concepts: Rewrite Post without Option

This gets rid of a lot of extra Option handling code, at the price of
having to rebuild instead of pass through non-changing states.

diff --git a/concepts/src/lib.rs b/concepts/src/lib.rs
index ca0624142dea4d751e4506184bf84508de7e0696..bb15eeb814ea45d9aee64ed43497f586df33ba65 100644 (file)
--- a/concepts/src/lib.rs
+++ b/concepts/src/lib.rs
@@ -110,14 +110,14 @@ impl Iterator for Counter5 {
 }
 
 pub struct Post {
-    state: Option<Box<dyn State>>,
+    state: Box<dyn State>,
     content: String,
 }
 
 impl Post {
     pub fn new() -> Post {
         Post {
-            state: Some(Box::new(Draft {})),
+            state: Box::new(Draft {}),
             content: String::new(),
         }
     }
@@ -127,27 +127,21 @@ impl Post {
     }
 
     pub fn content(&self) -> &str {
-        // as_ref() converts Option<Box<State>> to Option<&Box<State>>
-        // state can never be None, all state transitions return a new one
-        self.state.as_ref().unwrap().content(self)
+        self.state.content(self)
     }
 
     pub fn request_review(&mut self) {
-        if let Some(s) = self.state.take() {
-            self.state = Some(s.request_review());
-        }
+        self.state = self.state.request_review();
     }
 
     pub fn approve(&mut self) {
-        if let Some(s) = self.state.take() {
-            self.state = Some(s.approve());
-        }
+        self.state = self.state.approve();
     }
 }
 
 trait State {
-    fn request_review(self: Box::<Self>) -> Box<dyn State>;
-    fn approve(self: Box::<Self>) -> Box<dyn State>;
+    fn request_review(&self) -> Box<dyn State>;
+    fn approve(&self) -> Box<dyn State>;
 
     #[allow(unused_variables)]
     fn content<'a>(&self, post: &'a Post) -> &'a str {
@@ -157,33 +151,34 @@ trait State {
 
 struct Draft {}
 impl State for Draft {
-    fn request_review(self: Box::<Self>) -> Box<dyn State> {
+    fn request_review(&self) -> Box<dyn State> {
         Box::new(PendingReview {})
     }
 
-    fn approve(self: Box::<Self>) -> Box<dyn State> {
-        self
+    fn approve(&self) -> Box<dyn State> {
+        // don't change state
+        Box::new(Self {})
     }
 }
 
 struct PendingReview {}
 impl State for PendingReview {
-    fn request_review(self: Box::<Self>) -> Box<dyn State> {
-        self
+    fn request_review(&self) -> Box<dyn State> {
+        Box::new(Self {})
     }
 
-    fn approve(self: Box::<Self>) -> Box<dyn State> {
+    fn approve(&self) -> Box<dyn State> {
         Box::new(Published {})
     }
 }
 
 struct Published {}
 impl State for Published {
-    fn request_review(self: Box::<Self>) -> Box<dyn State> {
-        self
+    fn request_review(&self) -> Box<dyn State> {
+        Box::new(Self {})
     }
 
-    fn approve(self: Box::<Self>) -> Box<dyn State> {
+    fn approve(&self) -> Box<dyn State> {
         Box::new(Published {})
     }