Go Generics ปี 2026: Type Parameters, Constraints และคำถามสัมภาษณ์

// Basic generic function with type parameter T
func PrintSlice[T any](items []T) {
    for _, item := range items {
        fmt.Println(item)
    }
}

// Usage - type inference determines T automatically
func main() {
    PrintSlice([]int{1, 2, 3})        // T is int
    PrintSlice([]string{"a", "b"})     // T is string
}

// any: accepts all types (alias for interface{})
func Process[T any](value T) T {
    return value
}

// comparable: only types that support == and !=
func Contains[T comparable](slice []T, target T) bool {
    for _, v := range slice {
        if v == target {  // This comparison requires comparable
            return true
        }
    }
    return false
}

// This compiles
Contains([]int{1, 2, 3}, 2)

// This fails: slices are not comparable
// Contains([][]int{{1}, {2}}, []int{1})

// Custom constraint using type union
type Number interface {
    int | int8 | int16 | int32 | int64 |
    uint | uint8 | uint16 | uint32 | uint64 |
    float32 | float64
}

// Generic function constrained to numeric types
func Sum[T Number](values []T) T {
    var total T
    for _, v := range values {
        total += v  // + operator works because all Number types support it
    }
    return total
}

func main() {
    fmt.Println(Sum([]int{1, 2, 3}))         // 6
    fmt.Println(Sum([]float64{1.5, 2.5}))    // 4.0
}

// ~int matches int and any type with int as underlying type
type Signed interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64
}

// Custom type with int as underlying type
type UserID int
type Temperature int64

func Abs[T Signed](value T) T {
    if value < 0 {
        return -value
    }
    return value
}

func main() {
    var id UserID = -42
    var temp Temperature = -10
    
    fmt.Println(Abs(id))    // 42 - works because ~int matches UserID
    fmt.Println(Abs(temp))  // 10 - works because ~int64 matches Temperature
}

// Generic Stack type
type Stack[T any] struct {
    items []T
}

// Push adds an element to the stack
func (s *Stack[T]) Push(item T) {
    s.items = append(s.items, item)
}

// Pop removes and returns the top element
func (s *Stack[T]) Pop() (T, bool) {
    if len(s.items) == 0 {
        var zero T  // Zero value for type T
        return zero, false
    }
    index := len(s.items) - 1
    item := s.items[index]
    s.items = s.items[:index]
    return item, true
}

func main() {
    intStack := Stack[int]{}
    intStack.Push(10)
    intStack.Push(20)
    
    val, ok := intStack.Pop()  // val=20, ok=true
}

// This is NOT valid Go code
type Container[T any] struct {
    value T
}

// ERROR: methods cannot have type parameters
// func (c *Container[T]) Transform[U any](fn func(T) U) U {
//     return fn(c.value)
// }

// Valid alternative: use a standalone function
func Transform[T, U any](c *Container[T], fn func(T) U) U {
    return fn(c.value)
}

func Map[T, R any](input []T, transform func(T) R) []R {
    result := make([]R, len(input))
    for i, v := range input {
        result[i] = transform(v)
    }
    return result
}

func main() {
    numbers := []int{1, 2, 3}
    
    // Type inference: T=int, R=string inferred from arguments
    strings := Map(numbers, func(n int) string {
        return fmt.Sprintf("%d", n)
    })
    
    // Explicit types sometimes required for complex cases
    // Map[int, string](numbers, converter)
}

import (
    "cmp"
    "slices"
)

func main() {
    numbers := []int{3, 1, 4, 1, 5, 9}
    
    // slices.Sort uses cmp.Ordered constraint internally
    slices.Sort(numbers)  // [1, 1, 3, 4, 5, 9]
    
    // Binary search on sorted slice
    index, found := slices.BinarySearch(numbers, 4)
    
    // cmp.Compare returns -1, 0, or 1
    result := cmp.Compare(3, 5)  // -1
    
    // cmp.Or returns first non-zero value
    value := cmp.Or(0, 0, 42, 100)  // 42
}

import (
    "sync"
    "time"
)

type Cache[K comparable, V any] struct {
    mu    sync.RWMutex
    items map[K]cacheItem[V]
}

type cacheItem[V any] struct {
    value      V
    expiration time.Time
}

func NewCache[K comparable, V any]() *Cache[K, V] {
    return &Cache[K, V]{
        items: make(map[K]cacheItem[V]),
    }
}

func (c *Cache[K, V]) Set(key K, value V, ttl time.Duration) {
    c.mu.Lock()
    defer c.mu.Unlock()
    c.items[key] = cacheItem[V]{
        value:      value,
        expiration: time.Now().Add(ttl),
    }
}

func (c *Cache[K, V]) Get(key K) (V, bool) {
    c.mu.RLock()
    defer c.mu.RUnlock()
    
    item, exists := c.items[key]
    if !exists || time.Now().After(item.expiration) {
        var zero V
        return zero, false
    }
    return item.value, true
}

// Return zero value when slice is empty
func First[T any](slice []T) T {
    if len(slice) == 0 {
        var zero T  // Zero value: 0 for int, "" for string, nil for pointers
        return zero
    }
    return slice[0]
}

// Alternative: return pointer to avoid ambiguity
func FirstOrNil[T any](slice []T) *T {
    if len(slice) == 0 {
        return nil
    }
    return &slice[0]
}

// Constraint requiring both ordering and string conversion
type Stringable interface {
    String() string
}

type OrderedStringable interface {
    cmp.Ordered
    Stringable
}

// Alternative: use type parameters with multiple constraints inline
func PrintSorted[T interface{ cmp.Ordered; fmt.Stringer }](items []T) {
    slices.Sort(items)
    for _, item := range items {
        fmt.Println(item.String())
    }
}