How to assert on error types in Go tests?

Question

I have a error type like below defined

type RetryableError struct {
    msg string
}

func (a *RetryableError) Error() string {
    return a.msg
}

In a unit test, what is the Go way of asserting if the error returned is of RetryableError type?

Answer 1

Use type assertion:

err := someFunc()
if retryable, ok := err.(*RetryableError); ok {
   // use retryable
}

Your RetryableError is not an error, but *RetryableError is. To correct:

func (a RetryableError) Error() string {
    return a.msg
}