concepts: Add alternative Post implementation with states as types
[learn-rust.git] / concepts / src / lib.rs
index b7e5eb8f1df11116f8d94fd31fb1afe29491d087..5f1feea9404c8116e1cbb1106b825e5a6d52c677 100644 (file)
@@ -108,3 +108,148 @@ impl Iterator for Counter5 {
         }
     }
 }
+
+pub struct Post {
+    state: Box<dyn State>,
+    content: String,
+}
+
+impl Post {
+    pub fn new() -> Post {
+        Post {
+            state: 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.content(self)
+    }
+
+    pub fn request_review(&mut self) {
+        self.state = self.state.request_review();
+    }
+
+    pub fn approve(&mut self) {
+        self.state = self.state.approve();
+    }
+
+    pub fn reject(&mut self) {
+        self.state = self.state.reject();
+    }
+}
+
+trait State {
+    fn request_review(&self) -> Box<dyn State>;
+    fn approve(&mut self) -> Box<dyn State>;
+    fn reject(&self) -> Box<dyn State>;
+
+    #[allow(unused_variables)]
+    fn content<'a>(&self, post: &'a Post) -> &'a str {
+        ""
+    }
+}
+
+struct Draft {}
+impl State for Draft {
+    fn request_review(&self) -> Box<dyn State> {
+        Box::new(PendingReview {acks: 0})
+    }
+
+    fn approve(&mut self) -> Box<dyn State> {
+        // don't change state
+        Box::new(Self {})
+    }
+
+    fn reject(&self) -> Box<dyn State> {
+        Box::new(Self {})
+    }
+}
+
+struct PendingReview {
+    acks: u32,
+}
+
+impl State for PendingReview {
+    fn request_review(&self) -> Box<dyn State> {
+        Box::new(Self {acks: self.acks})
+    }
+
+    fn approve(&mut self) -> Box<dyn State> {
+        if self.acks >= 1 {
+            Box::new(Published {})
+        } else {
+            Box::new(Self {acks: self.acks + 1})
+        }
+    }
+
+    fn reject(&self) -> Box<dyn State> {
+        Box::new(Draft {})
+    }
+}
+
+struct Published {}
+impl State for Published {
+    fn request_review(&self) -> Box<dyn State> {
+        Box::new(Self {})
+    }
+
+    fn approve(&mut self) -> Box<dyn State> {
+        Box::new(Published {})
+    }
+
+    fn reject(&self) -> Box<dyn State> {
+        Box::new(Self {})
+    }
+
+    fn content<'a>(&self, post: &'a Post) -> &'a str {
+        &post.content
+    }
+}
+
+// state encoded as types; this is the "approved" state
+pub struct TPost {
+    content: String,
+}
+
+impl TPost {
+    pub fn new() -> TPostDraft {
+        TPostDraft {content: String::new()}
+    }
+
+    pub fn content(&self) -> &str {
+        &self.content
+    }
+}
+
+pub struct TPostDraft {
+    content: String,
+}
+
+impl TPostDraft {
+    pub fn add_text(&mut self, text: &str) {
+        self.content.push_str(text);
+    }
+
+    pub fn request_review(self) -> TPostReview {
+        TPostReview {content: self.content}
+    }
+}
+
+pub struct TPostReview {
+    content: String,
+}
+
+impl TPostReview {
+    pub fn approve(self) -> TPost {
+        TPost {content: self.content}
+    }
+
+    pub fn reject(self) -> TPostDraft {
+        TPostDraft {content: self.content}
+    }
+}