From 127a97dc297b675047b871e9a72b8dd8ffe2ba8b Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 14:59:12 +0800
Subject: [PATCH 01/22] Functional language features

---
 13-functional-language-features/README.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 13-functional-language-features/README.md

diff --git a/13-functional-language-features/README.md b/13-functional-language-features/README.md
new file mode 100644
index 0000000..f719f39
--- /dev/null
+++ b/13-functional-language-features/README.md
@@ -0,0 +1 @@
+# 13. Functional language features: iterators and closures

From 7cdedcf032d3548d2575af449feb977da684c766 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 15:04:50 +0800
Subject: [PATCH 02/22] --wip-- [skip ci]

---
 .../13-1-closures/README.md                            | 10 ++++++++++
 1 file changed, 10 insertions(+)
 create mode 100644 13-functional-language-features/13-1-closures/README.md

diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
new file mode 100644
index 0000000..aa21b90
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -0,0 +1,10 @@
+# 13.1. Closures: Anonymous functions that can capture their environment
+
+Closures:
+
+- can be saved in a variable
+- can be passed as arguments to functions
+- can be created in one place, and then later called to evaluate it in a different context
+- can capture values from the scope in which they're defined (unlike functions)
+- allow for code reuse
+- allow for behavior customisation

From 09629a4ae5150e8d079c422ec90dc3dc3954afd6 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 15:52:22 +0800
Subject: [PATCH 03/22] --wip-- [skip ci]

---
 .../workout_generator/Cargo.toml              |  8 +++++
 .../workout_generator/src/lib.rs              |  1 +
 .../workout_generator/src/main.rs             |  8 +++++
 .../workout_generator/src/workout.rs          | 29 +++++++++++++++++++
 4 files changed, 46 insertions(+)
 create mode 100644 13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml
 create mode 100644 13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs
 create mode 100644 13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
 create mode 100644 13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs

diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml
new file mode 100644
index 0000000..56f4002
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "workout_generator"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs
new file mode 100644
index 0000000..e3c76b4
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs
@@ -0,0 +1 @@
+pub mod workout;
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
new file mode 100644
index 0000000..97804d3
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
@@ -0,0 +1,8 @@
+use workout_generator::workout;
+
+fn main() {
+    let simulated_user_specified_value = 10;
+    let simulated_random_number = 7;
+
+    workout::generate_workout(simulated_user_specified_value, simulated_random_number);
+}
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
new file mode 100644
index 0000000..253d631
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
@@ -0,0 +1,29 @@
+use std::{thread, time::Duration};
+
+fn simulated_expensive_calculation(intensity: u32) -> u32 {
+    println!("calculating slowly...");
+    thread::sleep(Duration::from_secs(2)); // like timeout, timer
+    intensity
+}
+
+pub fn generate_workout(intensity: u32, random_number: u32) {
+    if intensity < 25 {
+        println!(
+            "Today, do {} pushups!",
+            simulated_expensive_calculation(intensity)
+        );
+        println!(
+            "Next, do {} situps!",
+            simulated_expensive_calculation(intensity)
+        );
+    } else {
+        if random_number == 3 {
+            println!("Take a break today! Remeber to stay hydrated!");
+        } else {
+            println!(
+                "Today, run for {} minutes",
+                simulated_expensive_calculation(intensity)
+            );
+        }
+    }
+}

From 3bd1aa2d3b5f1ba9f114fd7599f11ff89adab599 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 15:52:43 +0800
Subject: [PATCH 04/22] --wip-- [skip ci]

---
 .../01-abstract-behavior/workout_generator/src/main.rs          | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
index 97804d3..f3a727f 100644
--- a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
@@ -2,7 +2,7 @@ use workout_generator::workout;
 
 fn main() {
     let simulated_user_specified_value = 10;
-    let simulated_random_number = 7;
+    let simulated_random_number = 3;
 
     workout::generate_workout(simulated_user_specified_value, simulated_random_number);
 }

From 29e6bdb9ca6b5684dca767c1fa3845bfc5a24891 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 15:53:48 +0800
Subject: [PATCH 05/22] --wip-- [skip ci]

---
 .../01-abstract-behavior/workout_generator/src/main.rs          | 2 +-
 .../01-abstract-behavior/workout_generator/src/workout.rs       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
index f3a727f..97804d3 100644
--- a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
@@ -2,7 +2,7 @@ use workout_generator::workout;
 
 fn main() {
     let simulated_user_specified_value = 10;
-    let simulated_random_number = 3;
+    let simulated_random_number = 7;
 
     workout::generate_workout(simulated_user_specified_value, simulated_random_number);
 }
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
index 253d631..23d36e4 100644
--- a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
@@ -21,7 +21,7 @@ pub fn generate_workout(intensity: u32, random_number: u32) {
             println!("Take a break today! Remeber to stay hydrated!");
         } else {
             println!(
-                "Today, run for {} minutes",
+                "Today, run for {} minutes!",
                 simulated_expensive_calculation(intensity)
             );
         }

From 2970132046fe92bca9280a35a571d80aaabcd99c Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 16:40:32 +0800
Subject: [PATCH 06/22] --wip-- [skip ci]

---
 .../workout_generator/src/workout.rs          | 61 +++++++++++++++----
 1 file changed, 49 insertions(+), 12 deletions(-)

diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
index 23d36e4..d121dbd 100644
--- a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
@@ -6,24 +6,61 @@ fn simulated_expensive_calculation(intensity: u32) -> u32 {
     intensity
 }
 
+// Iteration #3: Define a closure, and store this definition in a variable so that it can be called later.
+// This pattern seems similar to memoization - the first time `expensive_clousure` is called.
+// the result is stored inside the variable itself.
 pub fn generate_workout(intensity: u32, random_number: u32) {
+    let expensive_closure = |num| simulated_expensive_calculation(num);
+
     if intensity < 25 {
-        println!(
-            "Today, do {} pushups!",
-            simulated_expensive_calculation(intensity)
-        );
-        println!(
-            "Next, do {} situps!",
-            simulated_expensive_calculation(intensity)
-        );
+        println!("Today, do {} pushups!", expensive_closure(intensity));
+        println!("Next, do {} situps!", expensive_closure(intensity));
     } else {
         if random_number == 3 {
             println!("Take a break today! Remeber to stay hydrated!");
         } else {
-            println!(
-                "Today, run for {} minutes!",
-                simulated_expensive_calculation(intensity)
-            );
+            println!("Today, run for {} minutes!", expensive_closure(intensity));
         }
     }
 }
+
+// Iteration #2 - refactor by storing the result of duplicated function calls in a variable.
+// Still not ideal as expensive result gets run at least once, although it's not
+// needed by the `random_number == 3` `if` block.
+// pub fn generate_workout(intensity: u32, random_number: u32) {
+//     let expensive_result = simulated_expensive_calculation(intensity);
+
+//     if intensity < 25 {
+//         println!("Today, do {} pushups!", expensive_result);
+//         println!("Next, do {} situps!", expensive_result);
+//     } else {
+//         if random_number == 3 {
+//             println!("Take a break today! Remeber to stay hydrated!");
+//         } else {
+//             println!("Today, run for {} minutes!", expensive_result);
+//         }
+//     }
+// }
+
+// Iteration #1 - expensive calculation called multiple times instead of once.
+// pub fn generate_workout(intensity: u32, random_number: u32) {
+//     if intensity < 25 {
+//         println!(
+//             "Today, do {} pushups!",
+//             simulated_expensive_calculation(intensity)
+//         );
+//         println!(
+//             "Next, do {} situps!",
+//             simulated_expensive_calculation(intensity)
+//         );
+//     } else {
+//         if random_number == 3 {
+//             println!("Take a break today! Remeber to stay hydrated!");
+//         } else {
+//             println!(
+//                 "Today, run for {} minutes!",
+//                 simulated_expensive_calculation(intensity)
+//             );
+//         }
+//     }
+// }

From be3aecd80b8e8e9187e888d6fabd00d6a9e3f80c Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 16:49:58 +0800
Subject: [PATCH 07/22] --wip-- [skip ci]

---
 .../01-abstract-behavior/workout_generator/src/workout.rs     | 4 ++--
 13-functional-language-features/13-1-closures/README.md       | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
index d121dbd..5a3f598 100644
--- a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
@@ -7,8 +7,8 @@ fn simulated_expensive_calculation(intensity: u32) -> u32 {
 }
 
 // Iteration #3: Define a closure, and store this definition in a variable so that it can be called later.
-// This pattern seems similar to memoization - the first time `expensive_clousure` is called.
-// the result is stored inside the variable itself.
+// This reintroduces the problem of the expensive calculation bein executed multiple times.
+// The solution will be presented in the next crate in this series.
 pub fn generate_workout(intensity: u32, random_number: u32) {
     let expensive_closure = |num| simulated_expensive_calculation(num);
 
diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
index aa21b90..88dde1a 100644
--- a/13-functional-language-features/13-1-closures/README.md
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -8,3 +8,4 @@ Closures:
 - can capture values from the scope in which they're defined (unlike functions)
 - allow for code reuse
 - allow for behavior customisation
+- need to be called at least once for the compiler to infer the types of their parameters & return value, if no arguments from the outer scope are passed into the definition

From 7e6dcd8b273afce592fbaac4fd4a59bec204d2c1 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 29 Mar 2022 20:04:50 +0800
Subject: [PATCH 08/22] --wip-- [skip ci]

---
 .../workout_generator/Cargo.toml              |  8 +++
 .../workout_generator/src/cacher.rs           |  0
 .../workout_generator/src/lib.rs              |  2 +
 .../workout_generator/src/main.rs             |  8 +++
 .../workout_generator/src/workout.rs          | 66 +++++++++++++++++++
 .../13-1-closures/README.md                   |  6 ++
 6 files changed, 90 insertions(+)
 create mode 100644 13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml
 create mode 100644 13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
 create mode 100644 13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs
 create mode 100644 13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
 create mode 100644 13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs

diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml
new file mode 100644
index 0000000..56f4002
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "workout_generator"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
new file mode 100644
index 0000000..e69de29
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs
new file mode 100644
index 0000000..f09b820
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod cacher;
+pub mod workout;
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
new file mode 100644
index 0000000..97804d3
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
@@ -0,0 +1,8 @@
+use workout_generator::workout;
+
+fn main() {
+    let simulated_user_specified_value = 10;
+    let simulated_random_number = 7;
+
+    workout::generate_workout(simulated_user_specified_value, simulated_random_number);
+}
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
new file mode 100644
index 0000000..5a3f598
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
@@ -0,0 +1,66 @@
+use std::{thread, time::Duration};
+
+fn simulated_expensive_calculation(intensity: u32) -> u32 {
+    println!("calculating slowly...");
+    thread::sleep(Duration::from_secs(2)); // like timeout, timer
+    intensity
+}
+
+// Iteration #3: Define a closure, and store this definition in a variable so that it can be called later.
+// This reintroduces the problem of the expensive calculation bein executed multiple times.
+// The solution will be presented in the next crate in this series.
+pub fn generate_workout(intensity: u32, random_number: u32) {
+    let expensive_closure = |num| simulated_expensive_calculation(num);
+
+    if intensity < 25 {
+        println!("Today, do {} pushups!", expensive_closure(intensity));
+        println!("Next, do {} situps!", expensive_closure(intensity));
+    } else {
+        if random_number == 3 {
+            println!("Take a break today! Remeber to stay hydrated!");
+        } else {
+            println!("Today, run for {} minutes!", expensive_closure(intensity));
+        }
+    }
+}
+
+// Iteration #2 - refactor by storing the result of duplicated function calls in a variable.
+// Still not ideal as expensive result gets run at least once, although it's not
+// needed by the `random_number == 3` `if` block.
+// pub fn generate_workout(intensity: u32, random_number: u32) {
+//     let expensive_result = simulated_expensive_calculation(intensity);
+
+//     if intensity < 25 {
+//         println!("Today, do {} pushups!", expensive_result);
+//         println!("Next, do {} situps!", expensive_result);
+//     } else {
+//         if random_number == 3 {
+//             println!("Take a break today! Remeber to stay hydrated!");
+//         } else {
+//             println!("Today, run for {} minutes!", expensive_result);
+//         }
+//     }
+// }
+
+// Iteration #1 - expensive calculation called multiple times instead of once.
+// pub fn generate_workout(intensity: u32, random_number: u32) {
+//     if intensity < 25 {
+//         println!(
+//             "Today, do {} pushups!",
+//             simulated_expensive_calculation(intensity)
+//         );
+//         println!(
+//             "Next, do {} situps!",
+//             simulated_expensive_calculation(intensity)
+//         );
+//     } else {
+//         if random_number == 3 {
+//             println!("Take a break today! Remeber to stay hydrated!");
+//         } else {
+//             println!(
+//                 "Today, run for {} minutes!",
+//                 simulated_expensive_calculation(intensity)
+//             );
+//         }
+//     }
+// }
diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
index 88dde1a..29519d0 100644
--- a/13-functional-language-features/13-1-closures/README.md
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -9,3 +9,9 @@ Closures:
 - allow for code reuse
 - allow for behavior customisation
 - need to be called at least once for the compiler to infer the types of their parameters & return value, if no arguments from the outer scope are passed into the definition
+
+## Sample code
+
+[Naively call expensive function multiple times as needed](./01-abstract-behavior/workout_generator/src/workout.rs)
+
+[Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct](./02-memoization-with-struct/workout_generator/src/)

From 91647e6ce1d1d6cd4edbd52cdea3e8236bf72300 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 5 Apr 2022 17:39:34 +0800
Subject: [PATCH 09/22] --wip-- [skip ci]

---
 13-functional-language-features/13-1-closures/README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
index 29519d0..e9f1b24 100644
--- a/13-functional-language-features/13-1-closures/README.md
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -14,4 +14,4 @@ Closures:
 
 [Naively call expensive function multiple times as needed](./01-abstract-behavior/workout_generator/src/workout.rs)
 
-[Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct](./02-memoization-with-struct/workout_generator/src/)
+[Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct / `Fn` trait](./02-memoization-with-struct/workout_generator/src/)

From 924b65ec01689a557d2871916ceb924f9580f8ad Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 5 Apr 2022 17:45:25 +0800
Subject: [PATCH 10/22] --wip-- [skip ci]

---
 .../workout_generator/src/cacher.rs                       | 8 ++++++++
 13-functional-language-features/13-1-closures/README.md   | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
index e69de29..c970ed2 100644
--- a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
@@ -0,0 +1,8 @@
+pub struct Cacher<T>
+where
+    T: Fn(u32) -> u32,
+{
+    // trait bounds on `T` specify that `calculation` field is a closure by using the `Fn` trait
+    pub calculation: T,
+    pub value: Option<u32>,
+}
diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
index e9f1b24..2084b91 100644
--- a/13-functional-language-features/13-1-closures/README.md
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -14,4 +14,4 @@ Closures:
 
 [Naively call expensive function multiple times as needed](./01-abstract-behavior/workout_generator/src/workout.rs)
 
-[Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct / `Fn` trait](./02-memoization-with-struct/workout_generator/src/)
+[Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct typed with a `Fn` trait bound that closures must match](./02-memoization-with-struct/workout_generator/src/)

From b345b5749a6fb73c403231c41e2d97a5235b569e Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Sun, 10 Apr 2022 12:55:02 +0800
Subject: [PATCH 11/22] --wip-- [skip ci]

---
 .../workout_generator/src/cacher.rs                       | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
index c970ed2..e5e158f 100644
--- a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
@@ -1,8 +1,14 @@
+// `Cacher` struct:
+// - holds a closure in `calculation` field
+//   - `calculation` field is of the generic type `T`
+//   - trait bounds on `T` specify that it's a closure by using the `Fn` trait
+//   - closure stored on `calculation` field must have on `u32` parameter and
+//     must return a `u32`
+// - holds an optional result in `value` field
 pub struct Cacher<T>
 where
     T: Fn(u32) -> u32,
 {
-    // trait bounds on `T` specify that `calculation` field is a closure by using the `Fn` trait
     pub calculation: T,
     pub value: Option<u32>,
 }

From 56b53bd30edff0be87e45e0b0e58936c66bc1635 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 19 Apr 2022 15:11:31 +0800
Subject: [PATCH 12/22] --wip-- [skip ci]

---
 .../workout_generator/src/cacher.rs             | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
index e5e158f..6513372 100644
--- a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
@@ -1,14 +1,21 @@
 // `Cacher` struct:
 // - holds a closure in `calculation` field
-//   - `calculation` field is of the generic type `T`
-//   - trait bounds on `T` specify that it's a closure by using the `Fn` trait
-//   - closure stored on `calculation` field must have on `u32` parameter and
-//     must return a `u32`
-// - holds an optional result in `value` field
+// - holds an optional result, `Option<u32>`, in the `value` field
 pub struct Cacher<T>
 where
     T: Fn(u32) -> u32,
 {
+    //   - `calculation` field is of the generic type `T`
+    //   - trait bounds on `T` specify that it's a closure by using the `Fn` trait
+    //   - closure stored on `calculation` field must have on `u32` parameter and
+    //     must return a `u32`
     pub calculation: T,
+
+    //   - before the closure is executed, `value` will be `None`
+    //   - when code using a `Cacher` askd for the result of the closure, the `Cacher`
+    //     will execute the closure and store the result within a `Some` variant in the
+    //     `value` field
+    //   - if the code asks for the result of the clousure again, instead of executing the
+    //     closure again, the `Cacher` will return the result held in the `Some` variant
     pub value: Option<u32>,
 }

From 2544414784398f9862f65f56a773ad7db4457f52 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Tue, 19 Apr 2022 15:30:05 +0800
Subject: [PATCH 13/22] --wip-- [skip ci]

---
 .../workout_generator/src/cacher.rs           | 30 ++++++++-
 .../workout_generator/src/main.rs             | 12 ++++
 .../workout_generator/src/workout.rs          | 67 ++++---------------
 3 files changed, 53 insertions(+), 56 deletions(-)

diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
index 6513372..63cfa33 100644
--- a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
@@ -9,13 +9,39 @@ where
     //   - trait bounds on `T` specify that it's a closure by using the `Fn` trait
     //   - closure stored on `calculation` field must have on `u32` parameter and
     //     must return a `u32`
-    pub calculation: T,
+    calculation: T,
 
+    //   The following logic is defined in `impl<T> Cacher<T>`:
     //   - before the closure is executed, `value` will be `None`
     //   - when code using a `Cacher` askd for the result of the closure, the `Cacher`
     //     will execute the closure and store the result within a `Some` variant in the
     //     `value` field
     //   - if the code asks for the result of the clousure again, instead of executing the
     //     closure again, the `Cacher` will return the result held in the `Some` variant
-    pub value: Option<u32>,
+    value: Option<u32>,
+}
+
+impl<T> Cacher<T>
+where
+    T: Fn(u32) -> u32,
+{
+    // takes a generic parameter `T`, which is defined as having the same trait bound
+    // as the `Cacher` struct
+    pub fn new(calculation: T) -> Cacher<T> {
+        Cacher {
+            calculation,
+            value: None,
+        }
+    }
+
+    pub fn value(&mut self, arg: u32) -> u32 {
+        match self.value {
+            Some(v) => v,
+            None => {
+                let v = (self.calculation)(arg);
+                self.value = Some(v);
+                v
+            }
+        }
+    }
 }
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
index 97804d3..f077a05 100644
--- a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
@@ -1,8 +1,20 @@
 use workout_generator::workout;
 
 fn main() {
+    // calculating slowly...
+    // Today, do 10 pushups!
+    // Next, do 10 situps!
     let simulated_user_specified_value = 10;
     let simulated_random_number = 7;
 
+    // calculating slowly...
+    // Today, run for 100 minutes!
+    // let simulated_user_specified_value = 100;
+    // let simulated_random_number = 7;
+
+    // Take a break today! Remeber to stay hydrated!
+    // let simulated_user_specified_value = 100;
+    // let simulated_random_number = 3;
+
     workout::generate_workout(simulated_user_specified_value, simulated_random_number);
 }
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
index 5a3f598..56bd51a 100644
--- a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
@@ -1,66 +1,25 @@
+use crate::cacher::Cacher;
 use std::{thread, time::Duration};
 
-fn simulated_expensive_calculation(intensity: u32) -> u32 {
-    println!("calculating slowly...");
-    thread::sleep(Duration::from_secs(2)); // like timeout, timer
-    intensity
-}
-
-// Iteration #3: Define a closure, and store this definition in a variable so that it can be called later.
-// This reintroduces the problem of the expensive calculation bein executed multiple times.
-// The solution will be presented in the next crate in this series.
+// Iteration #: Using `Cacher` struct
 pub fn generate_workout(intensity: u32, random_number: u32) {
-    let expensive_closure = |num| simulated_expensive_calculation(num);
+    let mut expensive_result = Cacher::new(|num| {
+        println!("calculating slowly...");
+        thread::sleep(Duration::from_secs(2));
+        num
+    });
 
     if intensity < 25 {
-        println!("Today, do {} pushups!", expensive_closure(intensity));
-        println!("Next, do {} situps!", expensive_closure(intensity));
+        println!("Today, do {} pushups!", expensive_result.value(intensity));
+        println!("Next, do {} situps!", expensive_result.value(intensity));
     } else {
         if random_number == 3 {
             println!("Take a break today! Remeber to stay hydrated!");
         } else {
-            println!("Today, run for {} minutes!", expensive_closure(intensity));
+            println!(
+                "Today, run for {} minutes!",
+                expensive_result.value(intensity)
+            );
         }
     }
 }
-
-// Iteration #2 - refactor by storing the result of duplicated function calls in a variable.
-// Still not ideal as expensive result gets run at least once, although it's not
-// needed by the `random_number == 3` `if` block.
-// pub fn generate_workout(intensity: u32, random_number: u32) {
-//     let expensive_result = simulated_expensive_calculation(intensity);
-
-//     if intensity < 25 {
-//         println!("Today, do {} pushups!", expensive_result);
-//         println!("Next, do {} situps!", expensive_result);
-//     } else {
-//         if random_number == 3 {
-//             println!("Take a break today! Remeber to stay hydrated!");
-//         } else {
-//             println!("Today, run for {} minutes!", expensive_result);
-//         }
-//     }
-// }
-
-// Iteration #1 - expensive calculation called multiple times instead of once.
-// pub fn generate_workout(intensity: u32, random_number: u32) {
-//     if intensity < 25 {
-//         println!(
-//             "Today, do {} pushups!",
-//             simulated_expensive_calculation(intensity)
-//         );
-//         println!(
-//             "Next, do {} situps!",
-//             simulated_expensive_calculation(intensity)
-//         );
-//     } else {
-//         if random_number == 3 {
-//             println!("Take a break today! Remeber to stay hydrated!");
-//         } else {
-//             println!(
-//                 "Today, run for {} minutes!",
-//                 simulated_expensive_calculation(intensity)
-//             );
-//         }
-//     }
-// }

From c42d31dd19272911e669fd1a6b0e45c274e9ac1d Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Wed, 20 Apr 2022 07:38:58 +0800
Subject: [PATCH 14/22] --wip-- [skip ci]

---
 13-functional-language-features/13-1-closures/README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
index 2084b91..28154e1 100644
--- a/13-functional-language-features/13-1-closures/README.md
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -15,3 +15,5 @@ Closures:
 [Naively call expensive function multiple times as needed](./01-abstract-behavior/workout_generator/src/workout.rs)
 
 [Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct typed with a `Fn` trait bound that closures must match](./02-memoization-with-struct/workout_generator/src/)
+
+[Capturing the environment with Closures](TODO)

From 1a22adf4c15262cc930e67925071eda173b3d9e8 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Wed, 20 Apr 2022 07:54:57 +0800
Subject: [PATCH 15/22] --wip-- [skip ci]

---
 .../13-2-iterators/README.md                  | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 13-functional-language-features/13-2-iterators/README.md

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
new file mode 100644
index 0000000..a57fbf1
--- /dev/null
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -0,0 +1,26 @@
+# Processing a series of items with iterators
+
+- iterator pattern is code that performs some task on a sequence of items in turn
+- an iterator is responsible for:
+  - the logic of iterating over each item
+  - determining when a sequence is finished
+- an iterator is lazy:
+  - it has no effect until...
+  - ...the code calls methods that consume the iterator to use it up
+
+```rust
+let v1 = vec![1, 2, 3]; // vector `v1`
+
+// - call `iter` method defined on `Vec<T>`
+//   -> creates iterator `v1_iter` over items in vector `v1`
+// - at this point, the code does not do anything useful;
+//   - remember that an iterator is lazy
+let v1_iter = v1.iter();
+
+// use `v1_iter` with a `for` loop to run code on each item on every iteration
+for val in v1_iter {
+  println!("Got: {}", val);
+}
+```
+
+## The `Iterator` trait and the `next` method

From 2902fbba3af144bf3c1033b0ea6a69f24933d8e9 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Wed, 20 Apr 2022 08:02:08 +0800
Subject: [PATCH 16/22] --wip-- [skip ci]

---
 .../13-2-iterators/README.md                  | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
index a57fbf1..3218476 100644
--- a/13-functional-language-features/13-2-iterators/README.md
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -24,3 +24,23 @@ for val in v1_iter {
 ```
 
 ## The `Iterator` trait and the `next` method
+
+Iterators implement a trait named `Iterator` that is defined in the standard library:
+
+```rust
+pub trait Iterator {
+  // new syntax (A): `type Item`
+  type Item;
+
+  // new syntax (A): `Self::Item`
+  fn next(&mut self) -> Option<Self::Item>;
+
+  // methods with default implementations elided
+}
+```
+
+`new syntax (A)` defines an associated type with this trait:
+
+- implementing the `Iterator` trait requires an `Item` type to be defined
+- the `Item` type is used in the return type of the `next` method
+- the `Item` type will be the type returned from the iterator

From 491672dd08cbcabeec91c98bef420eb1a928ee27 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Wed, 20 Apr 2022 08:06:26 +0800
Subject: [PATCH 17/22] --wip-- [skip ci]

---
 13-functional-language-features/13-2-iterators/README.md | 2 +-
 19-advanced-features/19-2-advanced-traits/README.md      | 7 +++++++
 19-advanced-features/README.md                           | 1 +
 3 files changed, 9 insertions(+), 1 deletion(-)
 create mode 100644 19-advanced-features/19-2-advanced-traits/README.md
 create mode 100644 19-advanced-features/README.md

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
index 3218476..4103fc6 100644
--- a/13-functional-language-features/13-2-iterators/README.md
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -39,7 +39,7 @@ pub trait Iterator {
 }
 ```
 
-`new syntax (A)` defines an associated type with this trait:
+`new syntax (A)` defines an [associated type](../../19-advanced-features/19-2-advanced-traits/README.md/#specifying-placeholder-types-in-trait-definitions-with-associated-types) with this trait:
 
 - implementing the `Iterator` trait requires an `Item` type to be defined
 - the `Item` type is used in the return type of the `next` method
diff --git a/19-advanced-features/19-2-advanced-traits/README.md b/19-advanced-features/19-2-advanced-traits/README.md
new file mode 100644
index 0000000..9085a1a
--- /dev/null
+++ b/19-advanced-features/19-2-advanced-traits/README.md
@@ -0,0 +1,7 @@
+# Advanced traits
+
+## Specifying placeholder types in trait definitions with associated types
+
+Referenced by:
+
+[The `Iterator` trait and the `next` method](../../13-functional-language-features/13-2-iterators/README.md/#the-iterator-trait-and-the-next-method)
diff --git a/19-advanced-features/README.md b/19-advanced-features/README.md
new file mode 100644
index 0000000..7c935bd
--- /dev/null
+++ b/19-advanced-features/README.md
@@ -0,0 +1 @@
+# Advanced features

From 2f84844759ff3f1af360eba31ede689dff80582d Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Wed, 20 Apr 2022 08:07:08 +0800
Subject: [PATCH 18/22] --wip-- [skip ci]

---
 19-advanced-features/19-2-advanced-traits/README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/19-advanced-features/19-2-advanced-traits/README.md b/19-advanced-features/19-2-advanced-traits/README.md
index 9085a1a..1cee064 100644
--- a/19-advanced-features/19-2-advanced-traits/README.md
+++ b/19-advanced-features/19-2-advanced-traits/README.md
@@ -4,4 +4,4 @@
 
 Referenced by:
 
-[The `Iterator` trait and the `next` method](../../13-functional-language-features/13-2-iterators/README.md/#the-iterator-trait-and-the-next-method)
+- [The `Iterator` trait and the `next` method](../../13-functional-language-features/13-2-iterators/README.md/#the-iterator-trait-and-the-next-method)

From a63903ae7cf098b7d751c11bfdc4207f38f7c5de Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Thu, 21 Apr 2022 17:41:40 +0800
Subject: [PATCH 19/22] --wip-- [skip ci]

---
 .../13-2-iterators/README.md                  | 44 +++++++++++++++++++
 .../13-2-iterators/iterators/Cargo.toml       |  8 ++++
 .../13-2-iterators/iterators/src/main.rs      |  9 ++++
 3 files changed, 61 insertions(+)
 create mode 100644 13-functional-language-features/13-2-iterators/iterators/Cargo.toml
 create mode 100644 13-functional-language-features/13-2-iterators/iterators/src/main.rs

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
index 4103fc6..353f8d4 100644
--- a/13-functional-language-features/13-2-iterators/README.md
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -44,3 +44,47 @@ pub trait Iterator {
 - implementing the `Iterator` trait requires an `Item` type to be defined
 - the `Item` type is used in the return type of the `next` method
 - the `Item` type will be the type returned from the iterator
+
+Implementors of the `Iterator` trait must define one method, the `next` method:
+
+- which returns one item of the iterator at a time wrapped in `Some`,
+- and, when the iteration is over, returns `None`
+
+The `next` method can be called on iterators directly:
+
+```rust
+#[test]
+fn iterator_demonstration() {
+  let v1 = vec![1, 2, 3];
+
+  // Note (A)
+  // v1_iter must be mutable, otherwise error on `v1_iter.next()`:
+  //   cannot borrow `v1_iter` as mutable, as it is not declared as mutable
+  let mut v1_iter = v1.iter();
+
+  // Note (B)
+  // `Some(&1)`, otherwise error:
+  //   expected `&{integer}`, found integer
+  assert_eq!(v1_iter.next(), Some(&1));
+  assert_eq!(v1_iter.next(), Some(&2));
+  assert_eq!(v1_iter.next(), Some(&3));
+  assert_eq!(v1_iter.next(), None);
+}
+```
+
+Note (A): Iterator `v1_iter` needs to be made mutable:
+
+- calling the `next` method changes the internal state used by the iterator:
+  - the internal state is used to keep track of where the iterator is in the sequence
+- calling `next`:
+  - consumes, or uses up, the iterator
+  - eats up an iterm from the iterator
+- `v1_iter` needn't be made mutable when used in a `for` loop because:
+  - the loop takes ownership of `v1_iter` and makes it mutable behind the scenes
+
+Note (B): Values obtained from calls to `next`:
+
+- are immutable references to the values in the vector `v1`
+- the `iter` method in `v1.iter()` produces an iterator over immutable references:
+  - to create an iterator that takes ownership of `v1` and returns owned values, use `into_iter`
+  - to create an iterator over mutable references, use `iter_mut`
diff --git a/13-functional-language-features/13-2-iterators/iterators/Cargo.toml b/13-functional-language-features/13-2-iterators/iterators/Cargo.toml
new file mode 100644
index 0000000..069541a
--- /dev/null
+++ b/13-functional-language-features/13-2-iterators/iterators/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "iterators"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/13-functional-language-features/13-2-iterators/iterators/src/main.rs b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
new file mode 100644
index 0000000..35bd387
--- /dev/null
+++ b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
@@ -0,0 +1,9 @@
+fn main() {
+    let v = vec![1, 2, 3];
+    let mut v_iter = v.iter();
+    assert_eq!(v_iter.next(), Some(&1));
+    assert_eq!(v_iter.next(), Some(&2));
+    assert_eq!(v_iter.next(), Some(&3));
+    assert_eq!(v_iter.next(), None);
+    println!("{:?}", v_iter.next());
+}

From 1f19bcd5d19738a6116ae8d8c609455e63d6b2b7 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Thu, 21 Apr 2022 17:52:14 +0800
Subject: [PATCH 20/22] --wip-- [skip ci]

---
 .../13-2-iterators/README.md                  | 26 +++++++++++++++----
 .../13-2-iterators/iterators/src/main.rs      | 15 ++++++++++-
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
index 353f8d4..8c37547 100644
--- a/13-functional-language-features/13-2-iterators/README.md
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -39,7 +39,7 @@ pub trait Iterator {
 }
 ```
 
-`new syntax (A)` defines an [associated type](../../19-advanced-features/19-2-advanced-traits/README.md/#specifying-placeholder-types-in-trait-definitions-with-associated-types) with this trait:
+**`new syntax (A)`** defines an [associated type](../../19-advanced-features/19-2-advanced-traits/README.md/#specifying-placeholder-types-in-trait-definitions-with-associated-types) with this trait:
 
 - implementing the `Iterator` trait requires an `Item` type to be defined
 - the `Item` type is used in the return type of the `next` method
@@ -69,10 +69,26 @@ fn iterator_demonstration() {
   assert_eq!(v1_iter.next(), Some(&2));
   assert_eq!(v1_iter.next(), Some(&3));
   assert_eq!(v1_iter.next(), None);
+
+  // Note (C) `into_iter`
+  let v1 = vec![1, 2, 3];
+  let mut v1_iter = v1.into_iter();
+  assert_eq!(v1_iter.next(), Some(1));
+  assert_eq!(v1_iter.next(), Some(2));
+  assert_eq!(v1_iter.next(), Some(3));
+  assert_eq!(v1_iter.next(), None);
+
+  // Note (D) `iter_mut`
+  let mut v1 = vec![1, 2, 3];
+  let mut v1_iter = v1.iter_mut();
+  assert_eq!(v1_iter.next(), Some(&mut 1));
+  assert_eq!(v1_iter.next(), Some(&mut 2));
+  assert_eq!(v1_iter.next(), Some(&mut 3));
+  assert_eq!(v1_iter.next(), None);
 }
 ```
 
-Note (A): Iterator `v1_iter` needs to be made mutable:
+**Note (A)**: Iterator `v1_iter` needs to be made mutable:
 
 - calling the `next` method changes the internal state used by the iterator:
   - the internal state is used to keep track of where the iterator is in the sequence
@@ -82,9 +98,9 @@ Note (A): Iterator `v1_iter` needs to be made mutable:
 - `v1_iter` needn't be made mutable when used in a `for` loop because:
   - the loop takes ownership of `v1_iter` and makes it mutable behind the scenes
 
-Note (B): Values obtained from calls to `next`:
+**Note (B)**: Values obtained from calls to `next`:
 
 - are immutable references to the values in the vector `v1`
 - the `iter` method in `v1.iter()` produces an iterator over immutable references:
-  - to create an iterator that takes ownership of `v1` and returns owned values, use `into_iter`
-  - to create an iterator over mutable references, use `iter_mut`
+  - **Note (C)**: to create an iterator that takes ownership of `v1` and returns owned values, use `into_iter`
+  - **Note (D)**: to create an iterator over mutable references, use `iter_mut`
diff --git a/13-functional-language-features/13-2-iterators/iterators/src/main.rs b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
index 35bd387..4018584 100644
--- a/13-functional-language-features/13-2-iterators/iterators/src/main.rs
+++ b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
@@ -5,5 +5,18 @@ fn main() {
     assert_eq!(v_iter.next(), Some(&2));
     assert_eq!(v_iter.next(), Some(&3));
     assert_eq!(v_iter.next(), None);
-    println!("{:?}", v_iter.next());
+
+    let v = vec![1, 2, 3];
+    let mut v_iter = v.into_iter();
+    assert_eq!(v_iter.next(), Some(1));
+    assert_eq!(v_iter.next(), Some(2));
+    assert_eq!(v_iter.next(), Some(3));
+    assert_eq!(v_iter.next(), None);
+
+    let mut v = vec![1, 2, 3];
+    let mut v_iter = v.iter_mut();
+    assert_eq!(v_iter.next(), Some(&mut 1));
+    assert_eq!(v_iter.next(), Some(&mut 2));
+    assert_eq!(v_iter.next(), Some(&mut 3));
+    assert_eq!(v_iter.next(), None);
 }

From 0d72169f1fa2e697fe212f25492c5417eb079b3d Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Thu, 21 Apr 2022 17:56:48 +0800
Subject: [PATCH 21/22] --wip-- [skip ci]

---
 13-functional-language-features/13-2-iterators/README.md    | 6 +++++-
 .../13-2-iterators/iterators/src/main.rs                    | 5 +++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
index 8c37547..163a2b2 100644
--- a/13-functional-language-features/13-2-iterators/README.md
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -70,9 +70,13 @@ fn iterator_demonstration() {
   assert_eq!(v1_iter.next(), Some(&3));
   assert_eq!(v1_iter.next(), None);
 
-  // Note (C) `into_iter`
+  // Note (C) `into_iter` takes ownership of v1
   let v1 = vec![1, 2, 3];
   let mut v1_iter = v1.into_iter();
+  //        Error (E1) ----------- `v1` moved due to this method call
+  // println!("{:?}", v1); // Error (E1) will occur
+  //       Error (E1) ^^ value borrowed here after move
+
   assert_eq!(v1_iter.next(), Some(1));
   assert_eq!(v1_iter.next(), Some(2));
   assert_eq!(v1_iter.next(), Some(3));
diff --git a/13-functional-language-features/13-2-iterators/iterators/src/main.rs b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
index 4018584..b46182b 100644
--- a/13-functional-language-features/13-2-iterators/iterators/src/main.rs
+++ b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
@@ -6,8 +6,9 @@ fn main() {
     assert_eq!(v_iter.next(), Some(&3));
     assert_eq!(v_iter.next(), None);
 
-    let v = vec![1, 2, 3];
-    let mut v_iter = v.into_iter();
+    let v1 = vec![1, 2, 3];
+    let mut v_iter = v1.into_iter();
+    println!("{:?}", v1);
     assert_eq!(v_iter.next(), Some(1));
     assert_eq!(v_iter.next(), Some(2));
     assert_eq!(v_iter.next(), Some(3));

From f9f2d5f223fc0ca4cb23f324051388eddd0b9574 Mon Sep 17 00:00:00 2001
From: David Chin <dlcmhd@me.com>
Date: Sat, 23 Apr 2022 13:29:25 +0800
Subject: [PATCH 22/22] --wip-- [skip ci]

---
 13-functional-language-features/13-2-iterators/README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
index 163a2b2..802f546 100644
--- a/13-functional-language-features/13-2-iterators/README.md
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -106,5 +106,5 @@ fn iterator_demonstration() {
 
 - are immutable references to the values in the vector `v1`
 - the `iter` method in `v1.iter()` produces an iterator over immutable references:
-  - **Note (C)**: to create an iterator that takes ownership of `v1` and returns owned values, use `into_iter`
-  - **Note (D)**: to create an iterator over mutable references, use `iter_mut`
+  - **Note (C)**: `into_iter` - creates an iterator that takes ownership of `v1` and returns owned values
+  - **Note (D)**: `iter_mut` - creates an iterator over mutable references