number of CPU cores.
ch := make(chan type, value)
// if value == 0 -> unbuffered
// if value > 0 -> buffer value elementsx, ok = <-ch
/* Where ok is set to True the channel is not closed & we've read something
Otherwise ok is set to False. In that case the channel was closed & the value
received is a zero value of the channel's type.io.Reader & io.Writer.io.Reader is an important interface in the language Go. A lot (if not all) functions that need to read from something take an io.Reader as input.io.Writer has the Write method.io.Reader or io.Writer interface, the whole standard Go library can be used on that type.os.Args.flag package has a more sophisticated interface, & also provided a way to parse flags.os/exec package has functions to run external commands, & is the premier way to execute commands from within a Go program.import "os/exec"
cmd := exec.Command("/bin/ls", "-l")
// Just run without doing anything with the returned data
err := cmd.Run()
// Capturing the standard output
buf, err := cmd.Output() // buf is byte slicenet.Dial. When you Dial into a remote system the function returns a Conn interface type, which can be used to send & receive information. The function Dial neatly abstracts away the network family & transport.conn, e := Dial("tcp", "192.0.32.10:80")
conn, e := Dial("udp", "192.0.32.10:80")
conn, e := Dial("tcp", "[2620:0:2d0:200::10]:80")Go 1.11 includes preliminary support for modules, Go's new dependency management system that makes dependency version information explicit and easier to manage.
go.mod file in the tree's root directory. Module source code may be located outside of GOPATH. There are four directives: module, require, replace, exclude.requirein go.mod, most go commands like 'go build' & 'go test' will automatically look up the proper module & add the highest version of that new direct dependency to your module's go.mod as a require directive. For example, if your new import corresponds to dependency M whose latest tagged release version is v1.2.3, your module's go.mod will end up with require M v1.2.3, which indicates module M is a dependency with allowed version >= v1.2.3 (and < v2, given v2 is considered incompatible with v1).v1.2.3)./vN at the end of the module paths used in go.mod files (e.g., module github.com/my/mod/v2, require github.com/my/mod/v2 v2.0.0) & in the package import path (e.g., import "github.com/my/mod/v2/mypkg").go.mod, provides the directory is outside $GOPATH/src. (Inside $GOPATH/src, for compatibility, the go command still runs in the old GOPATH mode, even if a go.mod is found)# Create a directory outside of your GOPATH:
$ mkdir -p /tmp/scratchpad/hello
$ cd /tmp/scratchpad/hello
# Initialize a new module:
$ go mod init github.com/you/hello
go: creating new go.mod: module github.com/you/hello
# Write your code
$ cat <<EOF > hello.go
package main
import (
"fmt"
"rsc.io/quote"
)
func main() {
fmt.Println(quote.Hello())
}
EOF
# Introduce `go mod tidy`
# Tidy makes sure go.mod matches the source code in the module.
# It adds any missing modules necessary to build the current module's
# packages and dependencies, and it removes unused modules that
# don't provide any relevant packages. It also adds any missing entries
# to go.sum and removes any unnecessary ones
$ go mod tidy t/s/hello ﳑ
go: finding module for package rsc.io/quote
go: downloading rsc.io/quote v1.5.2
go: found rsc.io/quote in rsc.io/quote v1.5.2
go: downloading rsc.io/sampler v1.3.0
go: downloading golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
$ cat go.mod
module github.com/you/hello
require rsc.io/quote v1.5.2
# Add a new dependency often brings in other indirect dependencies too
# List the current module and all its dependencies
$ go list -m all
github.com/you/hello
golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
rsc.io/quote v1.5.2
rsc.io/sampler v1.3.0
# In addition to go.mod, there is a go.sum file containing the expected
# cryptographic hashes of the content of specific module versions
$ cat go.sum t/s/hello ﳑ
golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:qgOY6WgZOaTkIIMiVjBQcw93ERBE4m30iBm00nkL0i8=
golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
rsc.io/quote v1.5.2 h1:w5fcysjrx7yqtD/aO+QwRjYZOKnaM9Uh2b40tElTs3Y=
rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
rsc.io/sampler v1.3.0 h1:7uVkIFmeBqHfdjD+gZwtXXI+RODJ2Wc4O7MPEh/QiW4=
rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
# Build & run
$ go build
$ ./hello
Hello, world.# From the output of go list -m all, we're using an untagged version of golang.org/x/text
# Let's upgrade to the latest tagged version
$ go get golang.org/x/text t/s/hello ﳑ
go: downloading golang.org/x/text v0.7.0
go: upgraded golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c => v0.7.0
$ cat go.mod t/s/hello ﳑ
module github.com/you/hello
go 1.19
require rsc.io/quote v1.5.2
require (
golang.org/x/text v0.7.0 // indirect
rsc.io/sampler v1.3.0 // indirect
)
$ go list -m all t/s/hello ﳑ
github.com/you/hello
golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4
golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f
golang.org/x/text v0.7.0
golang.org/x/tools v0.1.12
rsc.io/quote v1.5.2
rsc.io/sampler v1.3.0go mod tidy and Go does the rest.go tool provides go mod vendor command.
go mod vendor command constructs a directory named vendor in the main module's root directory that contains copies of all packages needed to support builds and tests of packages in the main modules.go mod vendor also creates the file vendor/modules.txt that contains a list of vendored packages and the module versions they were copied from.vendor to your Version Control System, then copy this around.$ go mod vendor
# Main module's directory structure
$ tree -L 3
├── go.mod
├── go.sum
├── hello.go
└── vendor
├── golang.org
│ └── x
├── modules.txt
└── rsc.io
├── quote
└── samplergo tool defaults to downloading modules from the public Go module mirror: https://proxy.golang.org and also defaults to validating downloaded modules (regardless of source) against the public Go checksum database at https://sum.golang.org.export GOPROXY=https://goproxy.io,directgo command defaults to downloading modules from the public Go module mirror, therefore if you have private code, you most likely should configure the GOPRIVATE setting (such as go env -w GOPRIVATE=*.corp.com,github.com/secret/repo), or the more fine-grained variants GONOPROXY or GONOSUMDB that support less frequent use cases. See the documentation for more details.go.work. The dependencies in this file can span multiple modules and anything declared in the go.work file will override dependencies in the module's go.mod.
go.work file that specifies relative paths to the module directories of each the modules in the workspace. When no go.work file exists, the workspace consists of the single module containing the current directory.
go.work files are defined in exactly the same way as for go.mod files.go 1.18
use ./my/first/thing
use ./my/second/thing
// or
// use (
// ./my/first/thing
// ./my/second/thing
// )
replace example.com/bad/thing v1.4.5 => example.com/good/thing v1.4.5$ mkdir workspace
$ cd workspace
# Create hello module
$ mkdir hello
$ cd hello
$ go mod init eaxmple.com/hello
go: creating new go.mod: module example.com/hello
$ cat <<EOF > hello.go
package main
import (
"fmt"
"golang.org/x/example/stringutil"
)
func main() {
fmt.Println(stringutil.Reverse("Hello"))
}
EOF
$ go mod tidy
go: finding module for package golang.org/x/example/stringutil
go: found golang.org/x/example/stringutil in golang.org/x/example v0.0.0-20220412213650-2e68773dfca0
$ go run example.com/hello
olleH
# Create the workspace
$ cd ../
$ go work init ./hello
$ tree
.
├── go.work
└── hello
├── go.mod
├── go.sum
└── hello.go
1 directory, 4 files
# Go command includes all the modules in the workspace as main modules. This allow us to refer to a package in the module
# even outside the module.
$ go run example.com/hello
olleH
# Download and modify the golang.org/x/example module
$ git clone https://go.googlesource.com/example
Cloning into 'example'...
remote: Total 165 (delta 27), reused 165 (delta 27)
Receiving objects: 100% (165/165), 434.18 KiB | 1022.00 KiB/s, done.
Resolving deltas: 100% (27/27), done.
# Add module to the workspace
$ go work use ./example
$ tree -L 1
.
├── example
├── go.work
└── hello
2 directories, 1 file
$ cat go.work
go 1.20
use (
./example
./hello
)
$ cd example/stringutil
# Create a new file
$ cat <<EOF > toupper.go
package stringutil
import "unicode"
// ToUpper uppercases all the runes in its argument string.
func ToUpper(s string) string {
r := []rune(s)
for i := range r {
r[i] = unicode.ToUpper(r[i])
}
return string(r)
}
EOF
# Modify hello program
$ cd ../../hello
$ cat <<EOF > hello.go
package main
import (
"fmt"
"golang.org/x/example/stringutil"
)
func main() {
fmt.Println(stringutil.ToUpper("Hello"))
}
EOF
$ cd ..
# Go command finds the example.com/hello module specified in the command line
# in the hello directory specified by the go.work file, and similiarly
# resolves the golang.org/x/example import using the go.work file.
$ go run example.com/hello
HELLOSource: https://go.dev/doc/modules/layout
Go projects can include packages, command-line programs or a combination of the two. This guide is organized by project type.
NOTE: throughout this document, file/package names are entirely arbitrary
project-root-directory/
go.mod
modname.go
modname_test.go
auth.go
auth_test.go
hash.go
hash_test.gomodname.go declares the package with:package modname
// ... package code hereproject-root-directory/
go.mod
auth.go
auth_test.go
client.go
main.gomain.go file contains func main, but this is just a convention. The “main” file can also be called modname.go (for an appropriate value of modname) or anything else.internal; this prevents other modules from depending on packages we don’t necessarily want to expose and support for external uses. Since other projects cannot import code from our internal directory, we’re free to refactor its API and generally move things around without breaking external users. The project structure for a package is thus:project-root-directory/
internal/
auth/
auth.go
auth_test.go
hash/
hash.go
hash_test.go
go.mod
modname.go
modname_test.goproject-root-directory/
go.mod
modname.go
modname_test.go
auth/
auth.go
auth_test.go
token/
token.go
token_test.go
hash/
hash.go
internal/
trace/
trace.gomodule github.com/someuser/modnameproject-root-directory/
go.mod
internal/
... shared internal packages
prog1/
main.go
prog2/
main.gomain. A top-level internal directory can contain shared packages used by all commands in the repository.cmd directory; while this isn’t strictly necessary in a repository that consists only of commands, it’s very useful in a mixed repository that has both commands and importable packages, as we will discuss next.project-root-directory/
go.mod
modname.go
modname_test.go
auth/
auth.go
auth_test.go
internal/
... internal packages
cmd/
prog1/
main.go
prog2/
main.gointernal directory. Moreover, since the project is likely to have many other directories with non-Go files, it’s a good idea to keep all Go commands together in a cmd directory:project-root-directory/
go.mod
internal/
auth/
...
metrics/
...
model/
...
cmd/
api-server/
main.go
metrics-analyzer/
main.go
...
... the project's other directories with non-Go codeGo models data input and output as a stream that flows from sources to targets. Data sources, such as files, network connections, or even some in-memory objects , can be modeled as streams of bytes from which data can be read or written to.
The most common usage of the fmt package is for writting to standard output and reading from standard input.
type metalloid struct {
name string
number int32
weight float64
}
func main() {
var metalloids = []metalloid{
{"Boron", 5, 10.81},
...
{"Polonium", 84, 209.0},
}
file, _ := os.Create("./metalloids.txt")
defer file.Close()
for _, m := range metalloids {
fmt.Fprintf(
file,
"%-10s %-10d %-10.3fn",
m.name, m.number, m.weight,
)
}
}The bufio package offers several functions to do buffered writing of IO streams using an `io.Writer interface.
In bytes package offers common primitives to achieve streaming IO on blocks of bytes stored in memory, represented by the bytes.Buffer byte. Since the bytes.Buffer type implements both io.Reader and io.Writer interfaces it is a great option to stream data into or out of memory using streaming IO primitives.
package main
import (
"encoding/json"
"fmt"
)
type Measurement struct {
Height int
Weight int
}
type Person struct {
Name string
Age int
Measurement Measurement // Nested object
}
func main() {
bob := &Person{
Name: "Bob",
Age: 20,
}
bobRaw, _ := json.Marshal(bob)
fmt.Println(string(bobRaw))
// Raw data without Measurement field
aliceRaw := []byte(`{"name": "Alice", "age": 23}`)
var alice Person
if err := json.Unmarshal(aliceRaw, &alice); err != nil {
panic(err)
}
fmt.Printf("%+vn", alice)
}
// {"Name":"Bob","Age":20,"Measurement":{"Height":190,"Weight":75}}
// {Name:Alice Age:23 Measurement:{Height:0 Weight:0}}package main
import (
"encoding/json"
"fmt"
)
type Measurement struct {
Height int `json:"height"`
Weight int `json:"weight"`
}
type Person struct {
Name string `json:"who"`
Age int `json:"how old"`
Measurement Measurement `json:"mm"`
}
func main() {
bob := &Person{
Name: "Bob",
Age: 20,
}
bobRaw, _ := json.Marshal(bob)
fmt.Println(string(bobRaw))
// Raw data without Measurement field
aliceRaw := []byte(`{"who": "Alice", "how old": 23, "mm": {"height": 150, "weight": 40}}`)
var alice Person
if err := json.Unmarshal(aliceRaw, &alice); err != nil {
panic(err)
}
fmt.Printf("%+v", alice)
}
// {"who":"Bob","how old":20,"mm":{"height":0,"weight":0}}
// {Name:Alice Age:23 Measurement:{Height:150 Weight:40}}package main
import (
"encoding/json"
"fmt"
)
func main() {
// Raw data without Measurement field
aliceRaw := []byte(`{"name": "Alice", "age": 23, "measurement": {"height": 150, "weight": 40}}`)
var alice map[string]interface{}
if err := json.Unmarshal(aliceRaw, &alice); err != nil {
panic(err)
}
// the object stored in the "mesurement" key is also stored
// as a map[string]interface{} type, and its type is asserted
// the interface{} type
measurement := alice["measurement"].(map[string]interface{})
fmt.Printf("%+vn", alice)
fmt.Printf("%+vn", measurement)
}
// map[age:23 measurement:map[height:150 weight:40] name:Alice]
// map[height:150 weight:40]omitempty property.package main
import (
"encoding/json"
"fmt"
)
type Measurement struct {
Height int `json:"height"`
Weight int `json:"weight"`
}
type Person struct {
Name string `json:"name"`
Age int `json:"age,omitempty"`
Measurement Measurement `json:"measurement"`
}
func main() {
bob := &Person{
Name: "Bob",
Measurement: Measurement{
Height: 190,
Weight: 75,
},
}
bobRaw, _ := json.Marshal(bob)
fmt.Println(string(bobRaw))
}
// Age field is ignored
// {"name":"Bob","measurement":{"height":190,"weight":75}}NOTE: There are a lot more helpful things in tips-notes. You may want to check it out.
Go Web Example
A basic HTTP server has a few key jobs to take care of:
package main
import (
"fmt"
"net/http"
)
func main() {
// Process dynamic request
http.HandleFunc("/", func (w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Welcome to my website!")
})
// Serving static assets
fs := http.FileServer(http.Dir("static/"))
http.Handle("/static/", http.StripPrefix("/static/", fs))
// Accept connections
http.ListenAndServe(":80", nil)
}A simple logging middleware.
// basic-middleware.go
package main
import (
"fmt"
"log"
"net/http"
)
func logging(f http.HandlerFunc) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
log.Println(r.URL.Path)
f(w, r)
}
}
func foo(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "foo")
}
func bar(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "bar")
}
func main() {
http.HandleFunc("/foo", logging(foo))
http.HandleFunc("/bar", logging(bar))
http.ListenAndServe(":8080", nil)
}A middleware in itself simple takes a http.HandleFunc as one of its parameters, wraps it & returns a new http.HandlerFunc for the server to call.
Define a new type Middleware which makes it eventually easier to chain multiple middlewares together.
How a new middleware is created, boilerplate code:
func newMiddleware() Middleware {
// Create a new Middleware
middleware := func(next http.HandlerFunc) http.HandlerFunc {
// Define the http.HandlerFunc which is called by the server eventually
handler := func(w http.ResponseWriter, r *http.Request) {
// ... do middleware things
// Call the next middleware/handler in chain
next(w, r)
}
// Return newly created handler
return handler
}
// Return newly created middleware
return middleware
}// advanced-middleware.go
package main
import (
"fmt"
"log"
"net/http"
"time"
)
type Middleware func(http.HandlerFunc) http.HandlerFunc
// Logging logs all requests with its path & the time it took to process
func Logging() Middleware {
// Create a new Middleware
return func(f http.HandlerFunc) http.HandlerFunc {
// Define the http.HandlerFunc
return func(w http.ResponseWriter, r *http.Request) {
// Do middleware things
start := time.Now()
defer func() { log.Println(r.URL.Path, time.Since(start)) }()
// Call the next middleware/handler in chain
f(w, r)
}
}
}
// Method ensures that url can only be requested with a specific method, else returns a 400 Bad Request
func Method(m string) Middleware {
// Create a new Middleware
return func(f http.HandlerFunc) http.HandlerFunc {
// Define the http.HandlerFunc
return func(w http.ResponseWriter, r *http.Request) {
// Do middleware things
if r.Method != m {
http.Error(w, http.StatusText(http.StatusBadRequest), http.StatusBadRequest)
return
}
// Call the next middleware/handler in chain
f(w, r)
}
}
}
// Chain applies middlewares to a http.HandlerFunc
func Chain(f http.HandlerFunc, middlewares ...Middleware) http.HandlerFunc {
for _, m := range middlewares {
f = m(f)
}
return f
}
func Hello(w http.ResponseWriter, r *http.Request) {
fmt.Fprintln(w, "hello world")
}
func main() {
http.HandleFunc("/", Chain(Hello, Method("GET"), Logging()))
http.ListenAndServe(":8080", nil)
}This section is mainly taken from: https://github.com/zalopay-oss/go-advanced/blob/master/ch3-rpc/ch3-01-rpc-go.md
// 13/rpc/rpcserver/main.go
package main
import (
"log"
"net"
"net/rpc"
)
type HelloService struct{}
// Only methods that satisfy these criteria will be made available for remote access; other methods will be ignored:
// - the method's type is exported.
// - the method is exported.
// - the method has two arguments, both exported (or builtin) types.
// - the method's second argument is a pointer.
// - the method has return type error.
// func (t *T) MethodName(argType T1, replyType *T2) error
func (p *HelloService) Hello(request string, reply *string) error {
*reply = "Hello " + request
return nil
}
func main() {
rpc.RegisterName("HelloService", new(HelloService))
listener, err := net.Listen("tcp", ":8081")
if err != nil {
log.Fatal("Listen TCP error:", err)
}
log.Println("Server is ready")
for {
// accept connection
conn, err := listener.Accept()
if err != nil {
log.Fatal("Accept error:", err)
}
// serve client in another goroutine
go func() {
log.Println("Accept new client:", conn.RemoteAddr())
rpc.ServeConn(conn)
}()
}
}// 13/rpc/rpcclient/main.go
package main
import (
"log"
"net/rpc"
)
func main() {
client, err := rpc.Dial("tcp", "localhost:8081")
if err != nil {
log.Fatal("Dialing error:", err)
}
var reply string
if err = client.Call("HelloService.Hello", "Kien", &reply); err != nil {
log.Fatal(err)
}
log.Println(reply)
}# Server
$ go run examples/13/rpc/rpcserver/main.go
2023/08/09 16:29:29 Server is ready
2023/08/09 16:29:30 Accept new client: 127.0.0.1:38728
2023/08/09 16:29:31 Accept new client: 127.0.0.1:38734
# Client
$ go run examples/13/rpc/rpcclient/main.go
2023/08/09 16:29:30 Hello Kien
$ go run examples/13/rpc/rpcclient/main.go
2023/08/09 16:29:31 Hello Kienprotoc:# Ubuntu
# https://grpc.io/docs/protoc-installation/#install-using-a-package-manager
$ sudo apt install -y protobuf-compiler
# install go plugin
$ go install github.com/golang/protobuf/protoc-gen-go@latesthello.proto:// version proto3
syntax = "proto3";
// generated package name
package main;
message String {
string value = 1;
}Generate Golang source code:
gRPC is a high performance, open-source remote procedure call (RPC) framework that can run anywhere. It enables client and server applications to communicate transparently, and makes it easier to build connected systems.
The gRPC server implements the service interface and runs an RPC server to handle client calls to its service methods. On the client side, the client has a stub (referred to as just a client in some languages) that provides the same methods as the server.
This section is about the new packages be added.
unique packageSource: https://go.dev/blog/unique
var internPool map[string]string
// Intern returns a string that is equal to s but that may share storage with
// a string previously passed to Intern.
func Intern(s string) string {
pooled, ok := internPool[s]
if !ok {
// Clone the string in case it's part of some much bigger string.
// This should be rare, if interning is being used well.
pooled = strings.Clone(s)
internPool[pooled] = pooled
}
return pooled
}unique package introduces a function similar to Intern called Make. But it also differs from Intern in two important ways:
Handle[T] has the property that two Handle[T] values are equal if and only if the values used to create them are equal. The comparison of two Handle[T] values is cheap: it comes down to a pointer comparison.net/netip package in the standard library, which interns values of type addrDetail, part of the netip.Addr structure.
netip.Addr, while the fact that they're canonicalized mean netip.Addr values are more efficient to compare, since comparing zone names becaomes a simple pointer comparison.// Addr represents an IPv4 or IPv6 address (with or without a scoped
// addressing zone), similar to net.IP or net.IPAddr.
type Addr struct {
// Other irrelevant unexported fields...
// Details about the address, wrapped up together and canonicalized.
z unique.Handle[addrDetail]
}
// addrDetail indicates whether the address is IPv4 or IPv6, and if IPv6,
// specifies the zone name for the address.
type addrDetail struct {
isV6 bool // IPv4 is false, IPv6 is true.
zoneV6 string // May be != "" if IsV6 is true.
}
var z6noz = unique.Make(addrDetail{isV6: true})
// WithZone returns an IP that's the same as ip but with the provided
// zone. If zone is empty, the zone is removed. If ip is an IPv4
// address, WithZone is a no-op and returns ip unchanged.
func (ip Addr) WithZone(zone string) Addr {
if !ip.Is6() {
return ip
}
if zone == "" {
ip.z = z6noz
return ip
}
ip.z = unique.Make(addrDetail{isV6: true, zoneV6: zone})
return ip
}There is the page lists a few resources for programmers interested in learning about the Golang.
Oops, actually you can refer to awesome-go for a complete list.
