Add links to source code for reference

intro/README.md

@@ -8,7 +8,7 @@ expression-based programming, recursion, immutability and higher-order
 functions.
 
 Getting Started
--------
+---------------
 
 Open file `src/main/scala/Main.scala` in your editor of choice. To run the 
 program start `sbt` from the terminal and type `run` in SBT prompt.
@@ -50,6 +50,14 @@ public class Loop {
 }
 ```
 
+<center>
+  <small>
+    _(source: [Loop1.java](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/java/Loop1.java))_
+  </small>
+</center>
+<br>
+
+
 This program is on purpose easy enough to understand. Consider however that 
 in your real programming work you would iterate over some complex domain 
 objects instead on numbers, maybe over two or three arrays of those 
@@ -98,6 +106,13 @@ object Main extends App {    // 1.
 }
 ```
 
+<center>
+  <small>
+    _(source: [Loop2.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop2.scala))_
+  </small>
+</center>
+<br>
+
 First you notice that Scala is less verbose: no semicolons needed, no `public
  static void main`, no explicit reassignment of `x` in `for` loop. Here are 
  other notable differences:
@@ -170,6 +185,13 @@ def sum(xs: List[Int]): Int = {
 val result = sum(square(filterEven(iterate(10))))
 ```
 
+<center>
+  <small>
+    _(source: [Loop3.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop3.scala))_
+  </small>
+</center>
+<br>
+
 Wow! The number of lines of code just exploded and we introduced a lot of 
 duplication along the way. Let's review new Scala syntax first:
 
@@ -309,6 +331,13 @@ def sum(xs: List[Int]): Int = {
 }
 ```
 
+<center>
+  <small>
+    _(source: [Loop4.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop4.scala))_
+  </small>
+</center>
+<br>
+
 Here we pattern match on a list instead of integer. Note how we used the same
 list construction syntax in the `case` expression to capture list's head `x` 
 and its `tail`. `++` concatenates two lists.
@@ -355,6 +384,13 @@ def square(xs: List[Int]): List[Int] =
   flatMap(xs, x => List(x * x))
 ```
 
+<center>
+  <small>
+    _(source: [Loop5.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop5.scala))_
+  </small>
+</center>
+<br>
+
 1. `flatMap` takes function `f` as its second argument;
 2. `A => B` is type annotation for a function that takes an argument of type 
 `A` and returns a value of type `B`;
@@ -404,6 +440,13 @@ def sum(xs: List[Int]): Int =
   foldLeft(xs)(0)((acc, x) => acc + x)
 ```
 
+<center>
+  <small>
+    _(source: [Loop6.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop6.scala))_
+  </small>
+</center>
+<br>
+
 1. `foldLeft[A, R]` is parametrized on type of the input list `A` as well as 
 the type of its return value `R` thus `foldLeft` is a *generic* function;
 2. Scala functions can have multiple parameter lists. Here it's useful for 
@@ -447,6 +490,13 @@ def sumOfEvenSquares(max: Int): Int = {
 }
 ```
 
+<center>
+  <small>
+    _(source: [Loop7.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop7.scala))_
+  </small>
+</center>
+<br>
+
 Here `square` really does only what it says and doesn't mess with lists 
 anymore, that's responsibility of `map` now.
 
@@ -470,6 +520,13 @@ def sumOfEvenSquares(max: Int): Int = {
 }
 ```
 
+<center>
+  <small>
+    _(source: [Loop8.scala](https://github.com/SingaporeScalaProgrammers/scala-workshop/blob/master/intro/src/main/scala/Loop8.scala))_
+  </small>
+</center>
+<br>
+
 We use `1 to 10` directly instead of our own `iterate` function. `filter`, 
 `map`, `sum` as well as `flatMap`, `foldLeft` and many more functions are 
 available as methods on any Scala collection. This allows using the fluid API