Golang函数式编程深入分析实例

定义集合功能函数

WorkWith

// WorkWith is the struct we"ll
// be implementing collections for
type WorkWith struct {
	Data    string
	Version int
}

针对该结构体定义filter和map函数：

// 基于判断函数过滤集合，返回符合条件的集合元素
func Filter(ws []WorkWith, f func(w WorkWith) bool) []WorkWith {
	// depending on results, smaller size for result
	// is len == 0
	result := make([]WorkWith, 0)
	for _, w := range ws {
		if f(w) {
			result = append(result, w)
		}
	}
	return result
}
// 基于转换函数转换集合元素，返回集合的元素为转换后的元素
func Map(ws []WorkWith, f func(w WorkWith) WorkWith) []WorkWith {
	// the result should always be the same
	// length
	result := make([]WorkWith, len(ws))
	for pos, w := range ws {
		newW := f(w)
		result[pos] = newW
	}
	return result
}

实现具体功能函数

import "strings"
// LowerCaseData does a ToLower to the
// Data string of a WorkWith
func LowerCaseData(w WorkWith) WorkWith {
	w.Data = strings.ToLower(w.Data)
	return w
}
// IncrementVersion increments a WorkWiths
// Version
func IncrementVersion(w WorkWith) WorkWith {
	w.Version++
	return w
}
// OldVersion returns a closures
// that validates the version is greater than
// the specified amount
func OldVersion(v int) func(w WorkWith) bool {
	return func(w WorkWith) bool {
		return w.Version >= v
	}
}

上面定义了三个函数，LowerCaseData修改WorkWith中Data值为小写形式，IncrementVersion让WorkWith中版本增加1，OldVersion基于参数过滤版本。

测试集合功能

定义测试用例文件：

import (
	"fmt"
	"testing"
)
func TestMap(t *testing.T) {
	ws := []WorkWith{
		{"Example", 1},
		{"Example 2", 2},
	}
	fmt.Printf("Initial list: %#v
", ws)
	// first lower case the list
	ws = Map(ws, LowerCaseData)
	fmt.Printf("After LowerCaseData Map: %#v
", ws)
	// next increment all versions
	ws = Map(ws, IncrementVersion)
	fmt.Printf("After IncrementVersion Map: %#v
", ws)
	// lastly remove all versions older than 3
	ws = Filter(ws, OldVersion(3))
	fmt.Printf("After OldVersion Filter: %#v
", ws)
}

运行 go test . -v

输出结果如下：

Initial list: []collections.WorkWith{collections.WorkWith{Data:"Example", Version:1}, collections.WorkWith{Data:"Example 2", Version:2}}

After LowerCaseData Map: []collections.WorkWith{collections.WorkWith{Data:"example", Version:1}, collections.WorkWith{Data:"example 2", Version:2}}

After IncrementVersion Map: []collections.WorkWith{collections.WorkWith{Data:"example", Version:2}, collections.WorkWith{Data:"example 2", Version:3}}

After OldVersion Filter: []collections.WorkWith{collections.WorkWith{Data:"example 2", Version:3}}

上面示例中，我们注意到函数都没有返回任何error对象，这遵循函数式编程思想，尽可能让函数纯粹：不修改原集合元素，即对原集合无副作用，而是生成新的集合。如果需要对集合应用多个功能，那么这种模式能够省去很多麻烦，并且测试也很简单。我们还可以将映射和过滤器链接在一起，让代码更简洁可读。

	ws := []WorkWith{
		{"Example", 1},
		{"Example 2", 2},
	}
	fmt.Printf("Initial list: %#v
", ws)
	result := Filter(Map(Map(ws, LowerCaseData), IncrementVersion), OldVersion(3))
	fmt.Printf("After OldVersion Filter: %#v
", result)

如果功能函数定义为集合类型的方法，并返回集合类型，则上述代码会更优雅。

泛型实现

上面代码仅能在特定类型上使用，我们自然想实现泛型函数，下面通过一个简单示例进行说明：

func map2[T, U any](data []T, f func(T) U) []U {
    res := make([]U, 0, len(data))
    for _, e := range data {
        res = append(res, f(e))
    }
    return res
}

该函数接收类型T，转换后返回类型U，当然两者类型也可以一样。下面测试函数功能：

    // 字符串转大写
    words := []string{"war", "cup", "water", "tree", "storm"}
    result := map2(words, func(s string) string {
        return strings.ToUpper(s)
    })
    fmt.Println(result)
    // 生成原集合元素的平方集合
    fmt.Println("-------------------")
    numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
    squares := map2(numbers, func(n int) int {
        return n * n
    })
    fmt.Println(squares)
    // 数值转为字符串
    fmt.Println("-------------------")
    as_strings := map2(numbers, func(n int) string {
        return strconv.Itoa(n)
    })
    fmt.Printf("%q", as_strings)