dev(extensions): [+] hellodriver

The hellodriver extension provides a useful interface for generating
greetings at the same time as a simple example of how to implement
drivers.

extensions/hellodriver.js

@@ -0,0 +1,50 @@
+/**
+ * Here we create an interface called 'hello-world'. This interface
+ * specifies that any implementation of 'hello-world' should implement
+ * a method called `greet`. The greet method has a couple of optional
+ * parameters including `subject` and `locale`. The `locale` parameter
+ * is not implemented by the driver implementation in the proceeding
+ * definition, showing how driver implementations don't always need
+ * to support optional features.
+ *
+ * subject: the person to greet
+ * locale: a standard locale string (ex: en_US.UTF-8)
+ */
+extension.on('create.interfaces', event => {
+    event.createInterface('hello-world', {
+        description: 'Provides methods for generating greetings',
+        methods: {
+            greet: {
+                description: 'Returns a greeting',
+                parameters: {
+                    subject: {
+                        type: 'string',
+                        optional: true,
+                    },
+                    locale: {
+                        type: 'string',
+                        optional: true,
+                    },
+                },
+            },
+        },
+    });
+});
+
+/**
+ * Here we register an implementation of the `hello-world` driver
+ * interface. This implementation is called "no-frills" which is
+ * the most basic reasonable implementation of the interface. The
+ * default return value is "Hello, World!", but if subject is
+ * provided it will be "Hello, <subject>!".
+ */
+extension.on('create.drivers', event => {
+    event.createDriver('hello-world', 'no-frills', {
+        greet ({ subject }) {
+            if ( subject === '%fail%' ) {
+                throw new Error('failing on purpose');
+            }
+            return `Hello, ${subject ?? 'World'}!`;
+        },
+    });
+});