Problem: You want to provide additional information and context to an error you receive before returning it as another error.
Solution: Wrap the error you receive with another error you create before returning it.
There are a couple of ways to wrap errors. The easiest is to use fmt.Errorf again and provide an error as part of the parameter:
err1 := errors . New ( "Oops something happened." ) err2 := fmt . Errorf ( "An error was encountered - %w" , err1 )
The %w verb allows you to place an error within the format string. In the example, err2 wraps err1 . But how do you extract err1 out of err2 ?
The errors package has an Unwrap function that does precisely this:
err := errors . Unwrap ( err2 )
This will give you back err1 .
Another way of wrapping an error with an error is to create a customized error struct like this:
type ConnectionError struct { Host string Port int Err error } func ( err * ConnectionError ) Error () string { return fmt . Sprintf ( "Error connecting to %s at port %d" , err . Host , err . Port ) }
Remember, to make it an error, the struct should have an Error method. To allow the struct to be unwrapped, you need to implement an Unwrap function:
func ( err * ConnectionError ) Unwrap () error { return err . Err }