Add [Modules]

solutions/crate-module/module.md

@@ -0,0 +1,148 @@
+1.
+```rust
+// in lib.rs
+mod front_of_house {
+    mod hosting {
+        fn add_to_waitlist() {}
+
+        fn seat_at_table() {}
+    }
+
+    mod serving {
+        fn take_order() {}
+
+        fn serve_order() {}
+
+        fn take_payment() {}
+
+        fn complain() {} 
+    }
+}
+```
+
+2.
+```rust
+// in lib.rs
+pub mod front_of_house {
+    pub mod hosting {
+        pub fn add_to_waitlist() {}
+
+        pub fn seat_at_table() {}
+    }
+
+    pub mod serving {
+        pub fn take_order() {}
+
+        pub fn serve_order() {}
+
+        pub fn take_payment() {}
+
+        // Maybe you don't want the guest hearing the your complaining about them
+        // So just make it private
+        fn complain() {} 
+    }
+}
+
+pub fn eat_at_restaurant() {
+    // 绝对路径
+    crate::front_of_house::hosting::add_to_waitlist();
+
+    // 相对路径
+    front_of_house::hosting::add_to_waitlist();
+}
+```
+
+3.
+```rust
+mod back_of_house {
+    fn fix_incorrect_order() {
+        cook_order();
+        super::front_of_house::serving::serve_order();
+    }
+
+    fn cook_order() {}
+}
+```
+
+```rust
+mod back_of_house {
+    fn fix_incorrect_order() {
+        cook_order();
+        crate::front_of_house::serving::serve_order();
+    }
+
+    fn cook_order() {}
+}
+```
+
+
+
+4.
+```rust
+// in src/lib.rs
+
+mod front_of_house;
+mod back_of_house;
+pub fn eat_at_restaurant() -> String {
+    front_of_house::hosting::add_to_waitlist();
+
+    back_of_house::cook_order();
+
+    String::from("yummy yummy!")
+}
+```
+
+```rust
+// in src/back_of_house.rs
+
+use crate::front_of_house;
+pub fn fix_incorrect_order() {
+    cook_order();
+    front_of_house::serving::serve_order();
+}
+
+pub fn cook_order() {}
+```
+
+
+```rust
+// in src/front_of_house/mod.rs
+
+pub mod hosting;
+pub mod serving;
+```
+
+```rust
+// in src/front_of_house/hosting.rs
+
+pub fn add_to_waitlist() {}
+
+pub fn seat_at_table() -> String {
+    String::from("sit down please")
+}
+```
+
+```rust,editable
+// in src/front_of_house/serving.rs
+
+pub fn take_order() {}
+
+pub fn serve_order() {}
+
+pub fn take_payment() {}
+
+// Maybe you don't want the guest hearing the your complaining about them
+// So just make it private
+fn complain() {} 
+```
+
+
+5.
+```rust
+mod front_of_house;
+
+fn main() {
+    assert_eq!(front_of_house::hosting::seat_at_table(), "sit down please");
+    assert_eq!(hello_package::eat_at_restaurant(),"yummy yummy!");
+}
+```