Rewrote unified types tour

Author: travissarles

tutorials/tour/Makefile

@@ -0,0 +1,2 @@
+dot:
+	dot -Tsvg unified-types-diagram.dot -o unified-types-diagram.svg

tutorials/tour/_posts/2017-02-13-unified-types.md

@@ -11,15 +11,15 @@ next-page: classes
 previous-page: basics
 ---
 
-In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates the class hierarchy.
+In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates a subset the class hierarchy.
 
-![Scala Type Hierarchy]({{ site.baseurl }}/resources/images/classhierarchy.img_assist_custom.png)
+![Scala Type Hierarchy]({{ site.baseurl }}/tutorial/tour/unified-types-diagram.svg)
 
 ## Scala Class Hierarchy ##
 
-The superclass of all classes `scala.Any` has two direct subclasses: `scala.AnyVal` and `scala.AnyRef`.
+`scala.Any` is the base type of all types. It defines certain universal methods: `==`, `!=`, `equals`, `hashCode`, `toString`. `scala.Any` has two direct subclasses: `scala.AnyVal` and `scala.AnyRef`.
 
-`scala.AnyVal` represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages. Note that the diagram above also shows implicit conversions between the value classes.
+`scala.AnyVal` represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages: `Byte`, `Short`, `Char`, `Int`, `Long`, `Float`, `Double`, and `Boolean`. Scala adds the `Unit` type which is similar to Java's `void`.
 
 `scala.AnyRef` represents reference classes. All non-value classes are defined as reference class. Every user-defined class in Scala implicitly extends `scala.AnyRef`. If Scala is used in the context of a Java runtime environment, `scala.AnyRef` corresponds to `java.lang.Object`.
 
@@ -35,7 +35,7 @@ val list: List[Any] = List(
 )
 
 list.foreach(element => println(element))
-````
+```
 
 It defines a variable `list` of type `List[Any]`. The list is initialized with elements of various types, but they all are instance of `scala.Any`, so you can add them to the list.
 
@@ -48,3 +48,15 @@ c
 true
 <function>
 ```
+
+## Nothing and Null
+`Nothing` is a subtype of all types. There is no value that has type `Nothing`. There are two uses: The first is to signal abnormal termination such as an exception (i.e. it doesn't return). The second type usage is an element type of empty collections (e.g. `Set[Nothing]`).
+
+`Null` is a subtype of all subtypes of `AnyRef`. The class name is `Null` but the literal identifier is `null`. You can assign any reference type to null.
+```tut
+val name: String = null
+```
+However, you can not assign value types to null:
+```
+val number: Int = null  // does not compile
+```

tutorials/tour/dot-hot-reload.sh

@@ -0,0 +1 @@
+ls *.dot | entr make dot

tutorials/tour/unified-types-diagram.dot

@@ -0,0 +1,13 @@
+digraph UnifiedTypes {
+  rankdir="BT"
+  AnyVal -> Any;
+  "scala.AnyRef (java.lang.Object)" -> Any;
+
+  "scala.Double", "scala.Float", "scala.Long", "scala.Int", "scala.Unit", "scala.Boolean", "scala.Char" -> AnyVal;
+  "scala.List", "scala.Seq", "scala.Iterable" -> "scala.AnyRef (java.lang.Object)"
+
+  "scala.Null" -> {"scala.List" "scala.Seq" "scala.Iterable"}
+  "scala.Nothing" -> {"scala.Double", "scala.Float", "scala.Long", "scala.Int", "scala.Unit", "scala.Boolean", "scala.Char", "scala.Null"}
+
+  {rank = same; "scala.Null"; "scala.Nothing"}
+}