How to define and use sentinel errors for specific error conditions in Go?

In Go, you can use sentinel errors to define and handle specific error conditions. Sentinel errors are predefined error values that can be returned by a function to indicate a specific error condition. Here's how you can define and use sentinel errors in Go:

Define a sentinel error value: You can define sentinel errors as constants of the error type. For example, let's define a sentinel error for a "file not found" condition:

var ErrFileNotFound = errors.New("file not found")

Use the sentinel error: Inside your function, when you encounter a specific error condition, you can return the sentinel error value. For example, if a file is not found, you can return the ErrFileNotFound value:

func OpenFile(filename string) error {
    if fileNotFound(filename) {
        return ErrFileNotFound
    }
    // ...Other code
}

Handle the sentinel error: When calling the function, you can check if the returned error matches the sentinel error value to handle specific error conditions. For example:

func ReadFile() {
    err := OpenFile("test.txt")
    if err != nil {
        if err == ErrFileNotFound {
            // Handle file not found error
        } else {
            // Handle other errors
        }
    }
}

By using sentinel errors, you can define and handle specific error conditions in a clear and concise way. However, it's important to note that sentinel errors should be exported if you want to compare them in other packages. Otherwise, you can define them as unexported constants within the same package.