box1 solution

exercises/19_smart_pointers/box1.rs

@@ -4,45 +4,43 @@
 // `Box` - a smart pointer used to store data on the heap, which also allows us
 // to wrap a recursive type.
 //
-// The recursive type we're implementing in this exercise is the `cons list` - a
+// The recursive type we're implementing in this exercise is the "cons list", a
 // data structure frequently found in functional programming languages. Each
-// item in a cons list contains two elements: the value of the current item and
+// item in a cons list contains two elements: The value of the current item and
 // the next item. The last item is a value called `Nil`.
-//
-// Step 1: use a `Box` in the enum definition to make the code compile
-// Step 2: create both empty and non-empty cons lists by replacing `todo!()`
-//
-// Note: the tests should not be changed
 
+// TODO: Use a `Box` in the enum definition to make the code compile.
 #[derive(PartialEq, Debug)]
 enum List {
     Cons(i32, List),
     Nil,
 }
 
-fn main() {
-    println!("This is an empty cons list: {:?}", create_empty_list());
-    println!(
-        "This is a non-empty cons list: {:?}",
-        create_non_empty_list()
-    );
-}
-
+// TODO: Create an empty cons list.
 fn create_empty_list() -> List {
     todo!()
 }
 
+// TODO: Create a non-empty cons list.
 fn create_non_empty_list() -> List {
     todo!()
 }
 
+fn main() {
+    println!("This is an empty cons list: {:?}", create_empty_list());
+    println!(
+        "This is a non-empty cons list: {:?}",
+        create_non_empty_list(),
+    );
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
     fn test_create_empty_list() {
-        assert_eq!(List::Nil, create_empty_list());
+        assert_eq!(create_empty_list(), List::Nil);
     }
 
     #[test]

rustlings-macros/info.toml

@@ -969,21 +969,16 @@ a different method that could make your code more compact than using `fold`."""
 name = "box1"
 dir = "19_smart_pointers"
 hint = """
-Step 1:
-
-The compiler's message should help: since we cannot store the value of the
+The compiler's message should help: Since we cannot store the value of the
 actual type when working with recursive types, we need to store a reference
 (pointer) to its value.
 
-We should, therefore, place our `List` inside a `Box`. More details in the book
-here: https://doc.rust-lang.org/book/ch15-01-box.html#enabling-recursive-types-with-boxes
+We should, therefore, place our `List` inside a `Box`. More details in The Book:
+https://doc.rust-lang.org/book/ch15-01-box.html#enabling-recursive-types-with-boxes
 
-Step 2:
+Creating an empty list should be fairly straightforward (Hint: Read the tests).
 
-Creating an empty list should be fairly straightforward (hint: peek at the
-assertions).
-
-For a non-empty list keep in mind that we want to use our `Cons` "list builder".
+For a non-empty list, keep in mind that we want to use our `Cons` list builder.
 Although the current list is one of integers (`i32`), feel free to change the
 definition and try other types!"""
 

solutions/19_smart_pointers/box1.rs

@@ -1 +1,47 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+// At compile time, Rust needs to know how much space a type takes up. This
+// becomes problematic for recursive types, where a value can have as part of
+// itself another value of the same type. To get around the issue, we can use a
+// `Box` - a smart pointer used to store data on the heap, which also allows us
+// to wrap a recursive type.
+//
+// The recursive type we're implementing in this exercise is the "cons list", a
+// data structure frequently found in functional programming languages. Each
+// item in a cons list contains two elements: The value of the current item and
+// the next item. The last item is a value called `Nil`.
+
+#[derive(PartialEq, Debug)]
+enum List {
+    Cons(i32, Box<List>),
+    Nil,
+}
+
+fn create_empty_list() -> List {
+    List::Nil
+}
+
+fn create_non_empty_list() -> List {
+    List::Cons(42, Box::new(List::Nil))
+}
+
+fn main() {
+    println!("This is an empty cons list: {:?}", create_empty_list());
+    println!(
+        "This is a non-empty cons list: {:?}",
+        create_non_empty_list(),
+    );
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_create_empty_list() {
+        assert_eq!(create_empty_list(), List::Nil);
+    }
+
+    #[test]
+    fn test_create_non_empty_list() {
+        assert_ne!(create_empty_list(), create_non_empty_list());
+    }
+}