diff --git a/tensorflow-nn/src/TensorFlow/NN.hs b/tensorflow-nn/src/TensorFlow/NN.hs
index 5cb5fda..c7d7c28 100644
--- a/tensorflow-nn/src/TensorFlow/NN.hs
+++ b/tensorflow-nn/src/TensorFlow/NN.hs
@@ -35,6 +35,7 @@ import TensorFlow.Tensor        ( Tensor(..)
                                 , Value(..)
                                 )
 import TensorFlow.Types         ( TensorType(..)
+                                , TensorProtoLens
                                 , OneOf
                                 )
 import TensorFlow.Ops           ( zerosLike
@@ -70,7 +71,7 @@ import TensorFlow.Ops           ( zerosLike
 --
 --  `logits` and `targets` must have the same type and shape.
 sigmoidCrossEntropyWithLogits
-  :: (OneOf '[Float, Double] a, TensorType a, Num a)
+  :: (OneOf '[Float, Double] a, TensorType a, TensorProtoLens a, Num a)
      => Tensor Value a          -- ^ __logits__
      -> Tensor Value a          -- ^ __targets__
      -> Build (Tensor Value a)
diff --git a/tensorflow-ops/src/TensorFlow/EmbeddingOps.hs b/tensorflow-ops/src/TensorFlow/EmbeddingOps.hs
index 9eb396b..24ceb91 100644
--- a/tensorflow-ops/src/TensorFlow/EmbeddingOps.hs
+++ b/tensorflow-ops/src/TensorFlow/EmbeddingOps.hs
@@ -27,7 +27,7 @@ import Data.List (genericLength)
 import TensorFlow.Build (Build, colocateWith, render)
 import TensorFlow.Ops ()  -- Num instance for Tensor
 import TensorFlow.Tensor (Tensor, Value)
-import TensorFlow.Types (OneOf, TensorType)
+import TensorFlow.Types (OneOf, TensorType, TensorProtoLens)
 import qualified TensorFlow.GenOps.Core as CoreOps
 
 -- | Looks up `ids` in a list of embedding tensors.
@@ -46,6 +46,7 @@ import qualified TensorFlow.GenOps.Core as CoreOps
 -- tensor. The returned tensor has shape `shape(ids) + shape(params)[1:]`.
 embeddingLookup :: forall a b v .
                    ( TensorType a
+                   , TensorProtoLens b
                    , OneOf '[Int64, Int32] b
                    , Num b
                    )
diff --git a/tensorflow-ops/src/TensorFlow/Gradient.hs b/tensorflow-ops/src/TensorFlow/Gradient.hs
index f863e36..f629510 100644
--- a/tensorflow-ops/src/TensorFlow/Gradient.hs
+++ b/tensorflow-ops/src/TensorFlow/Gradient.hs
@@ -94,13 +94,13 @@ import TensorFlow.Tensor
     , tensorOutput
     , tensorAttr
     )
-import TensorFlow.Types (OneOf, TensorType, attrLens)
+import TensorFlow.Types (OneOf, TensorType, TensorProtoLens, attrLens)
 import Proto.Tensorflow.Core.Framework.NodeDef
     (NodeDef, attr, input, op, name)
 
 type GradientCompatible a =
     -- TODO(fmayle): MaxPoolGrad doesn't support Double for some reason.
-    (Num a, OneOf '[ Float, Complex Float, Complex Double ] a)
+    (Num a, OneOf '[ Float, Complex Float, Complex Double ] a, TensorProtoLens a)
 
 -- TODO(fmayle): Support control flow.
 -- TODO(fmayle): Support gate_gradients-like option to avoid race conditions.
diff --git a/tensorflow-ops/src/TensorFlow/Ops.hs b/tensorflow-ops/src/TensorFlow/Ops.hs
index 3fff01d..af69c88 100644
--- a/tensorflow-ops/src/TensorFlow/Ops.hs
+++ b/tensorflow-ops/src/TensorFlow/Ops.hs
@@ -138,6 +138,7 @@ import qualified Prelude (abs)
 -- "neg 1 :: Tensor Value Float", it helps find the type of the subexpression
 -- "1".
 instance ( TensorType a
+         , TensorProtoLens a
          , Num a
          , v ~ Value
          , OneOf '[ Double, Float, Int32, Int64
@@ -191,7 +192,7 @@ initializedVariable initializer = do
 
 -- | Creates a zero-initialized variable with the given shape.
 zeroInitializedVariable
-  :: (TensorType a, Num a) =>
+  :: (TensorType a, TensorProtoLens a, Num a) =>
      TensorFlow.Types.Shape -> Build (Tensor TensorFlow.Tensor.Ref a)
 zeroInitializedVariable = initializedVariable . zeros
 
@@ -238,7 +239,8 @@ restore path x = do
 --   element 0:   index (0, ..., 0)
 --   element 1:   index (0, ..., 1)
 --   ...
-constant :: forall a . TensorType a => Shape -> [a] -> Tensor Value a
+constant :: forall a . (TensorType a, TensorProtoLens a)
+            => Shape -> [a] -> Tensor Value a
 constant (Shape shape') values
     | invalidLength = error invalidLengthMsg
     | otherwise = buildOp $ opDef "Const"
@@ -258,11 +260,11 @@ constant (Shape shape') values
                 & tensorVal .~ values
 
 -- | Create a constant vector.
-vector :: TensorType a => [a] -> Tensor Value a
+vector :: (TensorType a, TensorProtoLens a) => [a] -> Tensor Value a
 vector xs = constant [fromIntegral $ length xs] xs
 
 -- | Create a constant scalar.
-scalar :: forall a . TensorType a => a -> Tensor Value a
+scalar :: (TensorType a, TensorProtoLens a) => a -> Tensor Value a
 scalar x = constant [] [x]
 
 -- Random tensor from the unit normal distribution with bounded values.
@@ -273,7 +275,8 @@ truncatedNormal = buildOp $ opDef "TruncatedNormal"
                           & opAttr "dtype" .~ tensorType (undefined :: a)
                           & opAttr "T" .~ tensorType (undefined :: Int64)
 
-zeros :: forall a . (Num a, TensorType a) => Shape -> Tensor Value a
+zeros :: (Num a, TensorType a, TensorProtoLens a)
+ => Shape -> Tensor Value a
 zeros (Shape shape') = CoreOps.fill (vector $ map fromIntegral shape') (scalar 0)
 
 shape :: (TensorType t) => Tensor v1 t -> Tensor Value Int32
diff --git a/tensorflow-ops/tests/DataFlowOpsTest.hs b/tensorflow-ops/tests/DataFlowOpsTest.hs
index cd362c9..cb4f612 100644
--- a/tensorflow-ops/tests/DataFlowOpsTest.hs
+++ b/tensorflow-ops/tests/DataFlowOpsTest.hs
@@ -31,8 +31,9 @@ import qualified TensorFlow.Types as TF
 
 -- DynamicSplit is undone with DynamicStitch to get the original input
 -- back.
-testDynamicPartitionStitchInverse :: forall a.
-    (TF.TensorType a, Show a, Eq a) => StitchExample a -> Property
+testDynamicPartitionStitchInverse ::
+    forall a. (TF.TensorType a, TF.TensorProtoLens a, Show a, Eq a)
+    => StitchExample a -> Property
 testDynamicPartitionStitchInverse (StitchExample numParts values partitions) =
    let splitParts :: [TF.Tensor TF.Value a] =
            CoreOps.dynamicPartition numParts (TF.vector values) partTensor
diff --git a/tensorflow-ops/tests/EmbeddingOpsTest.hs b/tensorflow-ops/tests/EmbeddingOpsTest.hs
index 0a6b97d..4a7dc94 100644
--- a/tensorflow-ops/tests/EmbeddingOpsTest.hs
+++ b/tensorflow-ops/tests/EmbeddingOpsTest.hs
@@ -36,8 +36,9 @@ import qualified TensorFlow.Types as TF
 
 -- Verifies that direct gather is the same as dynamic split into
 -- partitions, followed by embedding lookup.
-testEmbeddingLookupUndoesSplit :: forall a. (TF.TensorType a, Show a, Eq a)
-                               => LookupExample a -> Property
+testEmbeddingLookupUndoesSplit ::
+    forall a. (TF.TensorType a, TF.TensorProtoLens a, Show a, Eq a)
+    => LookupExample a -> Property
 testEmbeddingLookupUndoesSplit
     (LookupExample numParts
                    shape@(TF.Shape (firstDim : restDims))
diff --git a/tensorflow/src/TensorFlow/Types.hs b/tensorflow/src/TensorFlow/Types.hs
index 3d47f39..604e06d 100644
--- a/tensorflow/src/TensorFlow/Types.hs
+++ b/tensorflow/src/TensorFlow/Types.hs
@@ -28,6 +28,7 @@
 
 module TensorFlow.Types
     ( TensorType(..)
+    , TensorProtoLens(..)
     , TensorData(..)
     , Shape(..)
     , protoShape
@@ -76,13 +77,12 @@ import Proto.Tensorflow.Core.Framework.AttrValue
     )
 import Proto.Tensorflow.Core.Framework.Tensor as Tensor
     ( TensorProto(..)
-    , floatVal
+    , boolVal
     , doubleVal
+    , floatVal
+    , int64Val
     , intVal
     , stringVal
-    , int64Val
-    , stringVal
-    , boolVal
     )
 import Proto.Tensorflow.Core.Framework.TensorShape
     ( TensorShapeProto(..)
@@ -101,7 +101,6 @@ newtype TensorData a = TensorData { unTensorData :: FFI.TensorData }
 class TensorType a where
     tensorType :: a -> DataType
     tensorRefType :: a -> DataType
-    tensorVal :: Lens' TensorProto [a]
     -- | Decode the bytes of a TensorData into a Vector.
     decodeTensorData :: TensorData a -> V.Vector a
     -- | Encode a Vector into a TensorData.
@@ -113,6 +112,25 @@ class TensorType a where
     --   ...
     encodeTensorData :: Shape -> V.Vector a -> TensorData a
 
+-- | Class of types that can be used for constructing constant tensors.
+class TensorProtoLens a where
+    tensorVal :: Lens' TensorProto [a]
+
+instance TensorProtoLens Float where
+    tensorVal = floatVal
+
+instance TensorProtoLens Double where
+    tensorVal = doubleVal
+
+instance TensorProtoLens Int32 where
+    tensorVal = intVal
+
+instance TensorProtoLens Int64 where
+    tensorVal = int64Val
+
+instance TensorProtoLens ByteString where
+    tensorVal = stringVal
+
 -- All types, besides ByteString, are encoded as simple arrays and we can use
 -- Vector.Storable to encode/decode by type casting pointers.
 
@@ -130,28 +148,24 @@ simpleEncode (Shape xs)
 instance TensorType Float where
     tensorType _ = DT_FLOAT
     tensorRefType _ = DT_FLOAT_REF
-    tensorVal = floatVal
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType Double where
     tensorType _ = DT_DOUBLE
     tensorRefType _ = DT_DOUBLE_REF
-    tensorVal = doubleVal
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType Int32 where
     tensorType _ = DT_INT32
     tensorRefType _ = DT_INT32_REF
-    tensorVal = intVal
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType Int64 where
     tensorType _ = DT_INT64
     tensorRefType _ = DT_INT64_REF
-    tensorVal = int64Val
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
@@ -161,40 +175,36 @@ integral = iso (fmap fromIntegral) (fmap fromIntegral)
 instance TensorType Word8 where
     tensorType _ = DT_UINT8
     tensorRefType _ = DT_UINT8_REF
-    tensorVal = intVal . integral
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType Word16 where
     tensorType _ = DT_UINT16
     tensorRefType _ = DT_UINT16_REF
-    tensorVal = intVal . integral
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType Int16 where
     tensorType _ = DT_INT16
     tensorRefType _ = DT_INT16_REF
-    tensorVal = intVal . integral
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType Int8 where
     tensorType _ = DT_INT8
     tensorRefType _ = DT_INT8_REF
-    tensorVal = intVal . integral
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType ByteString where
     tensorType _ = DT_STRING
     tensorRefType _ = DT_STRING_REF
-    tensorVal = stringVal
     -- Encoded data layout (described in third_party/tensorflow/c/c_api.h):
     --   table offsets for each element :: [Word64]
     --   at each element offset:
     --     string length :: VarInt64
     --     string data   :: [Word8]
+    -- C++ counterparts of these are TF_Tensor_{En,De}codeStrings.
     -- TODO(fmayle): Benchmark these functions.
     decodeTensorData tensorData =
         either (\err -> error $ "Malformed TF_STRING tensor; " ++ err) id $
@@ -244,21 +254,18 @@ instance TensorType ByteString where
 instance TensorType Bool where
     tensorType _ = DT_BOOL
     tensorRefType _ = DT_BOOL_REF
-    tensorVal = boolVal
     decodeTensorData = simpleDecode
     encodeTensorData = simpleEncode
 
 instance TensorType (Complex Float) where
     tensorType _ = DT_COMPLEX64
     tensorRefType _ = DT_COMPLEX64
-    tensorVal = error "TODO (Complex Float)"
     decodeTensorData = error "TODO (Complex Float)"
     encodeTensorData = error "TODO (Complex Float)"
 
 instance TensorType (Complex Double) where
     tensorType _ = DT_COMPLEX128
     tensorRefType _ = DT_COMPLEX128
-    tensorVal = error "TODO (Complex Double)"
     decodeTensorData = error "TODO (Complex Double)"
     encodeTensorData = error "TODO (Complex Double)"