Basics

Hello World

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

Run & Build

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

Module Init

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

Variables & Types

Declaration

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

Basic Types

bool	`true`, `false`
string	UTF-8 immutable byte sequence
int, int8..int64	Signed integers (platform / fixed width)
uint, uint8..uint64	Unsigned integers
float32, float64	IEEE-754 floating point
byte	Alias for `uint8`
rune	Alias for `int32` (Unicode code point)

Zero Values

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

Functions

Basic Function

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

Multiple Return Values

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

Variadic & Anonymous

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
}

Control Flow

If / Else

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

For Loop

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")
}

Structs & Methods

Struct Definition

type User struct {
    Name  string
    Email string
    Age   int
}
u := User{Name: "Alice", Email: "a@b.com", Age: 30}

Methods

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

Interfaces

Defining & Implementing

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

Common Interfaces

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)
}

Goroutines & Channels

Goroutines

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

Channels

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")
}

Patterns

sync.WaitGroup	Wait for multiple goroutines to finish
sync.Mutex	Mutual exclusion lock for shared state
context.Context	Cancellation, deadlines, request-scoped values

Error Handling

Basic Pattern

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

Custom Errors

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

errors Package

errors.New(msg)	Create simple error
fmt.Errorf("%w", err)	Wrap error with context
errors.Is(err, target)	Check error chain for match
errors.As(err, &target)	Extract typed error from chain

Slices & Maps

Slices

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

Maps

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 { }

Slice Operations

len(s)	Number of elements
cap(s)	Underlying array capacity
append(s, elems...)	Append elements, may reallocate
copy(dst, src)	Copy elements between slices
slices.Sort(s)	Sort slice (Go 1.21+ `slices` pkg)

Packages & Imports

Import Styles

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

Visibility

Uppercase first letter = exported (public). Lowercase first letter = unexported (package-private). No keywords like public/private needed.

Common Standard Library

fmt	Formatted I/O (Print, Sprintf, Errorf)
os	OS functions (files, env, args)
io	I/O primitives (Reader, Writer)
net/http	HTTP client and server
encoding/json	JSON encode/decode
strings	String manipulation functions
strconv	String ↔ number conversions
testing	Unit test framework

Generics

Type Parameters

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
}

Constraints

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

Testing

Basic Test

// 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)
    }
}

Test Commands

go test	Run tests in current package
go test ./...	Run all tests recursively
go test -v	Verbose output
go test -run TestAdd	Run specific test by name
go test -bench .	Run benchmarks
go test -cover	Show coverage percentage

GO QUICK REFERENCE