Dasar

Hello World

package main
import "fmt"
func main() {
    fmt.Println("Hello, World!")
}

Jalankan & Build

go run main.go        # compile and run
go build -o app .     # compile to binary
go test ./...         # run all tests

Inisialisasi Module

go mod init github.com/user/project
go mod tidy           # sync dependencies

Variabel & Tipe

Deklarasi

var name string = "Go"
age := 15               // short declaration
var x, y int = 1, 2
const Pi = 3.14159

Tipe Dasar

bool	`true`, `false`
string	Urutan byte UTF-8 immutable
int, int8..int64	Integer bertanda (platform / lebar tetap)
uint, uint8..uint64	Integer tak bertanda
float32, float64	Bilangan pecahan IEEE-754
byte	Alias untuk `uint8`
rune	Alias untuk `int32` (Unicode code point)

Nilai Zero

int, float	`0`
bool	`false`
string	`""` (string kosong)
pointer, slice, map	`nil`

Fungsi

Fungsi Dasar

func add(a, b int) int {
    return a + b
}

Beberapa Nilai Kembalian

func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("division by zero")
    }
    return a / b, nil
}

Variadic & Anonim

func sum(nums ...int) int {
    total := 0
    for _, n := range nums { total += n }
    return total
}
double := func(x int) int { return x * 2 }

Defer

func readFile(path string) {
    f, _ := os.Open(path)
    defer f.Close()   // runs when function returns
}

Alur Kontrol

If / Else

if x > 0 {
    fmt.Println("positive")
} else if x == 0 {
    fmt.Println("zero")
} else {
    fmt.Println("negative")
}

Perulangan For

for i := 0; i < 10; i++ { }  // classic
for x < 100 { x *= 2 }       // while-style
for { break }                 // infinite
for i, v := range slice { }   // range

Switch

switch day {
case "Mon", "Tue":
    fmt.Println("early week")
case "Fri":
    fmt.Println("TGIF")
default:
    fmt.Println("other")
}

Struct & Method

Definisi Struct

type User struct { Name string Email string Age int } u := User{Name: "Alice", Email: "[email protected]", Age: 30}

Method

func (u User) Greeting() string {
    return "Hi, " + u.Name
}
func (u *User) SetAge(age int) {
    u.Age = age   // pointer receiver mutates
}

Embedding

type Admin struct {
    User          // embedded struct
    Level string
}
a := Admin{User: User{Name: "Bob"}, Level: "super"}
fmt.Println(a.Name)  // promoted field

Interface

Definisi & Implementasi

type Stringer interface {
    String() string
}
// implicit implementation — no "implements" keyword
func (u User) String() string {
    return u.Name
}

Interface Umum

io.Reader	`Read(p []byte) (n int, err error)`
io.Writer	`Write(p []byte) (n int, err error)`
fmt.Stringer	`String() string`
error	`Error() string`

Type Assertion

var i interface{} = "hello"
s, ok := i.(string)   // ok == true
switch v := i.(type) {
case string: fmt.Println(v)
case int:    fmt.Println(v * 2)
}

Goroutine & Channel

Goroutine

go func() {
    fmt.Println("running concurrently")
}()
time.Sleep(time.Second)

Channel

ch := make(chan int)       // unbuffered
buf := make(chan int, 5)   // buffered
ch <- 42                   // send
val := <-ch                // receive

Select

select {
case msg := <-ch1:
    fmt.Println(msg)
case ch2 <- 42:
    fmt.Println("sent")
case <-time.After(time.Second):
    fmt.Println("timeout")
}

Pola Umum

sync.WaitGroup	Menunggu beberapa goroutine selesai
sync.Mutex	Lock mutual exclusion untuk shared state
context.Context	Pembatalan, deadline, nilai request-scoped

Penanganan Error

Pola Dasar

result, err := doSomething()
if err != nil {
    return fmt.Errorf("failed: %w", err)
}

Custom Error

type NotFoundError struct {
    ID string
}
func (e *NotFoundError) Error() string {
    return "not found: " + e.ID
}

Package errors

errors.New(msg)	Buat error sederhana
fmt.Errorf("%w", err)	Bungkus error dengan konteks
errors.Is(err, target)	Periksa rantai error untuk kecocokan
errors.As(err, &target)	Ekstrak typed error dari rantai

Slice & Map

Slice

s := []int{1, 2, 3}
s = append(s, 4, 5)
sub := s[1:3]             // [2, 3]
cp := make([]int, len(s))
copy(cp, s)

Map

m := map[string]int{"a": 1, "b": 2}
m["c"] = 3
val, ok := m["a"]        // ok == true
delete(m, "b")
for k, v := range m { }

Operasi Slice

len(s)	Jumlah elemen
cap(s)	Kapasitas array dasar
append(s, elems...)	Tambah elemen, mungkin realokasi
copy(dst, src)	Salin elemen antar slice
slices.Sort(s)	Urutkan slice (Go 1.21+ package `slices`)

Package & Import

Gaya Import

import "fmt"
import (
    "os"
    "strings"
    "github.com/user/pkg"
)

Visibilitas

Huruf kapital di awal = exported (public). Huruf kecil di awal = unexported (package-private). Tidak perlu keyword seperti public/private.

Standard Library Umum

fmt	I/O terformat (Print, Sprintf, Errorf)
os	Fungsi OS (file, env, args)
io	Primitif I/O (Reader, Writer)
net/http	HTTP client dan server
encoding/json	Encode/decode JSON
strings	Fungsi manipulasi string
strconv	Konversi string ↔ angka
testing	Framework unit test

Generics

Parameter Tipe

func Map[T, U any](s []T, f func(T) U) []U {
    r := make([]U, len(s))
    for i, v := range s { r[i] = f(v) }
    return r
}

Constraint

type Number interface {
    ~int | ~float64
}
func Sum[T Number](nums []T) T {
    var total T
    for _, n := range nums { total += n }
    return total
}

Testing

Test Dasar

// file: math_test.go
func TestAdd(t *testing.T) {
    got := Add(2, 3)
    if got != 5 {
        t.Errorf("Add(2,3) = %d, want 5", got)
    }
}

Perintah Test

go test	Jalankan test di package saat ini
go test ./...	Jalankan semua test secara rekursif
go test -v	Output verbose
go test -run TestAdd	Jalankan test tertentu berdasarkan nama
go test -bench .	Jalankan benchmark
go test -cover	Tampilkan persentase coverage

REFERENSI CEPAT GO