Step 2.1: Rich types ensure correctness

2_idioms/2_1_type_safety/src/main.rs

@@ -1,3 +1,79 @@
+use crate::post::{New, Post, PostData};
+
 fn main() {
-    println!("Implement me!");
+    let post = Post::<New>::new(PostData::default());
+
+    // post.allow(); error
+    post.publish().allow().delete();
+
+    let post = Post::<New>::new(PostData::default());
+    post.publish().deny();
+}
+
+mod post {
+    use std::marker::PhantomData;
+
+    pub struct New;
+    pub struct Unmoderated;
+    pub struct Published;
+    pub struct Deleted;
+
+    #[derive(Default)]
+    pub struct PostData {
+        id: u64,
+        user_id: u64,
+        title: String,
+        body: String,
+    }
+
+    #[derive(Clone)]
+    pub struct Post<State> {
+        id: u64,
+        user_id: u64,
+        title: String,
+        body: String,
+        _marker: PhantomData<State>,
+    }
+    impl<State> Post<State> {
+        pub fn new(data: PostData) -> Post<New> {
+            Post {
+                id: data.id,
+                user_id: data.user_id,
+                title: data.title,
+                body: data.body,
+                _marker: PhantomData,
+            }
+        }
+    }
+    impl Post<New> {
+        pub fn publish(self) -> Post<Unmoderated> {
+            self.transition::<Unmoderated>()
+        }
+    }
+    impl Post<Unmoderated> {
+        pub fn allow(self) -> Post<Published> {
+            self.transition::<Published>()
+        }
+
+        pub fn deny(self) -> Post<Deleted> {
+            self.transition::<Deleted>()
+        }
+    }
+    impl Post<Published> {
+        pub fn delete(self) -> Post<Deleted> {
+            self.transition::<Deleted>()
+        }
+    }
+
+    impl<State> Post<State> {
+        pub fn transition<NextState>(self) -> Post<NextState> {
+            Post {
+                id: self.id,
+                user_id: self.user_id,
+                title: self.title,
+                body: self.body,
+                _marker: PhantomData,
+            }
+        }
+    }
 }

README.md

@@ -81,7 +81,7 @@ Each step must be performed as a separate [PR (pull request)][PR] with an approp
     - [ ] [1.8. Thread safety][Step 1.8] (1 day)
     - [ ] [1.9. Phantom types][Step 1.9] (1 day)
 - [ ] [2. Idioms][Step 2] (2 days, after all sub-steps)
-    - [ ] [2.1. Rich types ensure correctness][Step 2.1] (1 day)
+    - [x] [2.1. Rich types ensure correctness][Step 2.1] (1 day)
     - [ ] [2.2. Swapping values with `mem::replace`][Step 2.2] (1 day)
     - [ ] [2.3. Bound behavior, not data][Step 2.3] (1 day)
     - [ ] [2.4. Abstract type in, concrete type out][Step 2.4] (1 day)