8-5-2. Guards and 8-5-3. Bindings

8/8-5-2_guards.rs

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+fn guard_example1(pair: (i32, i32)) {
+    println!("Tell me about {:?}", pair);
+    match pair {
+        (x, y) if x == y => println!("These are twins"),
+        // The ^ `if condition` part is a guard
+        //     ^ `if`とそれに続く条件式がガードです。
+        (x, y) if x + y == 0 => println!("Antimatter, kaboom!"),
+        (x, _) if x % 2 == 1 => println!("The first one is odd"),
+        _ => println!("No correlation..."),
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+struct A {
+    x: i32,
+}
+
+fn main() {
+    let pair = (2, -2);
+    // TODO ^ Try different values for `pair`
+    // TODO ^ `pair`の値を変更してみましょう。
+    guard_example1(pair);
+
+    // When a expression matches multiple guards,
+    // the expression is captured by the topmost guard.
+    let pair = (0, 0);
+    guard_example1(pair);   // Satisfies `x == y` and `x + y == 0` but prints 'There are twins'.
+
+    // The compiler cannot check the match expression covers all possible conditions anymore,
+    // so you MUST use the `_` pattern at the end.
+    let number: u8 = 4;
+    match number {
+        i if i == 0 => println!("Zero"),
+        i if i > 0 => println!("Greater than zero"),
+        _ => panic!("Fell through"), // This should not be possible to reach
+    }
+}

8/8-5-3_bindings.rs

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+fn binding_example1(age: u32) {
+    match age {
+        0             => println!("I haven't celebrated my first birthday yet"),
+        // Could `match` 1 ..= 12 directly but then what age
+        // would the child be? Instead, bind to `n` for the
+        // sequence of 1 ..= 12. Now the age can be reported.
+        // `1 ... 12`の値を一挙に`match`させることができる。
+        // しかしその場合、子供は正確には何歳?
+        // マッチした値を`n`にバインディングすることで値を使用できる。
+        n @ 1  ..= 12 => println!("I'm a child of age {:?}", n),
+        n @ 13 ..= 19 => println!("I'm a teen of age {:?}", n),
+        // Nothing bound. Return the result.
+        // マッチしなかった場合の処理
+        n             => println!("I'm an old person of age {:?}", n),
+    }
+}
+
+#[derive(Debug)]
+struct Movable { x: i32 }
+
+#[derive(Clone, Copy, Debug)]
+struct Copyable { x: i32 }
+
+fn main() {
+    println!("Tell me what type of person you are");
+    binding_example1(8);   // Prints `I'm a child of age 8`.
+    binding_example1(15);  // Prints `I'm a teen of age 15`.
+    binding_example1(30);  // Prints `I'm an old person of age 30`.
+
+    // Capturing by move
+    let a = Movable { x: 100 };
+    println!("a: {:p}", &a);
+    println!("x: {:p}", &a.x);  // &a equals to &a.x, because `x` is the first member of `a`.
+    match a {
+        b @ Movable { x: y } => {
+            println!("Moved a:      {:p}", &b);
+            println!("Captured a.x: {:p}", &y); // &b not equals to &y, because `y` is newly captured variable.
+            // println!("Original a: {:p}", &a);
+            //                              ^^ value is borrowed after move
+        }
+    }
+
+    // Capturing by copy
+    let a = Copyable { x: 100 };
+    println!("a: {:p}", &a);
+    println!("x: {:p}", &a.x);
+    match a {
+        b @ Copyable { x: y } => {
+            println!("Copied a:     {:p}", &b);
+            println!("Captured a.x: {:p}", &y); // &b not equals to &y, because `y` is newly captured variable.
+            println!("Original a:   {:p}", &a); // `a` can be borrowed.
+        }
+    }
+
+    // Capturing by reference
+    let a = Movable { x: 100 };
+    println!("a: {:p}", &a);
+    println!("x: {:p}", &a.x);
+    match a {
+        ref b @ Movable { x: ref y } => {
+            println!("Referenced a:   {:p}", b);
+            println!("           a.x: {:p}", y);    // &b equals to &y, because `y` captures a reference of a.x.
+            println!("Original a:     {:p}", &a);
+        }
+    }
+}