Rewrote currying tour

tutorials/tour/_posts/2017-02-13-currying.md

@@ -13,29 +13,28 @@ previous-page: nested-functions
 
 Methods may define multiple parameter lists. When a method is called with a fewer number of parameter lists, then this will yield a function taking the missing parameter lists as its arguments.
 
-Here is an example:
-
 ```tut
-object CurryTest extends App {
+import scala.math.pow
 
-  def filter(xs: List[Int], p: Int => Boolean): List[Int] =
-    if (xs.isEmpty) xs
-    else if (p(xs.head)) xs.head :: filter(xs.tail, p)
-    else filter(xs.tail, p)
+def getTotal(interestRate: Double, years: Int)(principal: Double) = {
+  principal * pow(1 + interestRate, years)
+}
+val savingsAccountsPrincipals = Seq(1000.0, 2000.0, 3000.0)
+val totals = savingsAccountsPrincipals.map(getTotal(interestRate = .10, years = 20)) // List(6727.499949325611, 13454.999898651222, 20182.499847976833)
+```
+The method `getTotal` takes two argument lists. The first contains `interestRate` and `years` is the same among all of the savings accounts. The second list contains the `principal` which varies between accounts. When we call `getTotal(interestRate = .10, years = 20)`, this returns getTotal with the second parameter list `(principal: Double)`.
 
-  def modN(n: Int)(x: Int) = ((x % n) == 0)
+If you're not passing the curried function as an argument, you can use an `_` in place of the second parameter list.
 
-  val nums = List(1, 2, 3, 4, 5, 6, 7, 8)
-  println(filter(nums, modN(2)))
-  println(filter(nums, modN(3)))
-}
 ```
+import scala.math.pow
 
-_Note: method `modN` is partially applied in the two `filter` calls; i.e. only its first argument is actually applied. The term `modN(2)` yields a function of type `Int => Boolean` and is thus a possible candidate for the second argument of function `filter`._
+def getTotal(interestRate: Double, years: Int)(principal: Double) = {
+  principal * pow(1 + interestRate, years)
+}
 
-Here's the output of the program above:
+val getTotalInTwentyYears = getTotal(interestRate = .10, years = 20)_
 
+val total = getTotalInTwentyYears(2000.0)  // 13454.999898651222
 ```
-List(2,4,6,8)
-List(3,6)
-```
+Here `getTotalInTwentyYears` is a function which takes the second parameter list from `getTotal`.