Support fetching storable vectors + use them in benchmark (#50)

In addition, you can now fetch TensorData directly. This might be useful in
scenarios where you feed the result of a computation back in, like RNN.

Before:

benchmarking feedFetch/4 byte
time                 83.31 μs   (81.88 μs .. 84.75 μs)
                     0.997 R²   (0.994 R² .. 0.998 R²)
mean                 87.32 μs   (86.06 μs .. 88.83 μs)
std dev              4.580 μs   (3.698 μs .. 5.567 μs)
variance introduced by outliers: 55% (severely inflated)

benchmarking feedFetch/4 KiB
time                 114.9 μs   (111.5 μs .. 118.2 μs)
                     0.996 R²   (0.994 R² .. 0.998 R²)
mean                 117.3 μs   (116.2 μs .. 118.6 μs)
std dev              3.877 μs   (3.058 μs .. 5.565 μs)
variance introduced by outliers: 31% (moderately inflated)

benchmarking feedFetch/4 MiB
time                 109.0 ms   (107.9 ms .. 110.7 ms)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 108.6 ms   (108.2 ms .. 109.2 ms)
std dev              740.2 μs   (353.2 μs .. 1.186 ms)

After:

benchmarking feedFetch/4 byte
time                 82.92 μs   (80.55 μs .. 85.24 μs)
                     0.996 R²   (0.993 R² .. 0.998 R²)
mean                 83.58 μs   (82.34 μs .. 84.89 μs)
std dev              4.327 μs   (3.664 μs .. 5.375 μs)
variance introduced by outliers: 54% (severely inflated)

benchmarking feedFetch/4 KiB
time                 85.69 μs   (83.81 μs .. 87.30 μs)
                     0.997 R²   (0.996 R² .. 0.999 R²)
mean                 86.99 μs   (86.11 μs .. 88.15 μs)
std dev              3.608 μs   (2.854 μs .. 5.273 μs)
variance introduced by outliers: 43% (moderately inflated)

benchmarking feedFetch/4 MiB
time                 1.582 ms   (1.509 ms .. 1.677 ms)
                     0.970 R²   (0.936 R² .. 0.993 R²)
mean                 1.645 ms   (1.554 ms .. 1.981 ms)
std dev              490.6 μs   (138.9 μs .. 1.067 ms)
variance introduced by outliers: 97% (severely inflated)

tensorflow-mnist/app/Main.hs

@@ -22,14 +22,9 @@ import Data.List (genericLength)
 import qualified Data.Text.IO as T
 import qualified Data.Vector as V
 
-import qualified TensorFlow.Build as TF
+import qualified TensorFlow.Core as TF
-import qualified TensorFlow.ControlFlow as TF
 import qualified TensorFlow.Gradient as TF
-import qualified TensorFlow.Nodes as TF
 import qualified TensorFlow.Ops as TF
-import qualified TensorFlow.Session as TF
-import qualified TensorFlow.Tensor as TF
-import qualified TensorFlow.Types as TF
 
 import TensorFlow.Examples.MNIST.InputData
 import TensorFlow.Examples.MNIST.Parse

tensorflow-mnist/tests/ParseTest.hs

@@ -48,10 +48,9 @@ import TensorFlow.Tensor
     , tensorFromName
     )
 import TensorFlow.Ops
-import TensorFlow.Nodes (unScalar)
 import TensorFlow.Session
     (runSession, run, run_, runWithFeeds, build, buildAnd)
-import TensorFlow.Types (TensorType(..), Shape(..))
+import TensorFlow.Types (TensorDataType(..), Shape(..), unScalar)
 import Test.Framework (Test)
 import Test.Framework.Providers.HUnit (testCase)
 import Test.HUnit ((@=?), Assertion)

tensorflow-ops/tests/BuildTest.hs

@@ -42,7 +42,7 @@ import TensorFlow.Build
     , withNameScope
     )
 import TensorFlow.ControlFlow (named)
-import TensorFlow.Nodes (unScalar)
+import TensorFlow.Types (unScalar)
 import TensorFlow.Ops
     ( add
     , assign

tensorflow-ops/tests/DataFlowOpsTest.hs

@@ -12,6 +12,7 @@
 -- See the License for the specific language governing permissions and
 -- limitations under the License.
 
+{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 
 import Data.Int (Int32, Int64)
@@ -32,7 +33,7 @@ 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
+    (TF.TensorDataType V.Vector 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

tensorflow-ops/tests/EmbeddingOpsTest.hs

@@ -12,6 +12,7 @@
 -- See the License for the specific language governing permissions and
 -- limitations under the License.
 
+{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 
@@ -129,7 +130,8 @@ testEmbeddingLookupGradients = testCase "testEmbeddingLookupGradients" $ do
 
 -- 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)
+testEmbeddingLookupUndoesSplit ::
+    forall a. (TF.TensorDataType V.Vector a, Show a, Eq a)
     => LookupExample a -> Property
 testEmbeddingLookupUndoesSplit
     (LookupExample numParts

tensorflow-ops/tests/FeedFetchBench.hs

@@ -5,7 +5,7 @@ import Control.Exception (evaluate)
 import Control.Monad.IO.Class (liftIO)
 import Criterion.Main (defaultMain, bgroup, bench)
 import Criterion.Types (Benchmarkable(..))
-import qualified Data.Vector as V
+import qualified Data.Vector.Storable as S
 import qualified TensorFlow.Core as TF
 import qualified TensorFlow.Ops as TF
 
@@ -23,12 +23,12 @@ nfSession init x = Benchmarkable $ \m -> TF.runSession $ do
     go m
 
 -- | Benchmark feeding and fetching a vector.
-feedFetchBenchmark :: TF.Session (V.Vector Float -> TF.Session (V.Vector Float))
+feedFetchBenchmark :: TF.Session (S.Vector Float -> TF.Session (S.Vector Float))
 feedFetchBenchmark = do
     input <- TF.build (TF.placeholder (TF.Shape [-1]))
     output <- TF.build (TF.render (TF.identity input))
     return $ \v -> do
-        let shape = TF.Shape [fromIntegral (V.length v)]
+        let shape = TF.Shape [fromIntegral (S.length v)]
             inputData = TF.encodeTensorData shape v
             feeds = [TF.feed input inputData]
         TF.runWithFeeds feeds output
@@ -36,8 +36,8 @@ feedFetchBenchmark = do
 main :: IO ()
 main = defaultMain
     [ bgroup "feedFetch"
-        [ bench "4 byte" $ nfSession feedFetchBenchmark (V.replicate 1 0)
+        [ bench "4 byte" $ nfSession feedFetchBenchmark (S.replicate 1 0)
-        , bench "4 KiB" $ nfSession feedFetchBenchmark (V.replicate 1024 0)
+        , bench "4 KiB" $ nfSession feedFetchBenchmark (S.replicate 1024 0)
-        , bench "4 MiB" $ nfSession feedFetchBenchmark (V.replicate (1024^2) 0)
+        , bench "4 MiB" $ nfSession feedFetchBenchmark (S.replicate (1024^2) 0)
         ]
     ]

tensorflow-ops/tests/TypesTest.hs

@@ -85,7 +85,7 @@ instance Arbitrary a => Arbitrary (TensorDataInputs a) where
         return $ TensorDataInputs sizes (V.fromList elems)
 
 -- Test that a vector is unchanged after being encoded and decoded.
-encodeDecodeProp :: (TF.TensorType a, Eq a) => TensorDataInputs a -> Bool
+encodeDecodeProp :: (TF.TensorDataType V.Vector a, Eq a) => TensorDataInputs a -> Bool
 encodeDecodeProp (TensorDataInputs shape vec) =
     TF.decodeTensorData (TF.encodeTensorData (TF.Shape shape) vec) == vec
 

tensorflow-queue/tests/QueueTest.hs

@@ -20,7 +20,7 @@ module Main where
 import Control.Monad.IO.Class (liftIO)
 import Data.Int (Int64)
 import Google.Test (googleTest)
-import TensorFlow.Nodes (Scalar(..))
+import TensorFlow.Types (Scalar(..))
 import TensorFlow.Ops (scalar)
 import TensorFlow.Queue
 import TensorFlow.Session

tensorflow/src/TensorFlow/Core.hs

@@ -36,7 +36,6 @@ module TensorFlow.Core
     , buildWithSummary
       -- ** Running graphs
     , Fetchable
-    , Scalar(..)
     , Nodes
     , run
     , run_
@@ -64,8 +63,10 @@ module TensorFlow.Core
     , value
     , tensorFromName
       -- ** Element types
+    , TensorType
     , TensorData
-    , TensorType(decodeTensorData, encodeTensorData)
+    , TensorDataType(decodeTensorData, encodeTensorData)
+    , Scalar(..)
     , Shape(..)
     , OneOf
     , type (/=)

tensorflow/src/TensorFlow/Nodes.hs

@@ -12,8 +12,8 @@
 -- See the License for the specific language governing permissions and
 -- limitations under the License.
 
+{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
@@ -24,12 +24,10 @@ import Control.Applicative (liftA2, liftA3)
 import Data.Map.Strict (Map)
 import Data.Monoid ((<>))
 import Data.Set (Set)
-import Data.String (IsString)
 import Data.Text (Text)
 import Lens.Family2 ((^.))
 import qualified Data.Map.Strict as Map
 import qualified Data.Set as Set
-import qualified Data.Vector as V
 
 import TensorFlow.Build
 import TensorFlow.Output
@@ -101,18 +99,12 @@ instance a ~ () => Fetchable ControlNode a where
 instance Nodes (Tensor v a) where
     getNodes t = Set.singleton <$> getOrAddOp (t ^. tensorOutput . outputOp)
 
-fetchTensorList :: TensorType a => Tensor v a -> Build (Fetch (Shape, [a]))
-fetchTensorList t = fmap (fmap V.toList) <$> fetchTensorVector t
-
 fetchTensorVector :: forall a v . TensorType a
-                  => Tensor v a -> Build (Fetch (Shape, V.Vector a))
+                  => Tensor v a -> Build (Fetch (TensorData a))
 fetchTensorVector (Tensor _ o) = do
     outputName <- renderOutput o
     return $ Fetch (Set.singleton outputName) $ \tensors ->
         let tensorData = tensors Map.! outputName
-            shape = Shape $ FFI.tensorDataDimensions tensorData
-            vec = decodeTensorData $ TensorData tensorData
-
             expectedType = tensorType (undefined :: a)
             actualType = FFI.tensorDataType tensorData
             badTypeError = error $ "Bad tensor type: expected "
@@ -121,21 +113,12 @@ fetchTensorVector (Tensor _ o) = do
                                    ++ show actualType
         in if expectedType /= actualType
                then badTypeError
-               else (shape, vec)
+               else TensorData tensorData
 
 -- The constraint "a ~ a'" means that the input/output of fetch can constrain
 -- the TensorType of each other.
-instance (TensorType a, a ~ a') => Fetchable (Tensor v a) (V.Vector a') where
+instance (TensorType a, a ~ a') => Fetchable (Tensor v a) (TensorData a') where
-    getFetch t = fmap snd <$> fetchTensorVector t
+    getFetch = fetchTensorVector
 
-newtype Scalar a = Scalar {unScalar :: a}
+instance (TensorType a, TensorDataType s a, a ~ a') => Fetchable (Tensor v a) (s a') where
-    deriving (Show, Eq, Ord, Num, Fractional, Floating, Real, RealFloat,
+    getFetch t = fmap decodeTensorData <$> fetchTensorVector t
-              RealFrac, IsString)
-
-instance (TensorType a, a ~ a') => Fetchable (Tensor v a) (Scalar a') where
-    getFetch t = fmap (Scalar . headFromSingleton . snd) <$> fetchTensorList t
-      where
-        headFromSingleton [x] = x
-        headFromSingleton xs
-            = error $ "Unable to extract singleton from tensor of length "
-                          ++ show (length xs)

tensorflow/src/TensorFlow/Types.hs

@@ -16,6 +16,7 @@
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RankNTypes #-}
@@ -30,6 +31,8 @@
 module TensorFlow.Types
     ( TensorType(..)
     , TensorData(..)
+    , TensorDataType(..)
+    , Scalar(..)
     , Shape(..)
     , protoShape
     , Attribute(..)
@@ -50,11 +53,13 @@ import Data.Complex (Complex)
 import Data.Default (def)
 import Data.Int (Int8, Int16, Int32, Int64)
 import Data.Monoid ((<>))
+import Data.String (IsString)
 import Data.Word (Word8, Word16, Word64)
 import Foreign.Storable (Storable)
 import GHC.Exts (Constraint, IsList(..))
 import Lens.Family2 (Lens', view, (&), (.~))
 import Lens.Family2.Unchecked (iso)
+import Text.Printf (printf)
 import qualified Data.Attoparsec.ByteString as Atto
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as B
@@ -95,66 +100,31 @@ import Proto.Tensorflow.Core.Framework.Types (DataType(..))
 import TensorFlow.Internal.VarInt (getVarInt, putVarInt)
 import qualified TensorFlow.Internal.FFI as FFI
 
--- | Data about a tensor that is encoded for the TensorFlow APIs.
-newtype TensorData a = TensorData { unTensorData :: FFI.TensorData }
-
 -- | The class of scalar types supported by tensorflow.
 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.
-    --
-    -- The values should be in row major order, e.g.,
-    --
-    --   element 0:   index (0, ..., 0)
-    --   element 1:   index (0, ..., 1)
-    --   ...
-    encodeTensorData :: Shape -> V.Vector a -> TensorData a
-
--- All types, besides ByteString, are encoded as simple arrays and we can use
--- Vector.Storable to encode/decode by type casting pointers.
-
--- TODO(fmayle): Assert that the data type matches the return type.
-simpleDecode :: Storable a => TensorData a -> V.Vector a
-simpleDecode = S.convert . S.unsafeCast . FFI.tensorDataBytes . unTensorData
-
-simpleEncode :: forall a . (TensorType a, Storable a)
-             => Shape -> V.Vector a -> TensorData a
-simpleEncode (Shape xs)
-    = TensorData . FFI.TensorData xs dt . S.unsafeCast . S.convert
-  where
-    dt = tensorType (undefined :: a)
 
 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
 
 integral :: Integral a => Lens' [Int32] [a]
 integral = iso (fmap fromIntegral) (fmap fromIntegral)
@@ -163,40 +133,140 @@ 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
+
+instance TensorType Bool where
+    tensorType _ = DT_BOOL
+    tensorRefType _ = DT_BOOL_REF
+    tensorVal = boolVal
+
+instance TensorType (Complex Float) where
+    tensorType _ = DT_COMPLEX64
+    tensorRefType _ = DT_COMPLEX64
+    tensorVal = error "TODO (Complex Float)"
+
+instance TensorType (Complex Double) where
+    tensorType _ = DT_COMPLEX128
+    tensorRefType _ = DT_COMPLEX128
+    tensorVal = error "TODO (Complex Double)"
+
+
+-- | Tensor data with the correct memory layout for tensorflow.
+newtype TensorData a = TensorData { unTensorData :: FFI.TensorData }
+
+-- | Types that can be converted to and from 'TensorData'.
+--
+-- 'S.Vector' is the most efficient to encode/decode for most element types.
+class TensorType a => TensorDataType s a where
+    -- | Decode the bytes of a 'TensorData' into an 's'.
+    decodeTensorData :: TensorData a -> s a
+    -- | Encode an 's' into a 'TensorData'.
+    --
+    -- The values should be in row major order, e.g.,
+    --
+    --   element 0:   index (0, ..., 0)
+    --   element 1:   index (0, ..., 1)
+    --   ...
+    encodeTensorData :: Shape -> s a -> TensorData a
+
+-- All types, besides ByteString and Bool, are encoded as simple arrays and we
+-- can use Vector.Storable to encode/decode by type casting pointers.
+
+-- TODO(fmayle): Assert that the data type matches the return type.
+simpleDecode :: Storable a => TensorData a -> S.Vector a
+simpleDecode = S.unsafeCast . FFI.tensorDataBytes . unTensorData
+
+simpleEncode :: forall a . (TensorType a, Storable a)
+             => Shape -> S.Vector a -> TensorData a
+simpleEncode (Shape xs) v =
+    if product xs /= fromIntegral (S.length v)
+        then error $ printf
+            "simpleEncode: bad vector length for shape %v: expected=%d got=%d"
+            (show xs) (product xs) (S.length v)
+        else TensorData (FFI.TensorData xs dt (S.unsafeCast v))
+  where
+    dt = tensorType (undefined :: a)
+
+instance TensorDataType S.Vector Float where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Double where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Int8 where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Int16 where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Int32 where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Int64 where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Word8 where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+instance TensorDataType S.Vector Word16 where
+    decodeTensorData = simpleDecode
+    encodeTensorData = simpleEncode
+
+-- TODO: Haskell and tensorflow use different byte sizes for bools, which makes
+-- encoding more expensive. It may make sense to define a custom boolean type.
+instance TensorDataType S.Vector Bool where
+    decodeTensorData =
+        S.convert . S.map (/= 0) . FFI.tensorDataBytes . unTensorData
+    encodeTensorData (Shape xs) =
+        TensorData . FFI.TensorData xs DT_BOOL . S.map fromBool . S.convert
+      where
+        fromBool x = if x then 1 else 0 :: Word8
+
+instance {-# OVERLAPPABLE #-} (Storable a, TensorDataType S.Vector a)
+    => TensorDataType V.Vector a where
+    decodeTensorData = (S.convert :: S.Vector a -> V.Vector a) . decodeTensorData
+    encodeTensorData x = encodeTensorData x . (S.convert :: V.Vector a -> S.Vector a)
+
+instance {-# OVERLAPPING #-} TensorDataType V.Vector (Complex Float) where
+    decodeTensorData = error "TODO (Complex Float)"
+    encodeTensorData = error "TODO (Complex Float)"
+
+instance {-# OVERLAPPING #-} TensorDataType V.Vector (Complex Double) where
+    decodeTensorData = error "TODO (Complex Double)"
+    encodeTensorData = error "TODO (Complex Double)"
+
+instance {-# OVERLAPPING #-} TensorDataType V.Vector ByteString where
     -- 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]
-    -- TODO(fmayle): Benchmark these functions.
     decodeTensorData tensorData =
         either (\err -> error $ "Malformed TF_STRING tensor; " ++ err) id $
             if expected /= count
@@ -241,32 +311,21 @@ instance TensorType ByteString where
         -- Convert to Vector Word8.
         byteVector = S.fromList $ L.unpack $ Builder.toLazyByteString bytes
 
--- TODO: Haskell and tensorflow use different byte sizes for bools, which makes
+newtype Scalar a = Scalar {unScalar :: a}
--- encoding more expensive. It may make sense to define a custom boolean type.
+    deriving (Show, Eq, Ord, Num, Fractional, Floating, Real, RealFloat,
-instance TensorType Bool where
+              RealFrac, IsString)
-    tensorType _ = DT_BOOL
-    tensorRefType _ = DT_BOOL_REF
-    tensorVal = boolVal
-    decodeTensorData =
-        S.convert . S.map (/= 0) . FFI.tensorDataBytes . unTensorData
-    encodeTensorData (Shape xs) =
-        TensorData . FFI.TensorData xs DT_BOOL . S.map fromBool . S.convert
-      where
-        fromBool x = if x then 1 else 0 :: Word8
 
-instance TensorType (Complex Float) where
+instance TensorDataType V.Vector a => TensorDataType Scalar a where
-    tensorType _ = DT_COMPLEX64
+    decodeTensorData = Scalar . headFromSingleton . decodeTensorData
-    tensorRefType _ = DT_COMPLEX64
+    encodeTensorData x (Scalar y) = encodeTensorData x (V.fromList [y])
-    tensorVal = error "TODO (Complex Float)"
+
-    decodeTensorData = error "TODO (Complex Float)"
+headFromSingleton :: V.Vector a -> a
-    encodeTensorData = error "TODO (Complex Float)"
+headFromSingleton x
+    | V.length x == 1 = V.head x
+    | otherwise = error $
+                  "Unable to extract singleton from tensor of length "
+                  ++ show (V.length x)
 
-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)"
 
 -- | Shape (dimensions) of a tensor.
 newtype Shape = Shape [Int64] deriving Show