start on monad section

Chapter07/monad_pattern.rs

@@ -1,4 +1,17 @@
 
 fn main() 
 {
+   let v1 = Some(2).and_then(|x| Some(x+x)).and_then(|y| Some(y*y));
+   println!("{:?}", v1);
+
+   let v2 = None.or_else(|| None).or_else(|| Some(222));
+   println!("{:?}", v2);
+
+   let v3 = Some(2).and_then(|x| Some("abc"));
+   println!("{:?}", v3);
+
+   // or_else is not quite a monad
+   // does not permit polymorphic bind
+   //let v4 = Some(2).or_else(|| Some("abc"));
+   //println!("{:?}", v4);
 }