Wrapping structured errors

A structured error can be used to decorate another error with additional information by wrapping it. This recipe shows how to do that.

How to do it...

1. Keep an error member variable (or a slice of errors) to store the root cause in the structure.
2. Implement Unwrap() error (or Unwrap() []error) method.

How it works...

You can wrap the root cause error in a structured error. This allows you to add structured contextual information about the error:

type ErrFile struct {
   Name string
   When string
   Err error
}
func (err ErrFile) Error() string {
   return fmt.Sprintf("%s: file %s, when %s", err.Err, err.Name, err.
   When)
}
func (err ErrFile) Unwrap() error { return err.Err }
func ReadConfigFile(name string) error {
  f, err:=os.Open(name)
  if err!=nil {
     return ErrFile {
    ...