Merge pull request #1538 from JL102/patch-1

Fixed typos in CDE docs page, and added info on how config fragments in the chain can be overridden

docs/Advanced-Concepts/CDEs.rst

@@ -21,24 +21,37 @@ Consider the following example using CDEs.
 
     class WithX(b: Boolean) extends Config((site, here, up) => {
       case SomeKeyX => b
-    }
+    })
 
     class WithY(b: Boolean) extends Config((site, here, up) => {
       case SomeKeyY => b
-    }
+    })
 
 
 When forming a query based on a ``Parameters`` object, like ``p(SomeKeyX)``, the configuration system traverses the "chain" of config fragments until it finds a partial function which is defined at the key, and then returns that value.
 
 .. code:: scala
 
-    val params = Config(new WithX(true) ++ new WithY(true)) // "chain" together config fragments
+    val params = new Config(new WithX(true) ++ new WithY(true)) // "chain" together config fragments
     params(SomeKeyX) // evaluates to true
     params(SomeKeyY) // evaluates to true
     params(SomeKeyZ) // evaluates to false
 
 In this example, the evaluation of ``params(SomeKeyX)`` will terminate in the partial function defined in ``WithX(true)``, while the evaluation of ``params(SomeKeyY)`` will terminate in the partial function defined in ``WithY(true)``. Note that when no partial functions match, the evaluation will return the default value for that parameter.
 
+Config fragments take precedence from left to right, meaning that a fragment at the start of the chain can override the value of a fragment to the right. It helps to read the fragment chain from right to left.
+
+.. code:: scala
+
+    case object SomeKeyX extends Field[Int](0)
+
+    class WithX(n: Int) extends Config((site, here, up) => {
+      case SomeKeyX => n
+    })
+
+    val params = new Config(new WithX(10) ++ new WithX(5))
+    println(params(SomeKeyX)) // evaluates to 10
+
 The real power of CDEs arises from the ``(site, here, up)`` parameters to the partial functions, which provide useful "views" into the global parameterization that the partial functions may access to determine a parameterization.
 
 .. note::
@@ -54,10 +67,10 @@ Site
 
     class WithXEqualsYSite extends Config((site, here, up) => {
       case SomeKeyX => site(SomeKeyY) // expands to site(SomeKeyY, site)
-    }
+    })
 
-    val params_1 = Config(new WithXEqualsYSite ++ new WithY(true))
+    val params_1 = new Config(new WithXEqualsYSite ++ new WithY(true))
-    val params_2 = Config(new WithY(true) ++ new WithXEqualsYSite)
+    val params_2 = new Config(new WithY(true) ++ new WithXEqualsYSite)
     params_1(SomeKeyX) // evaluates to true
     params_2(SomeKeyX) // evaluates to true
 
@@ -75,10 +88,10 @@ Here
     class WithXEqualsYHere extends Config((site, here, up) => {
       case SomeKeyY => false
       case SomeKeyX => here(SomeKeyY, site)
-    }
+    })
 
-    val params_1 = Config(new WithXEqualsYHere ++ new WithY(true))
+    val params_1 = new Config(new WithXEqualsYHere ++ new WithY(true))
-    val params_2 = Config(new WithY(true) ++ new WithXEqualsYHere)
+    val params_2 = new Config(new WithY(true) ++ new WithXEqualsYHere)
 
     params_1(SomeKeyX) // evaluates to false
     params_2(SomeKeyX) // evaluates to false
@@ -95,10 +108,10 @@ Up
 
     class WithXEqualsYUp extends Config((site, here, up) => {
       case SomeKeyX => up(SomeKeyY, site)
-    }
+    })
 
-    val params_1 = Config(new WithXEqualsYUp ++ new WithY(true))
+    val params_1 = new Config(new WithXEqualsYUp ++ new WithY(true))
-    val params_2 = Config(new WithY(true) ++ new WithXEqualsYUp)
+    val params_2 = new Config(new WithY(true) ++ new WithXEqualsYUp)
 
     params_1(SomeKeyX) // evaluates to true
     params_2(SomeKeyX) // evaluates to false