total := 0
for _, n := range nums {
    if n%2 == 0 {
        total += n*n
    }
}

total := Sum(Map(Filter(nums, IsEven), Square))

add := func(a, b int) int { return a + b }
fmt.Println(add(2, 3)) // 5

// ส่งฟังก์ชันเข้าฟังก์ชัน
func Apply(f func(int) int, v int) int { return f(v) }
fmt.Println(Apply(func(x int) int { return x*2 }, 5)) // 10

// คืนฟังก์ชันออกมา
func Maker() func() int {
    n := 0
    return func() int { n++; return n }
}
c := Maker()
fmt.Println(c(), c()) // 1 2

type IntFormatter func(int) string

func Map[T, R any](s []T, f func(T) R) []R {
    out := make([]R, 0, len(s))
    for _, v := range s { out = append(out, f(v)) }
    return out
}
func Filter[T any](s []T, keep func(T) bool) []T {
    var out []T
    for _, v := range s { if keep(v) { out = append(out, v) } }
    return out
}
func Reduce[T, R any](s []T, init R, f func(R, T) R) R {
    acc := init
    for _, v := range s { acc = f(acc, v) }
    return acc
}

// ใช้
nums := []int{1, 2, 3, 4}
evens := Filter(nums, func(n int) bool { return n%2 == 0 }) // [2 4]
squares := Map(evens, func(n int) int { return n*n })       // [4 16]
sum := Reduce(squares, 0, func(a, b int) int { return a+b }) // 20

func Counter() func() int {
    n := 0
    return func() int { n++; return n }
}

func Add(a int) func(int) int {
    return func(b int) int { return a+b }
}
add5 := Add(5)
fmt.Println(add5(3)) // 8

func WithPrefix(p string) func(string) string {
    return func(s string) string { return p + s }
}
hello := WithPrefix("สวัสดี ")
fmt.Println(hello("สมชาย"))

// pure
func Add(a, b int) int { return a+b }
// ไม่ pure (แอบแก้ข้างนอก)
var total int
func AddToTotal(n int) { total += n }

func GenericFormatter[T any](v T) string { return fmt.Sprintf("value: %v", v) }
type IntFormatter func(int) string

ch := make(chan IntFormatter, 1)
ch <- GenericFormatter // 1.27 เดา T=int ให้เอง เมื่อก่อนต้องเขียน GenericFormatter[int]
formatters := []IntFormatter{GenericFormatter}

func gen(nums ...int) <-chan int {
    out := make(chan int)
    go func() { defer close(out); for _, n := range nums { out <- n } }()
    return out
}
func MapChan[T, R any](in <-chan T, f func(T) R) <-chan R {
    out := make(chan R)
    go func() { defer close(out); for v := range in { out <- f(v) } }()
    return out
}
// ใช้: for v := range MapChan(gen(1,2,3,4), func(n int) int { return n*n }) { fmt.Println(v) }

func Map[T any, U any](xs []T, f func(T) U) []U {
    out := make([]U, 0, len(xs))
    for _, x := range xs {
        out = append(out, f(x))
    }
    return out
}
names := Map([]int{1, 2, 3}, func(n int) string {
    return fmt.Sprint("เบอร์ ", n)
})

func Filter[T any](xs []T, keep func(T) bool) []T {
    var out []T
    for _, x := range xs {
        if keep(x) {
            out = append(out, x)
        }
    }
    return out
}
evens := Filter([]int{1, 2, 3, 4}, func(n int) bool { return n%2 == 0 })

func counter() func() int {
    n := 0
    return func() int {
        n++
        return n
    }
}
a, b := counter(), counter()
fmt.Println(a(), a(), b()) // 1 2 1 (แยกกัน ไม่แย่งกัน)