megaherz · October 13, 2022 08:15 · Oct 13, 2022
diff --git a/blog.rs b/blog.rs
@@ -0,0 +1,95 @@
+mod blog {
+    pub struct Post {
+        state: Option<Box<dyn State>>,
+        content: String,
+    }
+
+    impl Post {
+        pub fn new() -> Post {
+            Post {
+                state: Some(Box::new(Draft {})),
+                content: String::new(),
+            }
+        }
+
+        pub fn add_text(&mut self, text: &str) {
+            self.content.push_str(text);
+        }
+
+        pub fn content(&self) -> &str {
+            self.state.as_ref().unwrap().content(self)
+        }
+
+        pub fn request_review(&mut self) {
+            if let Some(s) = self.state.take() {
+                self.state = Some(s.request_review())
+            }
+        }
+        pub fn approve(&mut self) {
+            if let Some(s) = self.state.take() {
+                self.state = Some(s.approve())
+            }
+        }
+    }
+
+    trait State {
+        fn request_review(self: Box<Self>) -> Box<dyn State>;
+        fn approve(self: Box<Self>) -> Box<dyn State>;
+        fn content<'a>(&self, post: &'a Post) -> &'a str {
+            ""
+        }
+    }
+
+    struct Draft {}
+
+    impl State for Draft {
+        fn request_review(self: Box<Self>) -> Box<dyn State> {
+            Box::new(PendingReview {})
+        }
+        fn approve(self: Box<Self>) -> Box<dyn State> {
+            self
+        }
+    }
+
+    struct PendingReview {}
+
+    impl State for PendingReview {
+        fn request_review(self: Box<Self>) -> Box<dyn State> {
+            self
+        }
+        fn approve(self: Box<Self>) -> Box<dyn State> {
+            Box::new(Published {})
+        }
+    }
+
+    struct Published {}
+
+    impl State for Published {
+        fn request_review(self: Box<Self>) -> Box<dyn State> {
+            self
+        }
+
+        fn approve(self: Box<Self>) -> Box<dyn State> {
+            self
+        }
+
+        fn content<'a>(&self, post: &'a Post) -> &'a str {
+            &post.content
+        }
+    }
+}
+
+use blog::Post;
+
+fn main() {
+    let mut post = Post::new();
+
+    post.add_text("I ate a salad for lunch today");
+    assert_eq!("", post.content());
+
+    post.request_review();
+    assert_eq!("", post.content());
+
+    post.approve();
+    assert_eq!("I ate a salad for lunch today", post.content());
+}