add-chapter-on-defaults (#1580)

* add-chapter-on-defaults

* add-subtutles

src/SUMMARY.md

@@ -160,6 +160,7 @@
         - [Unpacking options with `?`](error/option_unwrap/question_mark.md)
         - [Combinators: `map`](error/option_unwrap/map.md)
         - [Combinators: `and_then`](error/option_unwrap/and_then.md)
+        - [Defaults: `or`, `or_else`, `get_or_insert`, 'get_or_insert_with`](error/option_unwrap/defaults.md)
     - [`Result`](error/result.md)
         - [`map` for `Result`](error/result/result_map.md)
         - [aliases for `Result`](error/result/result_alias.md)

src/error/option_unwrap/defaults.md

@@ -0,0 +1,123 @@
+# Unpacking options and defaults
+
+The is more than one way to unpack an `Option` and fall back on a default if it is `None`. To choose the one that meets our needs, we need to consider the following:
+* do we need eager or lazy evaluation?
+* do we need to keep the original empty value intact, or modify it in place?
+
+##  `or()` is chainable, evaluates eagerly, keeps empty value intact
+
+`or()`is chainable and eagerly evaluates its argument, as is shown in the following example. Note that because `or`'s arguments are evaluated eagerly, the variable passed to `or` is moved.
+
+```
+#[derive(Debug)] 
+enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }
+
+fn main() {
+    let apple = Some(Fruit::Apple);
+    let orange = Some(Fruit::Orange);
+    let no_fruit: Option<Fruit> = None;
+
+    let first_available_fruit = no_fruit.or(orange).or(apple);
+    println!("first_available_fruit: {:?}", first_available_fruit);
+    // first_available_fruit: Some(Orange)
+
+    // `or` moves its argument.
+    // In the example above, `or(orange)` returned a `Some`, so `or(apple)` was not invoked.
+    // But the variable named `apple` has been moved regardless, and cannot be used anymore.
+    // println!("Variable apple was moved, so this line won't compile: {:?}", apple);
+    // TODO: uncomment the line above to see the compiler error
+ }
+```
+
+##  `or_else()` is chainable, evaluates lazily, keeps empty value intact
+
+Another alternative is to use `or_else`, which is also chainable, and evaluates lazily, as is shown in the following example:
+
+```
+#[derive(Debug)] 
+enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }
+
+fn main() {
+    let apple = Some(Fruit::Apple);
+    let no_fruit: Option<Fruit> = None;
+    let get_kiwi_as_fallback = || {
+        println!("Providing kiwi as fallback");
+        Some(Fruit::Kiwi)
+    };
+    let get_lemon_as_fallback = || {
+        println!("Providing lemon as fallback");
+        Some(Fruit::Lemon)
+    };
+
+    let first_available_fruit = no_fruit
+        .or_else(get_kiwi_as_fallback)
+        .or_else(get_lemon_as_fallback);
+    println!("first_available_fruit: {:?}", first_available_fruit);
+    // Providing kiwi as fallback
+    // first_available_fruit: Some(Kiwi)
+}
+```
+
+##  `get_or_insert()` evaluates eagerly, modifies empty value im place
+
+To make sure that an `Option` contains a value, we can use `get_or_insert` to modify it in place with a fallback value, as is shown in the following example. Note that `get_or_insert` eagerly evaluaes its parameter, so variable `apple` is moved:
+
+```
+#[derive(Debug)] 
+enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }
+
+fn main() {
+    let mut my_fruit: Option<Fruit> = None;
+    let apple = Fruit::Apple;
+    let first_available_fruit = my_fruit.get_or_insert(apple);
+    println!("my_fruit is: {:?}", first_available_fruit);
+    println!("first_available_fruit is: {:?}", first_available_fruit);
+    // my_fruit is: Apple
+    // first_available_fruit is: Apple
+    //println!("Variable named `apple` is moved: {:?}", apple);
+    // TODO: uncomment the line above to see the compliler error
+}
+```
+
+##  `get_or_insert_with()` evaluates lazily, modifies empty value im place
+
+Instead of explicitly providing a value to fall back on, we can pass a closure to `get_or_insert_with`, as follows:
+```
+#[derive(Debug)] 
+enum Fruit { Apple, Orange, Banana, Kiwi, Lemon }
+
+fn main() {
+    let mut my_fruit: Option<Fruit> = None;
+    let get_lemon_as_fallback = || {
+        println!("Providing lemon as fallback");
+        Fruit::Lemon
+    };
+    let first_available_fruit = my_fruit
+        .get_or_insert_with(get_lemon_as_fallback);
+    println!("my_fruit is: {:?}", first_available_fruit);
+    println!("first_available_fruit is: {:?}", first_available_fruit);
+    // Providing lemon as fallback
+    // my_fruit is: Lemon
+    // first_available_fruit is: Lemon
+
+    // If the Option has a value, it is left unchanged, and the closure is not invoked
+    let mut my_apple = Some(Fruit::Apple);
+    let should_be_apple = my_apple.get_or_insert_with(get_lemon_as_fallback);
+    println!("should_be_apple is: {:?}", should_be_apple);
+    println!("my_apple is unchanged: {:?}", my_apple);
+    // The output is a follows. Note that the closure `get_lemon_as_fallback` is not invoked
+    // should_be_apple is: Apple
+    // my_apple is unchanged: Some(Apple)
+}
+```
+
+### See also:
+
+[`closures`][closures], [`get_or_insert`][get_or_insert], [`get_or_insert_with`][get_or_insert_with], ,[`moved variables`][moved], [`or`][or], [`or_else`][or_else]
+
+[closures]: https://doc.rust-lang.org/book/ch13-01-closures.html
+[get_or_insert]: https://doc.rust-lang.org/core/option/enum.Option.html#method.get_or_insert
+[get_or_insert_with]: https://doc.rust-lang.org/core/option/enum.Option.html#method.get_or_insert_with
+[moved]: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html
+[or]: https://doc.rust-lang.org/core/option/enum.Option.html#method.or
+[or_else]: https://doc.rust-lang.org/core/option/enum.Option.html#method.or_else