Add comments.

theano/scan_module/scan_opt.py

@@ -45,6 +45,15 @@ def info(*msg):
 
 @gof.local_optimizer([None])
 def remove_constants_and_unused_inputs_scan(node):
+    '''
+    Move constants into the inner graph, and remove unused inputs.
+
+    Constants that are in the outer graph are represented by a free symbolic
+    variable in the inner graph. If we move them into the inner graph,
+    constant-folding can happen in the inner graph.
+    This is applied only on sequences and non-sequences,
+    not on initial states.
+    '''
     if not isinstance(node.op, scan_op.Scan):
         return False
     op = node.op
@@ -54,9 +63,12 @@ def remove_constants_and_unused_inputs_scan(node):
                      op.tap_array[:(op.n_mit_mot+op.n_mit_sot)] ]))
     st += op.n_sit_sot
     st += op.n_shared_outs
-    op_ins, op_outs = scan_utils.reconstruct_graph(op.inputs, op.outputs,
-                                                   '')
+    op_ins, op_outs = scan_utils.reconstruct_graph(op.inputs, op.outputs)
+
+    # Corresponds to the initial states, which should stay untouched.
+    # We put those variables aside, and put them back at the end.
     out_stuff_inner = op_ins[op.n_seqs:st]
+
     non_seqs = op_ins[st:]
     st  = ( op.n_seqs +
            op.n_mit_mot +
@@ -67,9 +79,13 @@ def remove_constants_and_unused_inputs_scan(node):
     outer_non_seqs = node.inputs[st:]
     out_stuff_outer = node.inputs[1+op.n_seqs:st]
 
+    # To replace constants in the outer graph by clones in the inner graph
     givens   = {}
+    # All the inputs of the inner graph of the new scan
     nw_inner = []
+    # Same for the outer graph, initialized w/ number of steps
     nw_outer = [node.inputs[0]]
+
     all_ins = gof.graph.inputs(op_outs)
     for idx in xrange(op.n_seqs):
         if (isinstance(node.inputs[idx+1], tensor.TensorConstant) and
@@ -84,6 +100,7 @@ def remove_constants_and_unused_inputs_scan(node):
         elif op_ins[idx] in all_ins:
             nw_inner += [op_ins[idx]]
             nw_outer += [node.inputs[idx+1]]
+
     nw_n_seqs = len(nw_inner)
     # Add outputs stuff
     nw_inner += out_stuff_inner