Go - Wrapping an Error with Other Errors

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 
}