要創(chuàng)建自定義錯誤類型，需定義一個實現(xiàn)Error() string方法的類型，例如使用結(jié)構(gòu)體添加元數(shù)據(jù)，或使用字符串類型創(chuàng)建輕量錯誤，1. 定義包含Code、Message、Details字段的struct并實現(xiàn)Error方法，2. 可嵌入error字段實現(xiàn)錯誤包裝，3. 實現(xiàn)Unwrap方法支持errors.As和errors.Is，4. 返回指針以確保方法調(diào)用正確，5. 對于簡單場景可定義字符串類型并實現(xiàn)Error方法，最終使錯誤可編程檢查且富含上下文。

In Go, creating a custom error type gives you more control over error handling—like adding context, metadata, or custom behavior. Here’s how to do it step by step.

Define a Custom Error Type

The most common way is to define a new type that implements the error interface. The error interface is built into Go and has just one method:

type error interface {
    Error() string
}

So, any type that has an Error() string method automatically satisfies the error interface.

Here’s an example of a custom error type:

type MyError struct {
    Code    int
    Message string
    Details string
}

func (e *MyError) Error() string {
    return fmt.Sprintf("[%d] %s: %s", e.Code, e.Message, e.Details)
}

Now you can create and return errors of this type:

func somethingWentWrong() error {
    return &MyError{
        Code:    400,
        Message: "Invalid input",
        Details: "The provided value was out of range",
    }
}

And handle it:

if err := somethingWentWrong(); err != nil {
    fmt.Println(err) // Output: [400] Invalid input: The provided value was out of range
}

Add Context with Error Wrapping (Go 1.13 )

If you want to wrap another error (for example, an underlying system error), embed the error type or include an error field:

type NetworkError struct {
    Op      string
    Err     error
}

func (e *NetworkError) Error() string {
    return fmt.Sprintf("network error during %s: %v", e.Op, e.Err)
}

// Optional: Implement Unwrap() to support errors.Is and errors.As
func (e *NetworkError) Unwrap() error {
    return e.Err
}

Usage:

cause := fmt.Errorf("connection timeout")
err := &NetworkError{Op: "read", Err: cause}
fmt.Println(err) // network error during read: connection timeout

Now you can use errors.As to check for specific error types:

var netErr *NetworkError
if errors.As(err, &netErr) {
    fmt.Printf("Operation was: %s\n", netErr.Op)
}

When to Use Custom Errors

• You need to attach metadata (e.g., error codes, timestamps, IDs).
• You want to categorize errors (e.g., ValidationError, AuthError).
• You're building a library and want users to programmatically inspect errors.
• You need to wrap underlying errors while preserving context.

? Tip: Always return a pointer to your custom error if it’s a struct, so the Error() method works correctly (especially since it usually needs to be a pointer receiver).

Simple Named Error Type (Alternative)

Sometimes you just want a distinct error type without extra fields:

type NotFoundError string

func (e NotFoundError) Error() string {
    return string(e)   ": not found"
}

// Usage
return NotFoundError("user")

This is lightweight and works well for sentinel-like errors with dynamic messages.

Basically, just define a type with an Error() method, and Go treats it as an error. Adding fields, wrapping, and Unwrap() makes it more powerful.

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