Add advantages section comparing to the Rust Book

Author: jhwgh1968

functional/stateful-types.md

@@ -231,6 +231,25 @@ fn main() {
 They would get a syntax error. The type `Interpreter<Loaded>` does not
 implement `set_param()`, only the type `Interpreter<Init>` does.
 
+## Advantages
+
+This sort of type transformation to represent state is also described in [the
+Rust Book](https://doc.rust-lang.org/book/ch17-03-oo-design-patterns.html#implementing-transitions-as-transformations-into-different-types),
+except the types are not generic. Using generic types has a key advantage: code
+re-use.
+
+First, it allows fields that are common to multiple states to be de-duplicated.
+That is, if the interpreter had a common field across the `Init`, `Loaded`, and
+`Ready` states, making them three structs would require the field to be
+consistent three times. By making the non-shared fields a generic, they are
+implemented once.
+
+Second, it makes the `impl` blocks easier to read, because they are broken down
+by state. Methods common to all states are typed once in one block, and methods
+unique to one state are in a separate block.
+
+Both of these mean there is less code and it is better organized.
+
 ## Disadvantages
 
 This is a lot of typing. Depending on the amount of change caused by state