strict mode to avoid nested scan

theano/scan_module/scan.py

@@ -45,6 +45,7 @@ __contact__ = "Razvan Pascanu <r.pascanu@gmail>"
 import itertools
 import logging
 import numpy
+import warnings
 
 from theano.compile import SharedVariable, function
 from theano import compile
@@ -75,7 +76,8 @@ def scan(fn,
          mode=None,
          name=None,
          profile=False,
-         allow_gc=None):
+         allow_gc=None,
+         strict=False):
     """
     This function constructs and applies a Scan op to the provided
     arguments.
@@ -314,6 +316,10 @@ def scan(fn,
         Set the value of allow gc for the internal graph of scan.  If
         set to None, this will use the value of config.scan.allow_gc.
 
+    :param strict:
+        If true, all the shared variables used in ``fn`` must be provided as a
+        part of ``non_sequences``. 
+
     :rtype: tuple
     :return: tuple of the form (outputs, updates); ``outputs`` is either a
              Theano variable or a list of Theano variables representing the
@@ -910,6 +916,21 @@ def scan(fn,
                              not isinstance(arg, tensor.Constant))]
 
     givens.update(OrderedDict(zip(other_scan_args, other_inner_args)))
+
+    if strict:
+        non_seqs_set = set(non_sequences)
+
+        other_shared_scan_args = [arg.variable for arg
+                            in dummy_f.maker.expanded_inputs
+                            if (isinstance(arg.variable, SharedVariable) and
+                                not arg.update and
+                                arg.variable in non_seqs_set)]
+        other_shared_inner_args = [safe_new(arg.variable, '_copy') for arg
+                            in dummy_f.maker.expanded_inputs
+                            if (isinstance(arg.variable, SharedVariable) and
+                                not arg.update and
+                                arg.variable in non_seqs_set)]
+    else:
         other_shared_scan_args = [arg.variable for arg
                             in dummy_f.maker.expanded_inputs
                             if (isinstance(arg.variable, SharedVariable) and
@@ -990,6 +1011,10 @@ def scan(fn,
     info['as_while'] = as_while
     info['profile'] = profile
     info['allow_gc'] = allow_gc
+    info['strict'] = strict
+    if strict:
+        warnings.warn('In the strict mode, all neccessary shared variables '
+                      'must be passed as a part of non_sequences', Warning)
 
     local_op = scan_op.Scan(inner_inputs, new_outs, info)