Add dag?.

graph.rkt

@@ -35,6 +35,7 @@
 
            bfs bfs/generalized fewest-vertices-path
            dfs dfs/generalized
+           dag?
 
            graphviz)
 
@@ -170,6 +171,8 @@
      #:process-unvisited process-unvisited
      #:combine combine
      #:return finish))
+  (define (dag? g)
+    (g:dag? (gg g)))
 
   ;; 10 Graphviz
   (define (graphviz g #:output [output #f] #:colors [colors #f])
@@ -250,6 +253,7 @@
                          #:combine (-> Any Any Any)
                          #:return (-> Graph Any Any))
                         Any)]
+  [dag? (-> Graph Boolean)]
 
   ;; 10 Graphviz
   [graphviz (->* (Graph)
@@ -349,7 +353,9 @@
                   '((a . 7) (b . 6) (c . 3) (d . 2)))
 
     (check-equal? (dfs/generalized (directed-graph '((a b) (a c) (b d) (c d))))
-                  (void)))
+                  (void))
+    (check-true (dag? (directed-graph '((a b) (b c)))))
+    (check-false (dag? (directed-graph '((a b) (b a))))))
 
   (test-case "10 Graphviz"
     (define g (directed-graph '((a b) (b c))))