golang determines error type

Golang is a simple and efficient programming language. It has a powerful type system and rich libraries, which provides developers with great convenience. Error handling is a very important task in Go language. When an error occurs while the program is running, we need to be able to find the error quickly and accurately and handle it. In order to achieve this goal, we need to know how to determine the error type, let's find out together.

1. What are error types?

The error type refers to the type of error object in Golang. In the Go language, the error type is an interface type that defines a method named "Error()", which returns a string representing the current error information. In Golang code, the New() function of the errors package is usually used to create a new error object. The code example is as follows:

import "errors"
err := errors.New("this is an error")

2. How to determine the error type?

The way to determine the error type in Golang is very simple. We only need to use type assertion to type-convert the error object. In type assertion, we can assert an interface type to another interface type, such as asserting an error object to a custom error type. The code example is as follows:

type MyError struct {
    message string
}

func (e *MyError) Error() string {
    return e.message
}

func doSomething() error {
    return &MyError{message: "This is a custom error"}
}

func main() {
    err := doSomething()
    if e, ok := err.(*MyError); ok {
        // 错误类型是 MyError
        fmt.Println(e.message)
    } else if err != nil {
        // 其他类型的错误
        fmt.Println("error:", err)
    } else {
        // 没有发生错误
        fmt.Println("no error")
    }
}

In the above example code, we define a custom error type MyError, which implements the Error() method. Then, we return an instance of MyError in the doSomething() function. In the main() function, we convert this error object to the MyError type through type assertion. If the conversion is successful, it means that the error type is MyError, and we can handle it as needed. If the conversion fails, the error object is another type of error.

Note that the type assertion syntax here is "e, ok := err.(*MyError)", where ok indicates whether the type assertion is successful. If the conversion of err to the MyError type is successful, then ok will be true, otherwise ok will be false.

In addition to type assertions, you can also use the Is() and As() functions provided by the errors package to determine and convert error types. The specific usage is as follows:

if errors.Is(err, io.EOF) {
    // 错误类型是 io.EOF
}

var target *MyError
if errors.As(err, &target) {
    // 错误类型是 *MyError
}

3. How to handle error types

After we determine the specific type of an error, we can adopt different processing methods according to needs. Common error handling methods include:

3.1 Print error message

If there are just some minor problems, we can print the error message directly, for example:

func doSomething() error {
    if err := someFunc(); err != nil {
        return fmt.Errorf("调用 someFunc() 错误：%v", err)
    }
    return nil
}

In the above code, When the someFunc() function returns an error, we use the fmt.Errorf() function to print out the error information and return it to the caller.

3.2 Return the error directly

If the error is serious, we can directly return the error and let the upper caller handle the error, for example:

func doSomething() error {
    if err := someFunc(); err != nil {
        return err
    }
    return nil
}

In the above code, When the someFunc() function returns an error, we directly return the error and let the upper-level caller determine the error type and handle it.

3.3 Refer to the processing methods of the standard library

For some common error types, the standard library provides some processing methods for our reference. For example, when opening a file fails, the standard library will return an os.PathError type error. At this time, we can refer to the standard library's approach to handle it:

func doSomething(filename string) error {
    f, err := os.Open(filename)
    if err != nil {
        if os.IsNotExist(err) {
            return fmt.Errorf("文件 %s 不存在", filename)
        } else if os.IsPermission(err) {
            return fmt.Errorf("无权限访问文件 %s", filename)
        }
        return err
    }
    defer f.Close()
    // do something
    return nil
}

In the above code, we first try to open a file , when the opening fails, we use the os.IsNotExist() and os.IsPermission() functions to determine the error type and return different error information.

4. Summary

In Golang, error handling is a very important task. When an error occurs in a program, we need to be able to find the error quickly and accurately and handle it. Through the introduction of this article, we have learned how to determine the error type and mastered common error handling methods. At the same time, we also noticed some small details, such as the need to determine whether the conversion is successful during type assertion. I hope this article can be helpful to everyone when handling errors in Golang.

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