Update closures

Author: junderw

exercises/18_closures/closures1.rs

@@ -10,8 +10,6 @@
 //
 // Execute `rustlings hint closures1` or use the `hint` watch subcommand for a hint.
 
-// I AM NOT DONE
-
 
 trait Doorman {
     fn greet_customer(&self, customer_name: &str);

rustlings-macros/info.toml

@@ -877,8 +877,7 @@ https://doc.rust-lang.org/book/ch11-01-writing-tests.html#checking-for-panics-wi
 
 [[exercises]]
 name = "closures1"
-path = "exercises/18_closures/closures1.rs"
-mode = "compile"
+dir = "18_closures"
 hint = """
 Self is a concept that is only used in struct/enum methods.
 

solutions/18_closures/closures1.rs

@@ -0,0 +1,61 @@
+// closures1.rs
+//
+// "Why do we even need closures?" is a question that gets asked from time to time.
+// While it's true that most things that closures can do can also be done with
+// regular old structs and enums, closures can make things a lot more clear with a lot
+// less clutter compared to structs.
+//
+// Below is a good example of how one could implement a capturing closure using structs,
+// and how closures simplifies this greatly.
+//
+// Execute `rustlings hint closures1` or use the `hint` watch subcommand for a hint.
+
+trait Doorman {
+    fn greet_customer(&self, customer_name: &str);
+}
+
+struct GreeterWithState<'a> {
+    greeting: &'a str,
+}
+
+impl Doorman for GreeterWithState<'_> {
+    fn greet_customer(&self, customer_name: &str) {
+        println!("{}, {}?", self.greeting, customer_name);
+    }
+}
+
+fn greet_customers(doorman: impl Doorman) {
+    doorman.greet_customer("Bill");
+    doorman.greet_customer("Alex");
+    doorman.greet_customer("John");
+    doorman.greet_customer("Jessie");
+}
+
+fn greet_customers_closure(doorman: impl Fn(&str)) {
+    doorman("Bill");
+    doorman("Alex");
+    doorman("John");
+    doorman("Jessie");
+}
+
+fn main() {
+    let greeting = String::from("Hello! How are you");
+
+    // Method 1 for passing in functions with state.
+    // Just create a struct, store the state, and add a method.
+    // If you need to be generic, it can be a trait method.
+    let doorman = GreeterWithState {
+        greeting: &greeting,
+    };
+    greet_customers(doorman);
+
+    // Method 2 for passing in functions with state.
+    // Notice that the body of this closure is exactly the same
+    // as GreeterWithState's Doorman implementation.
+    //
+    // This makes things much cleaner with less clutter, but
+    // we are forgetting something very important.
+    greet_customers_closure(|customer_name| {
+        println!("{}, {}?", greeting, customer_name); // Capture greeting by reference
+    })
+}