type Counter struct { n int }

// 值接收者读取副本；指针接收者修改共享对象。
func (c Counter) Value() int { return c.n }
func (c *Counter) Inc() { c.n++ }

c := Counter{}
c.Inc()             // 编译器自动取地址
fmt.Println(c.Value())

// 接口由使用者定义，靠近消费端。
type OrderFinder interface {
    Find(ctx context.Context, id string) (Order, error)
}

type Service struct { orders OrderFinder }

func (s Service) Get(ctx context.Context, id string) (Order, error) {
    return s.orders.Find(ctx, id)
}

func Map[S ~[]E, E any, R any](src S, fn func(E) R) []R {
    out := make([]R, len(src))
    for i, item := range src {
        out[i] = fn(item)
    }
    return out
}

names := Map(users, func(u User) string { return u.Name })

tests := []struct {
    name string
    input string
    want int64
    wantErr bool
}{
    {"yuan", "12.30", 1230, false},
    {"invalid", "12.x", 0, true},
}
for _, tt := range tests {
    t.Run(tt.name, func(t *testing.T) { ... })
}

func FuzzParseAmount(f *testing.F) {
    f.Add("12.30")
    f.Fuzz(func(t *testing.T, input string) {
        cents, err := ParseAmount(input)
        if err == nil && cents < 0 {
            t.Fatalf("negative result: %d", cents)
        }
    })
}

func BenchmarkDecode(b *testing.B) {
    payload := loadFixture(b)
    b.ReportAllocs()
    for b.Loop() {
        _, err := Decode(payload)
        if err != nil { b.Fatal(err) }
    }
}