How Do Go's `Is()` and `As()` Functions Handle Recursive Error Wrapping?

Go Errors: Unraveling the Mysteries of Is() and As()

In Go, error handling is crucial for providing meaningful diagnostics and implementing robust programs. The errors package offers Is() and As() functions for determining error equality and extracting specific details, respectively. One common misconception is that these functions support recursive error wrapping, enabling the unwrapping of multiple nested errors.

However, upon closer inspection, the standard fmt.Errorf function does not provide recursive error wrapping. This means that wrapping errors using %w does not allow for the full traversal of an error chain.

To address this issue, custom error types can be defined that implement the error interface and implement their own Is() and As() methods. This allows for the recursive unwrapping of multiple nested errors.

Example:

type customError struct {
    err     error
    wrapped *customError
}

func (c *customError) Error() string {
    if c.err != nil {
        return c.err.Error()
    }
    return "Custom error without cause"
}

func (c *customError) Is(err error) bool {
    if c.err != nil {
        return errors.Is(c.err, err)
    }
    return false
}

func (c *customError) As(target interface{}) bool {
    if c.err != nil {
        return errors.As(c.err, target)
    }
    return false
}

func Wrap(errs ...error) error {
    var rootError *customError
    for i := len(errs) - 1; i >= 0; i-- {
        rootError = &customError{
            err:     errs[i],
            wrapped: rootError,
        }
    }
    return rootError
}

With this custom error type, you can easily wrap and unwrap multiple errors and perform recursive Is() and As() checks:

err := Wrap(Err1, Err2, Err3)
fmt.Println(errors.Is(err, Err1)) // true
fmt.Println(errors.Is(err, Err3)) // false

var errType ErrType
errors.As(err, &errType)
fmt.Println(errType) // "my error type"

While this custom implementation provides recursive error unwrapping, there is currently no built-in type in the Go standard library that provides this functionality out of the box. However, libraries like github.com/pkg/errors offer additional error-handling capabilities, including recursive unwrapping, which may be worth considering in your own projects.

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