The errors package located in github.com/pkg/errors is a drop-in replacement for the standard Go errors package. Additionally, it provides some very useful functionality for wrapping and handling errors. The typed and declared errors in the preceding recipe are a good example—they can be useful to add additional information to an error, but wrapping it in the standard way will change its type and break the type assertion:

// this wont work if you wrapped it 
// in a standard way. that is,
// fmt.Errorf("custom error: %s", err.Error())
if err == Package.ErrorNamed{
  //handle this error in a specific way
}

This recipe will demonstrate how to use the pkg/errors package to add annotation to errors throughout your code.