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course/6-errors/exercises/3-custom_errors/readme.md

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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:
+
+```go
+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:
+
+```go
+func sendSMS(msg, userName string) error {
+    if !canSendToUser(userName) {
+        return userError{name: userName}
+    }
+    ...
+}
+```
+
+## Assignment
+
+Our users are frequently trying 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 think it would be best to make a specific 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:
+
+```
+can not divide DIVIDEND by zero
+```
+
+Where `DIVIDEND` is the actual dividend of the `divideError`. Use the `%v` [verb](https://pkg.go.dev/fmt#hdr-Printing) to format the dividend as a float.