graph.rkt: Tidy up and structure in sections.

The sections are the same as in the documentation of the generic graph
library: https://docs.racket-lang.org/graph/index.html

graph.rkt

@@ -20,29 +20,58 @@
 
 (module graph-wrapper racket
   (require (prefix-in g: graph))
-  (provide graph?
+  (provide graph? has-vertex? has-edge?
+
            directed-graph
-           has-edge?
+
            graphviz)
 
+  ;; Wrap the opaque graph structure coming from the generic
+  ;; graph library.
   (struct graph (g))
 
-  (define (directed-graph es [ws #f])
-    (graph (g:directed-graph es ws)))
+  (define (has-vertex? g v)
+    (g:has-vertex? (graph-g g) v))
   (define (has-edge? g u v)
     (g:has-edge? (graph-g g) u v))
+
+  ;; 2 Graph constructors
+  ;; 2.2 Weighted graphs
+  (define (directed-graph es [ws #f])
+    (graph (g:directed-graph es ws)))
+
+  ;; 10 Graphviz
   (define (graphviz g #:output [output #f] #:colors [colors #f])
     (g:graphviz (graph-g g) #:output output #:colors colors)))
 
-(require/typed 'graph-wrapper
+
+(require/typed/provide 'graph-wrapper
   [#:opaque Graph graph?]
-  [directed-graph (->* ((Listof (List Any Any))) ((Listof Any)) Graph)]
+  [has-vertex? (-> Graph Any Boolean)]
   [has-edge? (-> Graph Any Any Boolean)]
+
+  ;; 2 Graph constructors
+  ;; 2.2 Weighted graphs
+  [directed-graph (->* ((Listof (List Any Any))) ((Listof Any)) Graph)]
+
+  ;; 10 Graphviz
   [graphviz (->* (Graph)
                  (#:output Output-Port
                   #:colors (HashTable Any Natural))
                  String)])
 
-(define g (directed-graph '((a b) (b c))))
-(has-edge? g 'a 'c)
-(graphviz g)
+
+(module+ test
+  ;; The goal of the tests is to check that all of the provided
+  ;; functions can be invoked without errors.  The tests do not check
+  ;; whether the results make sense.
+  (require typed/rackunit)
+
+  (test-case "1 Generic Graph Interface"
+    (define g (directed-graph '((a b) (b c))))
+    (check-true (has-edge? g 'a 'c))
+    (check-true (has-vertex? g 'a)))
+
+  (test-case "10 Graphviz"
+    (define g (directed-graph '((a b) (b c))))
+    (graphviz g)))