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course/6-errors/exercises/1-errors/readme.md

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+# The Error Interface
+
+Go programs express errors with `error` values. An Error is any type that implements the simple built-in [error interface](https://blog.golang.org/error-handling-and-go):
+
+```go
+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`.
+
+```go
+// 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.*