By default FAST_COMPILE do lazy evaluation

theano/compile/mode.py

@@ -323,7 +323,9 @@ class Mode(object):
 # If a string is passed as the mode argument in function or
 # FunctionMaker, the Mode will be taken from this dictionary using the
 # string as the key
-FAST_COMPILE = Mode('py', 'fast_compile')
+# Use VM_linker to allow lazy evaluation by default.
+FAST_COMPILE = Mode(theano.gof.vm.VM_Linker(use_cloop=False, c_thunks=False),
+                    'fast_compile')
 if theano.config.cxx:
     FAST_RUN = Mode('cvm', 'fast_run')
 else:

theano/gof/tests/test_vm.py

@@ -53,6 +53,24 @@ class TestCallbacks(unittest.TestCase):
         assert self.n_callbacks['IfElse'] == 2
 
 
+def test_c_thunks():
+    a = tensor.scalars('a')
+    b, c = tensor.vectors('bc')
+    cases = [False]
+    if theano.config.cxx:
+        cases.append(True)
+    for c_thunks in cases:
+        f = function([a, b, c], ifelse(a, a*b, b*c),
+                     mode=Mode(
+                         optimizer=None,
+                         linker=vm.VM_Linker(c_thunks=c_thunks,
+                                             use_cloop=False)))
+        f(1, [2], [3, 2])
+        from nose.tools import assert_raises
+        assert_raises(ValueError, f, 0, [2], [3, 4])
+        assert any([hasattr(t, 'cthunk') for t in f.fn.thunks]) == c_thunks
+
+
 def test_speed():
     if not theano.config.cxx:
         raise SkipTest("G++ not available, so we need to skip this test.")

theano/gof/vm.py

@@ -686,7 +686,7 @@ class VM_Linker(link.LocalLinker):
     """
 
     def __init__(self, allow_gc=None, use_cloop=False, callback=None,
-                 lazy=None, schedule=None):
+                 lazy=None, schedule=None, c_thunks=None):
         """
         allow_gc - force the virtual machine to clean up unnecessary
             references, in order to allow garbage collection on
@@ -705,6 +705,8 @@ class VM_Linker(link.LocalLinker):
             version. If lazy is True or False, we force the version used
             between Loop/LoopGC and Stack.
 
+        c_thunks - If None or True, don't change the default. If False,
+            don't compile c code for the thunks.
         """
         # Note: if more parameters are added to __init__, make sure to forward
         # them in the "type(self)(...)" call in the "accept" method below.
@@ -715,6 +717,7 @@ class VM_Linker(link.LocalLinker):
         self.use_cloop = use_cloop
         self.callback = callback
         self.lazy = lazy
+        self.c_thunks = c_thunks
         self.updated_vars = {}
         if schedule:
             self.schedule = schedule
@@ -1010,6 +1013,8 @@ class VM_Linker(link.LocalLinker):
 
         for node in order:
             try:
+                if self.c_thunks is False:
+                    node.op._op_use_c_code = False
                 thunks.append(node.op.make_thunk(node,
                                                  storage_map,
                                                  compute_map,