haskell: tryOne should actually backtrack on failure

haskell/nfker7h/lib/Language/Yanais/Parser.hs

@@ -218,10 +218,14 @@ tryOne f = do
   let (l, r) = U.splitAt 1 (peText env)
   case U.uncons l of
     Nothing -> return Nothing
-    Just (fi, r2) -> (
+    Just (fi, r2) ->
       (if not ((slen r2) == 0) then error "tryOne : unable to iterator over characters" else ())
       `seq`
-      (shiftEnv (slen l, r) >> return (f fi)))
+        case f fi of
+          Nothing -> return Nothing
+          Just x -> do
+            shiftEnv (slen l, r)
+            return . Just $ x
 
 -- | A case tree for `eats`
 data HandleTree x =