Rewrote sequence comprehensions section of tour

Author: travissarles

tutorials/tour/_posts/2017-02-13-sequence-comprehensions.md

@@ -13,58 +13,39 @@ previous-page: extractor-objects
 
 Scala offers a lightweight notation for expressing *sequence comprehensions*. Comprehensions have the form `for (enumerators) yield e`, where `enumerators` refers to a semicolon-separated list of enumerators. An *enumerator* is either a generator which introduces new variables, or it is a filter. A comprehension evaluates the body `e` for each binding generated by the enumerators and returns a sequence of these values.
 
-Here is an example:
- 
+Here's an example:
+
 ```tut
-object ComprehensionTest1 extends App {
-  def even(from: Int, to: Int): List[Int] =
-    for (i <- List.range(from, to) if i % 2 == 0) yield i
-  Console.println(even(0, 20))
-}
-```
- 
-The for-expression in function introduces a new variable `i` of type `Int` which is subsequently bound to all values of the list `List(from, from + 1, ..., to - 1)`. The guard `if i % 2 == 0` filters out all odd numbers so that the body (which only consists of the expression i) is only evaluated for even numbers. Consequently, the whole for-expression returns a list of even numbers.
+case class User(val name: String, val age: Int)
 
-The program yields the following output:
+val userBase = List(new User("Travis", 28),
+  new User("Kelly", 33),
+  new User("Jennifer", 44),
+  new User("Dennis", 23))
 
+val twentySomethings = for (user <- userBase if (user.age >=20 && user.age < 30))
+  yield user.name  // i.e. add this to a list
+
+twentySomethings.foreach(name => println(name))  // prints Travis Dennis
 ```
-List(0, 2, 4, 6, 8, 10, 12, 14, 16, 18)
-```
+The `for` loop used with a `yield` statement actually creates a `List`. Because we said `yield user.name`, it's a `List[String]`. `user <- userBase` is our iterator and `if (user.age >=20 && user.age < 30)` is a guard that filters out users who are in their 20s.
+
+Here is a more complicated example using two generators. It computes all pairs of numbers between `0` and `n-1` whose sum is equal to a given value `v`:
 
-Here is a more complicated example which computes all pairs of numbers between `0` and `n-1` whose sum is equal to a given value `v`:
- 
 ```tut
-object ComprehensionTest2 extends App {
-  def foo(n: Int, v: Int) =
-    for (i <- 0 until n;
-         j <- i until n if i + j == v) yield
-      (i, j);
-  foo(20, 32) foreach {
-    case (i, j) =>
-      println(s"($i, $j)")
-  }
+def foo(n: Int, v: Int) =
+   for (i <- 0 until n;
+        j <- i until n if i + j == v)
+     yield (i, j)
+
+foo(10, 10) foreach {
+  case (i, j) =>
+    print(s"($i, $j) ")  // prints (1, 9) (2, 8) (3, 7) (4, 6) (5, 5)
 }
-```
- 
-This example shows that comprehensions are not restricted to lists. The previous program uses iterators instead. Every datatype that supports the operations `withFilter`, `map`, and `flatMap` (with the proper types) can be used in sequence comprehensions.
-
-Here's the output of the program:
 
 ```
-(13, 19)
-(14, 18)
-(15, 17)
-(16, 16)
+Here `n == 10` and `v == 10`. On the first iteration, `i == 0` and `j == 0` so `i + j != v` and therefore nothing is yielded. `j` gets incremented 9 more times before `i` gets incremented to `1`. Without the `if` guard, this would simply print the following:
 ```
 
-There is also a special form of sequence comprehension which returns `Unit`. Here the bindings that are created from the list of generators and filters are used to perform side-effects. The programmer has to omit the keyword `yield` to make use of such a sequence comprehension.
-Here's a program which is equivalent to the previous one but uses the special for comprehension returning `Unit`:
- 
+(0, 0) (0, 1) (0, 2) (0, 3) (0, 4) (0, 5) (0, 6) (0, 7) (0, 8) (0, 9) (1, 1) ...
 ```
-object ComprehensionTest3 extends App {
-  for (i <- Iterator.range(0, 20);
-       j <- Iterator.range(i, 20) if i + j == 32)
-    println(s"($i, $j)")
-}
-```
-