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The Error Interface

Because errors are just interfaces, you can build your own custom types that implement the error interface. Here's an example of a userError struct that implements the error interface:

type userError struct {
    name string
}

func (e userError) Error() string {
    return fmt.Sprintf("%v has a problem with their account", e.name)
}

It can then be used as an error:

func sendSMS(msg, userName string) error {
    if !canSendToUser(userName) {
        return userError{name: userName}
    }
    ...
}

Assignment

Our users frequently try to run custom analytics queries on their message deliverability metrics, and end up writing a bad query that tries to divide a number by zero. It's become such a problem that we need to make a new type of error for division by zero.

Update the code so that the divideError type implements the error interface. Its Error() method should just return a string formatted in the following way:

cannot divide DIVIDEND by zero

Where DIVIDEND is the actual dividend of the divideError. Use the %v verb to format the dividend as a float.