Using standard library only, how to wrap error in custom error?

Question

Since Go 1.13 we have the ability to chain errors, unwrap them and check if any error in the chain matches any of expected errors via errors.Is() and errors.As().

To wrap an error all you have to do is use %w verb with fmt.Errorf() like below.

fmt.Errorf("Custom message: %w", err)

This is easy, it wraps err in another one with additional message. But let's say I need some more context than just a message. How do I wrap err in my own, structured, custom error? Using Go 1.13+ standard library only.

pepsighan · Accepted Answer · 2020-04-24 14:13:20Z

15

You may create a new error type which wraps other errors while providing additional structured information.

type MyError struct {
    Inner error
    // Provide any additional fields that you need.
    Message string
    AdditionalContext string 
}

// Error is mark the struct as an error.
func (e *MyError) Error() string {
    return fmt.Sprintf("error caused due to %v; message: %v; additional context: %v", e.Inner, e.Message, e.AdditionalContext)
}

// Unwrap is used to make it work with errors.Is, errors.As.
func (e *MyError) Unwrap() error {
    // Return the inner error.
    return e.Inner
}

// WrapWithMyError to easily create a new error which wraps the given error.
func WrapWithMyError(err error, message string, additionalContext string) error {
    return &MyError {
        Inner: err,
        Message: message,
        AdditionalContext: additionalContext,
    }
}

You need to implement Error and Unwrap interfaces to use the new features of errors.Is and errors.As.

answered Apr 24, 2020 at 14:13

pepsighan

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3 Comments

wst Over a year ago

Perfect, this is what I wanted. Additionally I've checked std lib fmt.Errorf code and it does exactly this. It creates a struct with inner error and Unwrap method. I wish Unwrap method was part of an interface, but it seems it isn't.

pepsighan Over a year ago

Unwrap is part of an ad-hoc interface at golang.org/src/errors/wrap.go?s=372:400#L4.

wst Over a year ago

I meant classical, exposed interface. Turns our there is no specific reason for that - they couldn't come up with a name. github.com/golang/go/issues/35306

Muhamed Keta · Accepted Answer · 2020-04-24 13:04:12Z

-2

Errors are an interface (satisfied by error() string) So you can make another error type as:

type myCustomError struct {
  Message    string
  StatusCode int
}

func (m myCustomError) error() string {
  return fmt.Sprintf("Code %d: \tMessage: %s", m.StatusCode, m.Message) 
}

Now you can use an error that is thrown as a custom error with

_, err := doStuff()
var v myCustomError
if errors.As(err, &v) {
   // now v is parsed 
}

answered Apr 24, 2020 at 13:04

Muhamed Keta

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1 Comment

wst Over a year ago

Question was not about creating customer error but wrapping existing error in the custom one to create an error chain, which can be later checked with errors.As and errors.Is functions.

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