added units example

Author: BartMassey

README.md

@@ -15,5 +15,6 @@ in a controlled repository now.
 * `rc`: examples with `RefCell` and `Rc`
 * `readline`: example of readline with error handling
 * `error`: example of creating a custom error
-* `modules`: example of Rust modules
-* `point.rs`: example of Rust "methods" and ownership
+* `modules`: example of modules
+* `point.rs`: example of "methods" and ownership
+* `units.rs`: example of newtype and deriving

rc/examples/03-refcell.rs

@@ -0,0 +1,16 @@
+// https://doc.rust-lang.org/std/cell/
+use rc::refcell::Counter;
+
+/// Increment the given counter. Note that this function
+/// takes `counter` by immutable reference.
+fn update_counter(counter: &Counter) {
+    counter.incr();
+}
+
+fn main() {
+    let counter = Counter::default();
+    println!("{}", counter.value());
+    update_counter(&counter);
+    println!("{}", counter.value());
+    println!("{}", counter.incr_value());
+}

units.rs

@@ -0,0 +1,49 @@
+// https://doc.rust-lang.org/stable/rust-by-example/trait/derive.html
+
+// A tuple struct that can be compared
+#[derive(PartialEq, PartialOrd)]
+struct Centimeters(f64);
+
+// A tuple struct that can be printed
+#[derive(Debug)]
+struct Inches(i32);
+
+impl Inches {
+    fn to_centimeters(&self) -> Centimeters {
+        let &Inches(inches) = self;
+
+        Centimeters(inches as f64 * 2.54)
+    }
+}
+
+// A vanilla tuple struct
+struct Seconds(i32);
+
+fn main() {
+    let _one_second = Seconds(1);
+
+    // Error! `Seconds` can't be printed, because it doesn't implement the
+    // `Debug` trait
+    //println!("One second looks like: {}", _one_second);
+    // TODO ^ Try uncommenting this line
+
+    // Error: `Seconds` can't be compared, because it doesn't implement the
+    // `PartialEq` trait
+    //let _this_is_true = _one_second == _one_second;
+    // TODO ^ Try uncommenting this line
+
+    let foot = Inches(12);
+
+    println!("One foot === {:?}", foot);
+
+    let meter = Centimeters(100.0);
+
+    let cmp =
+        if foot.to_centimeters() < meter {
+            "smaller"
+        } else {
+            "bigger"
+        };
+
+    println!("one foot is {} than one meter", cmp);
+}