Best of both worlds : ignore unknown characters but only on top state

src/Graph.hs

@@ -3,6 +3,7 @@
 module Graph (
       Graph(..)
     , Vertex(..)
+    , Zipper(..)
     , editLabel
     , follow
     , open

src/Transducer.hs

@@ -7,7 +7,7 @@ module Transducer (
 
 import Stream (Stream(..), merge)
 import qualified Stream (empty)
-import Graph (Graph(..), Vertex(..), editLabel, follow, open, rewind, singleton, weave)
+import Graph (Graph(..), Vertex(..), Zipper(..), editLabel, follow, open, rewind, singleton, weave)
 
 type Transducer input output = Graph input [output]
 
@@ -24,7 +24,9 @@ fromList = foldl add empty
 run :: (Ord input, Eq output) => Transducer input output -> Stream input -> Stream output
 run transducer (Stream inputs) = foldl (\(Stream outputs) (input, stream) ->
     case follow transducer input of
-      Nothing -> Stream.empty
+      Nothing -> case context transducer of
+        Top -> run (rewind transducer) stream
+        _ -> Stream.empty
       Just newState@(Graph {focus}) ->
         Stream ((emit stream <$> label focus) ++ outputs) `merge` run newState stream
   ) Stream.empty inputs