Added r operator for ScalarFromTensor as well as raising and error if the

theano/tensor/basic.py

@@ -1583,6 +1583,10 @@ class ScalarFromTensor(Op):
         s, = inp
         dt, = grads
         return [tensor_from_scalar(dt)]
+
+    def R_op(self, inputs, eval_points):
+        return None
+
     def __str__(self):
         return self.__class__.__name__
     def c_code(self, node, name, inputs, outputs, sub):
@@ -5329,6 +5333,9 @@ def Rop(f, wrt, eval_points):
     for out in f:
         if out in wrt:
             rval.append( eval_points[wrt.index(out)])
+        elif seen_nodes[out.owner][out.owner.outputs.index(out)] is None:
+            raise ValueError(( 'The function is not differentiable with '
+                              'respect to the provided inputs !'))
         else:
             rval.append(seen_nodes[out.owner][out.owner.outputs.index(out)] )