Improved scan error message

theano/scan_module/scan_utils.py

@@ -188,19 +188,47 @@ def get_updates_and_outputs(ls):
         else:
             return [x]
 
+    def filter(x):
+        """
+        Ensure `x` is made only of allowed data types.
+
+        Return True iff `x` is made only of lists, tuples, dictionaries, Theano
+        variables or `theano.scan_module.until` objects.
+        """
+        # Is `x` a container we can iterate on?
+        iter_on = None
+        if isinstance(x, list) or isinstance(x, tuple):
+            iter_on = x
+        elif isinstance(x, dict):
+            iter_on = x.iteritems()
+        if iter_on is not None:
+            return all(filter(y) for y in iter_on)
+        else:
+            return (isinstance(x, theano.Variable) or
+                    isinstance(x, theano.scan_module.until))
+
+    if not filter(ls):
+        raise ValueError(
+                'The return value of your scan lambda expression may only be '
+                'made of lists, tuples, or dictionaries containing Theano '
+                'variables (or `theano.scan_module.until` objects for '
+                'conditions). In particular if you need to use constant values, '
+                'you can use `tensor.constant` to turn them into Theano '
+                'variables.')
+
     if is_outputs(ls):
         return None, _list(ls), {}
     if is_updates(ls):
         return None, [], dict(ls)
+    error_msg = ('Scan cannot parse the return value of your lambda '
+                 'expression, which is: %s' % (ls,))
     if not isinstance(ls, (list, tuple)):
-        raise ValueError(('Scan can not parse the return value'
-                          ' of your lambda expression'))
+        raise ValueError(error_msg)
     ls = list(ls)
     deprication_msg = ('The return value of the lambda function'
                     ' has been restricted. you have to always return first the'
                     ' outputs (if any), afterwards the updates (if any) and'
                     ' at the end the conclusion')
-    error_msg = 'Scan can not parse the return value of your lambda expression'
     if len(ls) == 2:
         if is_outputs(ls[0]):
             if is_updates(ls[1]):
@@ -229,6 +257,8 @@ def get_updates_and_outputs(ls):
                 raise ValueError(error_msg)
         else:
             raise ValueError(error_msg)
+    else:
+        raise ValueError(error_msg)
 
 
 def isNaN_or_Inf_or_None(x):