100 exercises to learn Rust

book/src/05_ticket_v2/04_if_let.md

@@ -0,0 +1,70 @@
+# Concise branching
+
+Your solution to the previous exercise probably looks like this:
+
+```rust
+impl Ticket {
+    pub fn assigned_to(&self) -> &String {
+        match &self.status {
+            Status::InProgress { assigned_to } => assigned_to,
+            Status::Done | Status::ToDo => {
+                panic!("Only `In-Progress` tickets can be assigned to someone"),
+            }
+        }
+    }
+}
+```
+
+You only care about the `Status::InProgress` variant. 
+Do you really need to match on all the other variants?
+
+New constructs to the rescue!
+
+## `if let`
+
+The `if let` construct allows you to match on a single variant of an enum, 
+without having to handle all the other variants.
+
+Here's how you can use `if let` to simplify the `assigned_to` method:
+
+```rust
+impl Ticket {
+    pub fn assigned_to(&self) -> &String {
+        if let Status::InProgress { assigned_to } = &self.status {
+            assigned_to
+        } else {
+            panic!("Only `In-Progress` tickets can be assigned to someone");
+        }
+    }
+}
+```
+
+## `let/else`
+
+If the `else` branch is meant to return early (a panic counts as returning early!),
+you can use the `let/else` construct:
+
+```rust
+impl Ticket {
+    pub fn assigned_to(&self) -> &String {
+        let Status::InProgress { assigned_to } = &self.status else {
+            panic!("Only `In-Progress` tickets can be assigned to someone");
+        };
+        assigned_to
+    }
+}
+```
+
+It allows you to assign the destructured variable without incurring
+any "right drift", i.e. the variable is assigned at the same indentation level
+as the code that precedes it.
+
+## Style
+
+Both `if let` and `let/else` are idiomatic Rust constructs.  
+Use them as you see fit to improve the readability of your code, 
+but don't overdo it: `match` is always there when you need it.
+
+## References
+
+- The exercise for this section is located in `exercises/05_ticket_v2/04_if_let`