Add a test

Now that you've gotten your code to a stable place (nicely done, by the way), add a test. Testing your code during development can expose bugs that find their way in as you make changes. In this topic, you add a test for the Hello function.

Go's built-in support for unit testing makes it easier to test as you go. Specifically, using naming conventions, Go's testing package, and the go test command, you can quickly write and execute tests.

In the greetings directory, create a file called greetings_test.go.
Ending a file's name with _test.go tells the go test command that this file contains test functions.
In greetings_test.go, paste the following code and save the file.
```
package greetings

import (
    "testing"
    "regexp"
)

// TestHelloName calls greetings.Hello with a name, checking
// for a valid return value.
func TestHelloName(t *testing.T) {
    name := "Gladys"
    want := regexp.MustCompile(`\b`+name+`\b`)
    msg, err := Hello("Gladys")
    if !want.MatchString(msg) || err != nil {
        t.Fatalf(`Hello("Gladys") = %q, %v, want match for %#q, nil`, msg, err, want)
    }
}

// TestHelloEmpty calls greetings.Hello with an empty string,
// checking for an error.
func TestHelloEmpty(t *testing.T) {
    msg, err := Hello("")
    if msg != "" || err == nil {
        t.Fatalf(`Hello("") = %q, %v, want "", error`, msg, err)
    }
}
```
In this code, you:
- Implement test functions in the same package as the code you're testing.
- Create two test functions to test the greetings.Hello function. Test function names have the form TestName, where Name says something about the specific test. Also, test functions take a pointer to the testing package's testing.T type as a parameter. You use this parameter's methods for reporting and logging from your test.
- Implement two tests:
  - TestHelloName calls the Hello function, passing a name value with which the function should be able to return a valid response message. If the call returns an error or an unexpected response message (one that doesn't include the name you passed in), you use the t parameter's Fatalf method to print a message to the console and end execution.
  - TestHelloEmpty calls the Hello function with an empty string. This test is designed to confirm that your error handling works. If the call returns a non-empty string or no error, you use the t parameter's Fatalf method to print a message to the console and end execution.
At the command line in the greetings directory, run the go test command to execute the test.
The go test command executes test functions (whose names begin with Test) in test files (whose names end with _test.go). You can add the -v flag to get verbose output that lists all of the tests and their results.

The tests should pass.
```
$ go test
PASS
ok      example.com/greetings   0.364s

$ go test -v
=== RUN   TestHelloName
--- PASS: TestHelloName (0.00s)
=== RUN   TestHelloEmpty
--- PASS: TestHelloEmpty (0.00s)
PASS
ok      example.com/greetings   0.372s
```
Break the greetings.Hello function to view a failing test.
The TestHelloName test function checks the return value for the name you specified as a Hello function parameter. To view a failing test result, change the greetings.Hello function so that it no longer includes the name.

In greetings/greetings.go, paste the following code in place of the Hello function. Note that the highlighted lines change the value that the function returns, as if the name argument had been accidentally removed.
```
// Hello returns a greeting for the named person.
func Hello(name string) (string, error) {
    // If no name was given, return an error with a message.
    if name == "" {
        return name, errors.New("empty name")
    }
    // Create a message using a random format.
    // message := fmt.Sprintf(randomFormat(), name)
    message := fmt.Sprint(randomFormat())
    return message, nil
}
```
At the command line in the greetings directory, run go test to execute the test.
This time, run go test without the -v flag. The output will include results for only the tests that failed, which can be useful when you have a lot of tests. The TestHelloName test should fail -- TestHelloEmpty still passes.
```
$ go test
--- FAIL: TestHelloName (0.00s)
    greetings_test.go:15: Hello("Gladys") = "Hail, %v! Well met!", <nil>, want match for `\bGladys\b`, nil
FAIL
exit status 1
FAIL    example.com/greetings   0.182s
```

In the next (and last) topic, you'll see how to compile and install your code to run it locally.