The Error Interface

Go programs express errors with error values. An Error is any type that implements the simple built-in error interface:

type error interface {
    Error() string
}

When something can go wrong in a function, that function should return an error as its last return value. Any code that calls a function that can return an error should handle errors by testing whether the error is nil.

// Atoi converts a stringified number to an interger
i, err := strconv.Atoi("42b")
if err != nil {
    fmt.Println("couldn't convert:", err)
    // because "42b" isn't a valid integer, we print:
    // couldn't convert: strconv.Atoi: parsing "42b": invalid syntax
    // Note:
    // 'parsing "42b": invalid syntax' is returned by the .Error() method
    return
}
// if we get here, then
// i was converted successfully

A nil error denotes success; a non-nil error denotes failure.

Assignment

We offer a product that allows businesses that use Textio to send pairs of messages to couples. It is mostly used by flower shops and movie theaters.

Complete the sendSMSToCouple function. It should send 2 messages, first to the customer, then to the customer's spouse.

1. Use sendSMS() to send the msgToCustomer. If an error is encountered, return 0.0 and the error.
2. Do the same for the msgToSpouse
3. If both messages are sent successfully, return the total cost of the messages added together.

When you return a non-nil error in Go, it's conventional to return the "zero" values of all other return values.