Ch. 6: Introduce let-else after if let.

This is not technically part of the 2024 Edition, since it is available
on the 2021 edition as well, but it *will* be an important update for
the 2024 Edition revision of the print book!

listings/ch06-enums-and-pattern-matching/listing-06-07/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "enums"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

listings/ch06-enums-and-pattern-matching/listing-06-07/src/main.rs

@@ -0,0 +1,45 @@
+#[derive(Debug)] // so we can inspect the state in a minute
+enum UsState {
+    Alabama,
+    Alaska,
+    // --snip--
+}
+
+// ANCHOR: state
+impl UsState {
+    fn existed_in(&self, year: u16) -> bool {
+        match self {
+            UsState::Alabama => year >= 1819,
+            UsState::Alaska => year >= 1959,
+            // -- snip --
+        }
+    }
+}
+// ANCHOR_END: state
+
+enum Coin {
+    Penny,
+    Nickel,
+    Dime,
+    Quarter(UsState),
+}
+
+// ANCHOR: describe
+fn describe_state_quarter(coin: Coin) -> Option<String> {
+    if let Coin::Quarter(state) = coin {
+        if state.existed_in(1900) {
+            Some(format!("{state:?} is pretty old, for America!"))
+        } else {
+            Some(format!("{state:?} is relatively new."))
+        }
+    } else {
+        None
+    }
+}
+// ANCHOR_END: describe
+
+fn main() {
+    if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
+        println!("{desc}");
+    }
+}

listings/ch06-enums-and-pattern-matching/listing-06-08/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "enums"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

listings/ch06-enums-and-pattern-matching/listing-06-08/src/main.rs

@@ -0,0 +1,45 @@
+#[derive(Debug)] // so we can inspect the state in a minute
+enum UsState {
+    Alabama,
+    Alaska,
+    // --snip--
+}
+
+impl UsState {
+    fn existed_in(&self, year: u16) -> bool {
+        match self {
+            UsState::Alabama => year >= 1819,
+            UsState::Alaska => year >= 1959,
+            // -- snip --
+        }
+    }
+}
+
+enum Coin {
+    Penny,
+    Nickel,
+    Dime,
+    Quarter(UsState),
+}
+
+// ANCHOR: describe
+fn describe_state_quarter(coin: Coin) -> Option<String> {
+    let state = if let Coin::Quarter(state) = coin {
+        state
+    } else {
+        return None;
+    };
+
+    if state.existed_in(1900) {
+        Some(format!("{state:?} is pretty old, for America!"))
+    } else {
+        Some(format!("{state:?} is relatively new."))
+    }
+}
+// ANCHOR_END: describe
+
+fn main() {
+    if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
+        println!("{desc}");
+    }
+}

listings/ch06-enums-and-pattern-matching/listing-06-09/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "enums"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

listings/ch06-enums-and-pattern-matching/listing-06-09/src/main.rs

@@ -0,0 +1,43 @@
+#[derive(Debug)] // so we can inspect the state in a minute
+enum UsState {
+    Alabama,
+    Alaska,
+    // --snip--
+}
+
+impl UsState {
+    fn existed_in(&self, year: u16) -> bool {
+        match self {
+            UsState::Alabama => year >= 1819,
+            UsState::Alaska => year >= 1959,
+            // -- snip --
+        }
+    }
+}
+
+enum Coin {
+    Penny,
+    Nickel,
+    Dime,
+    Quarter(UsState),
+}
+
+// ANCHOR: describe
+fn describe_state_quarter(coin: Coin) -> Option<String> {
+    let Coin::Quarter(state) = coin else {
+        return None;
+    };
+
+    if state.existed_in(1900) {
+        Some(format!("{state:?} is pretty old, for America!"))
+    } else {
+        Some(format!("{state:?} is relatively new."))
+    }
+}
+// ANCHOR_END: describe
+
+fn main() {
+    if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
+        println!("{desc}");
+    }
+}

src/SUMMARY.md

@@ -33,7 +33,7 @@
 - [Enums and Pattern Matching](ch06-00-enums.md)
   - [Defining an Enum](ch06-01-defining-an-enum.md)
   - [The `match` Control Flow Construct](ch06-02-match.md)
-  - [Concise Control Flow with `if let`](ch06-03-if-let.md)
+  - [Concise Control Flow with `if let` and `let else`](ch06-03-if-let.md)
 
 ## Basic Rust Literacy
 

src/ch06-03-if-let.md

@@ -1,4 +1,4 @@
-## Concise Control Flow with `if let`
+## Concise Control Flow with `if let` and `let else`
 
 The `if let` syntax lets you combine `if` and `let` into a less verbose way to
 handle values that match one pattern while ignoring the rest. Consider the
@@ -62,8 +62,70 @@ Or we could use an `if let` and `else` expression, like this:
 {{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-14-count-and-announce-if-let-else/src/main.rs:here}}
 ```
 
+## Staying on the “happy path” with `let else`
+
+One common pattern is to perform some computation when a value is present and
+return a default value otherwise. Continuing on with our example of coins with a
+`UsState` value, if we wanted to say something funny depending on how old the
+state on the quarter was, we might introduce a method on `UsState` to check the
+age of a state, like so:
+
+```rust
+{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-07/src/main.rs:state}}
+```
+
+Then we might use `if let` to match on the type of coin, introducing a `state`
+variable within the body of the condition, as in Listing 6-7.
+
+<Listing number="6-7" caption="Using" file-name="src/main.rs">
+
+```rust
+{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-07/src/main.rs:describe}}
+```
+
+</Listing>
+
+That gets the job done, but it has pushed the work into the body of the `if let`
+statement, and if the work to be done is more complicated, it might be hard to
+follow exactly how the top-level branches relate. We could also take advantage
+of the fact that expressions produce a value either to produce the `state` from
+the `if let` or to return early, as in Listing 6-8. (You could do similar with a
+`match`, of course!)
+
+<Listing number="6-8" caption="Using `if let` to produce a value or return early." file-name="src/main.rs">
+
+```rust
+{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-08/src/main.rs:describe}}
+```
+
+</Listing>
+
+This is a bit annoying to follow in its own way, though! One branch of the `if
+let` produces a value, and the other one returns from the function entirely.
+
+To make this common pattern nicer to express, Rust has `let`-`else`. The
+`let`-`else` syntax takes a pattern on the left side and an expression on the
+right, very similar to `if let`, but it does not have an `if` branch, only an
+`else` branch. If the pattern matches, it will bind the value from the pattern
+in the outer scope. If the pattern does *not* match, the program will flow into
+the `else` arm, which must return from the function.
+
+In Listing 6-9, you can see how Listing 6-8 looks when using `let`-`else` in
+place of `if let`. Notice that it stays “on the happy path” in the main body of
+the function this way, without having significantly different control flow for
+two branches the way the `if let` did.
+
+<Listing number="6-9" caption="Using `let`-`else` to clarify the flow through the function." file-name="src/main.rs">
+
+```rust
+{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-09/src/main.rs:describe}}
+```
+
+</Listing>
+
 If you have a situation in which your program has logic that is too verbose to
-express using a `match`, remember that `if let` is in your Rust toolbox as well.
+express using a `match`, remember that `if let` and `let else` are in your Rust
+toolbox as well.
 
 ## Summary