05: extend generics example: impl vs T

Author: wprzytula

content/lessons/05_types_reasoning/generics.rs

@@ -51,9 +51,19 @@ where
     }
 }
 
-// Here information about the concrete underlying type is erased
-// We can only either format or clone the result
-fn cloning_machine(item: &(impl Clone + Debug)) -> impl Clone + Debug {
+// Here information about the concrete underlying type is preserved.
+fn cloning_machine<T: Clone + Debug>(item: &T) -> T {
+    item.clone()
+}
+
+// Here information about the concrete underlying type is erased.
+// We can only either format or clone the result.
+fn erasing_cloning_machine1(item: &(impl Clone + Debug)) -> impl Clone + Debug {
+    item.clone()
+}
+
+// Ditto.
+fn erasing_cloning_machine2<T: Clone + Debug>(item: &T) -> impl Clone + Debug {
     item.clone()
 }
 
@@ -64,7 +74,16 @@ fn main() {
     let _p2 = Tuple2::new(1, 2.5);
 
     let arr = [1, 2, 3];
+
     let arr2 = cloning_machine(&arr);
-    // arr2[0]; // won't compile: cannot index into a value of type `impl std::clone::Clone + std::fmt::Debug`
-    println!("{:?}", arr2)
+    let _x = arr2[0]; // This compiles, because `cloning_machine` preserves the type.
+    println!("{:?}", arr2);
+
+    let arr3 = erasing_cloning_machine1(&arr);
+    // arr3[0]; // won't compile: cannot index into a value of type `impl std::clone::Clone + std::fmt::Debug`
+    println!("{:?}", arr3);
+
+    let arr4 = erasing_cloning_machine2(&arr);
+    // arr4[0]; // won't compile: cannot index into a value of type `impl std::clone::Clone + std::fmt::Debug`
+    println!("{:?}", arr4);
 }