Doc + uniformity fix.

doc/extending/type.txt

@@ -167,6 +167,25 @@ overridden.
 For more details you can go see the documentation for :ref:`type`.
 
 
+Additional definitions
+----------------------
+
+For certain mechanisms, you can register functions and other such
+things to plus your type into theano's mechanisms.  These are optional
+but will allow people to use you type with familiar interfaces.
+
+`transfer()`
+~~~~~~~~~~~~
+
+To plug in additional options for the transfer target, define a
+function which takes a theano variable and a target argument and
+returns eitehr a new transferred variable (which can be the same as
+the input if no transfer is nessecary) or returns None if the transfer
+can't be done.
+
+Then register that function by calling :func:`register_transfer()`
+with it as argument.
+
 Defining double
 ===============
 

theano/sandbox/cuda/var.py

@@ -160,11 +160,14 @@ CudaNdarrayType.SharedVariable = CudaNdarraySharedVariable
 
 def cuda_shared_constructor(value, name=None, strict=False,
                             allow_downcast=None, borrow=False,
-                            broadcastable=None):
+                            broadcastable=None, target='gpu'):
     """
     SharedVariable Constructor for CudaNdarrayType.
 
     """
+    if target != 'gpu':
+        rause TypeError('not for gpu')
+
     # THIS CONSTRUCTOR TRIES TO CAST VALUE TO A FLOAT32, WHICH THEN GOES ONTO THE CARD
     # SO INT shared vars, float64 shared vars, etc. all end up on the card.
     # THIS IS NOT THE DEFAULT BEHAVIOUR THAT WE WANT.