Step 2.1: Rich types ensure correctness

[ziimakc]

Resolves Step 2.1

Task

For the Post type described above, assume the following behavior in our application:

+-----+              +-------------+            +-----------+
| New |--publish()-->| Unmoderated |--allow()-->| Published |
+-----+              +-------------+            +-----------+
                           |                          |
                         deny()                    delete()
                           |       +---------+        |
                           +------>| Deleted |<-------+
                                   +---------+

Implement this behavior using [typestates idiom][3], so that calling delete() on New post (or calling deny() on Deleted post) will be a compile-time error.

Solution

Implemented task as described.