Diego Avanzini | Custom errors in Go

In any programming language, it’s sometimes necessary to handle a custom error type and handle it in a custom way.

In Go, you can create a specific type that implements the “error” interface, so that when that error type is received, it can be recognized using the type assertion system. Here’s a practical example:

type CustomError struct {
	Inner error
}

func (e CustomError) Error() string {  
	return fmt.Sprintf("this is a custom error due to: %s", e.Inner.Error())  
}

to check the if returned error is a CustomError

if returnedError, isCustomError := err.(CustomError); isCustomError {  
	...  
}

otherwise (from go 1.13)

var got CustomError
isCustomError := errors.As(err, &got)
if !isCustomError {
	...
}