Go：为一组单独的结果实现ManyDecode

huangapple
go评论115阅读模式
英文:

Go: Implementing a ManyDecode for a "set" of individual results

问题

我已经实现了一个非常简单的解码方法（暂时使用gob.Decoder）- 这对于单个响应很有效 - 甚至对于切片也很有效，但我需要实现一个DecodeMany方法，它能够解码一组单独的响应（而不是切片）。

工作中的解码方法：

  1. var v MyType
  2. _ = Decode(&v)
  3. ...
  5. func Decode(v interface{}) error {
  6.    buf, _ := DoSomething() // func DoSomething() ([]byte, error)
  7.    // 省略错误处理
  8.    return gob.NewDecoder(bytes.NewReader(buf)).Decode(v)
  9. }

我尝试为DecodeMany方法处理一个不一定是切片的响应：

  1. var vv []MyType
  2. _ = DecodeMany(&vv)
  3. ...
  5. func DecodeMany(vv []interface{}) error {
  6.    for _, g := range DoSomething() { // func DoSomething() []struct{Buf []bytes}
  7.       
  8.       // 使用g.Buf作为单个的"interface{}"
  9.       // 希望像这样：
  10.       var v interface{} /* 如何创建单个vv类型的实例？ */
  11.       _ = gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(v)
  12.       vv = append(vv, v)
  13.    }
  14.    return
  15. }

除了无法编译之外，上述代码还有以下错误：

无法将&vv（类型为*[]MyType的值）作为DecodeMany的[]interface{}类型的参数使用。

英文:

I have implemented a very simple Decode method (using gob.Decoder for now) - this works well for single responses - it would even work well for slices, but I need to implement a DecodeMany method where it is able to decode a set of individual responses (not a slice).

Working Decode method:

  1. var v MyType
  2. _ = Decode(&v)
  3. ...
  5. func Decode(v interface{}) error {
  6.    buf, _ := DoSomething() // func DoSomething() ([]byte, error)
  7.    // error handling omitted for brevity
  8.    return gob.NewDecoder(bytes.NewReader(buf)).Decode(v)
  9. }

What I'm trying to do for a DecodeMany method is to deal with a response that isn't necessarily a slice:

  1. var vv []MyType
  2. _ = DecodeMany(&vv)
  3. ...
  5. func DecodeMany(vv []interface{}) error {
  6.    for _, g := range DoSomething() { // func DoSomething() []struct{Buf []bytes}
  7.       
  8.       // Use g.Buf as an individual "interface{}"
  9.       // want something like:
  10.       var v interface{} /* Somehow create instance of single vv type? */
  11.       _ = gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(v)
  12.       vv = append(vv, v)
  13.    }
  14.    return
  15. }

Besides not compiling the above also has the error of:
> cannot use &vv (value of type *[]MyType) as type []interface{} in argument to DecodeMany

答案1

得分: 1

如果你想修改传递的切片，它必须是一个指针，否则你必须返回一个新的切片。另外，如果函数声明为具有类型[]interface{}的参数，你只能传递类型为[]interface{}的值，而不能传递其他切片类型...除非你使用泛型...

这是一个使用Go 1.18引入的泛型的完美示例。

将DecodeMany()改为泛型函数，使用T类型参数作为切片元素类型：

当使用指针时

  1. func DecodeMany[T any](vv *[]T) error {
  2.     for _, g := range DoSomething() {
  3.         var v T
  4.         if err := gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(&v); err != nil {
  5.             return err
  6.         }
  7.         *vv = append(*vv, v)
  8.     }
  9.     return nil
  10. }

这是一个简单的应用程序来测试它：

  1. type MyType struct {
  2.     S int64
  3. }
  5. func main() {
  6.     var vv []MyType
  7.     if err := DecodeMany(&vv); err != nil {
  8.         panic(err)
  9.     }
  10.     fmt.Println(vv)
  11. }
  13. func DoSomething() (result []struct{ Buf []byte }) {
  14.     for i := 3; i < 6; i++ {
  15.         buf := &bytes.Buffer{}
  16.         v := MyType{S: int64(i)}
  17.         if err := gob.NewEncoder(buf).Encode(v); err != nil {
  18.             panic(err)
  19.         }
  20.         result = append(result, struct{ Buf []byte }{buf.Bytes()})
  21.     }
  22.     return
  23. }

这将输出（在Go Playground上尝试）：

  1. [{3} {4} {5}]

当返回一个切片时

如果你选择返回切片，你不需要传递任何东西，但你需要分配结果：

  1. func DecodeMany[T any]() ([]T, error) {
  2.     var result []T
  3.     for _, g := range DoSomething() {
  4.         var v T
  5.         if err := gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(&v); err != nil {
  6.             return result, err
  7.         }
  8.         result = append(result, v)
  9.     }
  10.     return result, nil
  11. }

使用它：

  1. vv, err := DecodeMany[MyType]()
  2. if err != nil {
  3.     panic(err)
  4. }
  5. fmt.Println(vv)

在Go Playground上尝试这个例子。

英文:

If you want to modify the passed slice, it must be a pointer, else you must return a new slice. Also if the function is declared to have a param of type []interface{}, you can only pass a value of type []interface{} and no other slice types... Unless you use generics...

This is a perfect example to start using generics introduced in Go 1.18.

Change DecodeMany() to be generic, having a T type parameter being the slice element type:

When taking a pointer

  1. func DecodeMany[T any](vv *[]T) error {
  2. 	for _, g := range DoSomething() {
  3. 		var v T
  4. 		if err := gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(&amp;v); err != nil {
  5. 			return err
  6. 		}
  7. 		*vv = append(*vv, v)
  8. 	}
  9. 	return nil
  10. }

Here's a simple app to test it:

  1. type MyType struct {
  2. 	S int64
  3. }
  5. func main() {
  6. 	var vv []MyType
  7. 	if err := DecodeMany(&amp;vv); err != nil {
  8. 		panic(err)
  9. 	}
  10. 	fmt.Println(vv)
  11. }
  13. func DoSomething() (result []struct{ Buf []byte }) {
  14. 	for i := 3; i &lt; 6; i++ {
  15. 		buf := &amp;bytes.Buffer{}
  16. 		v := MyType{S: int64(i)}
  17. 		if err := gob.NewEncoder(buf).Encode(v); err != nil {
  18. 			panic(err)
  19. 		}
  20. 		result = append(result, struct{ Buf []byte }{buf.Bytes()})
  21. 	}
  22. 	return
  23. }

This outputs (try it on the Go Playground):

  1. [{3} {4} {5}]

When returning a slice

If you choose to return the slice, you don't have to pass anything, but you need to assign the result:

  1. func DecodeMany[T any]() ([]T, error) {
  2. 	var result []T
  3. 	for _, g := range DoSomething() {
  4. 		var v T
  5. 		if err := gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(&amp;v); err != nil {
  6. 			return result, err
  7. 		}
  8. 		result = append(result, v)
  9. 	}
  10. 	return result, nil
  11. }

Using it:

  1. vv, err := DecodeMany[MyType]()
  2. if err != nil {
  3. 	panic(err)
  4. }
  5. fmt.Println(vv)

Try this one on the Go Playground.

huangapple
  • 本文由 发表于 2022年4月6日 00:36:20
  • 转载请务必保留本文链接：https://go.coder-hub.com/71755407.html
  • generics
  • go
  • gob
  • slice
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