Writer takes a generator, coroutines!

graphgif.scm

@@ -241,21 +241,24 @@
 ;; Output ;;
 ;;;;;;;;;;;;
 
-(define (output-to-file surfaces filename)
+(define (output-to-file filename surface-gen)
   (define pngdir (mkdtemp "/tmp/graphgif_XXXXXX"))
-  (vector-for-each
-   (lambda (i surface)
-     (cairo-surface-write-to-png
-      surface
-      (string-append pngdir "/img-" (number->string i) ".png")))
-   surfaces)
+  (let loop ([surface (surface-gen)]
+	     [i 1])
+    (when surface
+      (cairo-surface-write-to-png
+       surface
+       (string-append pngdir "/img-" (number->string i) ".png"))
+      (loop (surface-gen) (1+ i))))
   (system* "ffmpeg" "-y"
 	   "-i" (string-append pngdir "/img-%d.png")
 	   "-loop" "0" filename))
 
-(define-public (write-graphs-to-file graphs filename)
-  (output-to-file (vector-map (lambda (_ graph) (draw-abstract-graph graph)) graphs)
-		  filename))
+(define-public (write-graphs-to-file filename graph-gen)
+  (define (surface-gen)
+    (let ([graph (graph-gen)])
+      (and graph (draw-abstract-graph graph))))
+  (output-to-file filename surface-gen))
 
 ;; Local Variables:
 ;; geiser-scheme-implementation: guile