11.14 结构体、集合和高阶函数

然后我们就可以使用高阶函数，实际上也就是把函数作为定义所需方法（其他函数）的参数，例如：

1）定义一个通用的函数，它接收一个作用于每一辆 car 的 f 函数作参数：

// Process all cars with the given function f:
func (cs Cars) Process(f func(car *Car)) {
    for _, c := range cs {
        f(c)
    }
}

// Find all cars matching a given criteria.
func (cs Cars) FindAll(f func(car *Car) bool) Cars {
    cars := make([]*Car, 0)
    cs.Process(func(c *Car) {
        if f(c) {
            cars = append(cars, c)
        }
    })
    return cars
}

3）实现 Map 功能，产出除 car 对象以外的东西：

现在我们可以定义下面这样的具体查询：

allNewBMWs := allCars.FindAll(func(car *Car) bool {
    return (car.Manufacturer == "BMW") && (car.BuildYear > 2010)
})

func MakeSortedAppender(manufacturers []string)(func(car *Car),map[string]Cars) {
    // Prepare maps of sorted cars.
    sortedCars := make(map[string]Cars)
    for _, m := range manufacturers {
        sortedCars[m] = make([]*Car, 0)
    }
    sortedCars["Default"] = make([]*Car, 0)
    // Prepare appender function:
    appender := func(c *Car) {
        if _, ok := sortedCars[c.Manufacturer]; ok {
            sortedCars[c.Manufacturer] = append(sortedCars[c.Manufacturer], c)
        } else {
            sortedCars["Default"] = append(sortedCars["Default"], c)
        }
    }
    return appender, sortedCars
}

现在我们可以用它把汽车分类为独立的集合，像这样：

我们让这些代码在下面的程序 cars.go 中执行：

// cars.go
package main
)
type Any interface{}
type Car struct {
    Model        string
    Manufacturer string
    BuildYear    int
    // ...
}
type Cars []*Car
func main() {
    // make some cars:
    ford := &Car{"Fiesta", "Ford", 2008}
    bmw := &Car{"XL 450", "BMW", 2011}
    merc := &Car{"D600", "Mercedes", 2009}
    bmw2 := &Car{"X 800", "BMW", 2008}
    // query:
    allCars := Cars([]*Car{ford, bmw, merc, bmw2})
    allNewBMWs := allCars.FindAll(func(car *Car) bool {
        return (car.Manufacturer == "BMW") && (car.BuildYear > 2010)
    })
    fmt.Println("AllCars: ", allCars)
    fmt.Println("New BMWs: ", allNewBMWs)
    //
    manufacturers := []string{"Ford", "Aston Martin", "Land Rover", "BMW", "Jaguar"}
    sortedAppender, sortedCars := MakeSortedAppender(manufacturers)
    allCars.Process(sortedAppender)
    fmt.Println("Map sortedCars: ", sortedCars)
    BMWCount := len(sortedCars["BMW"])
    fmt.Println("We have ", BMWCount, " BMWs")
}
// Process all cars with the given function f:
func (cs Cars) Process(f func(car *Car)) {
    for _, c := range cs {
        f(c)
    }
}
// Find all cars matching a given criteria.
func (cs Cars) FindAll(f func(car *Car) bool) Cars {
    cs.Process(func(c *Car) {
        if f(c) {
            cars = append(cars, c)
        }
    })
    return cars
}
// Process cars and create new data.
func (cs Cars) Map(f func(car *Car) Any) []Any {
    result := make([]Any, len(cs))
    ix := 0
    cs.Process(func(c *Car) {
        result[ix] = f(c)
        ix++
    })
    return result
}
func MakeSortedAppender(manufacturers []string) (func(car *Car), map[string]Cars) {
    // Prepare maps of sorted cars.
    sortedCars := make(map[string]Cars)
    for _, m := range manufacturers {
        sortedCars[m] = make([]*Car, 0)
    }
    sortedCars["Default"] = make([]*Car, 0)
    // Prepare appender function:
    appender := func(c *Car) {
        if _, ok := sortedCars[c.Manufacturer]; ok {
            sortedCars[c.Manufacturer] = append(sortedCars[c.Manufacturer], c)
        } else {
            sortedCars["Default"] = append(sortedCars["Default"], c)
        }
    }
    return appender, sortedCars
}

输出：

AllCars:  [0xf8400038a0 0xf840003bd0 0xf840003ba0 0xf840003b70]
New BMWs:  [0xf840003bd0]
Map sortedCars:  map[Default:[0xf840003ba0] Jaguar:[] Land Rover:[] BMW:[0xf840003bd0 0xf840003b70] Aston Martin:[] Ford:[0xf8400038a0]]
We have  2  BMWs