Many ways to skin a conduit

April 17, 2012

GravatarBy Michael Snoyman

There's more than one way to skin a cat, and certainly more than one way to write code. The various options can sometimes be confusing. And in the case of the conduit library, there are also some routes that you shouldn't take. You'll see what I mean through the examples.

For the most part, using existing Sources, Sinks, and Conduits is straight-forward. The problem comes from writing them in the first place. Let's take a simple example: we want a Source that will enumerate the Ints 1 to 1000. For testing purposes, we'll connect it to a Sink that sums up all of its input. I came up with six different ways to write the Source, though two of those are using functions I haven't yet released.

import Criterion.Main
import Data.Conduit
import qualified Data.Conduit.List as CL
import qualified Data.List
import Data.Functor.Identity (runIdentity)

sourceList, unfold, enumft, yielder, raw, state
    :: Monad m
    => Int -- ^ stop
    -> Source m Int

sourceList stop = CL.sourceList [1..stop]

unfold stop =
    CL.unfold f 1
    f i
        | i > stop = Nothing
        | otherwise = Just (i, i + 1)

enumft stop = CL.enumFromTo 1 stop

yielder stop =
    go 1
    go i
        | i > stop = return ()
        | otherwise = do
            yield i
            go $ i + 1

raw stop =
    go 1
    go i
        | i > stop = Done Nothing ()
        | otherwise = HaveOutput (go $ i + 1) (return ()) i

state stop =
    sourceState 1 pull
    pull i
        | i > stop = return StateClosed
        | otherwise = return $ StateOpen (i + 1) i

main :: IO ()
main = do
    mapM_ test sources
    defaultMain $ map bench' sources
    sink :: Monad m => Sink Int m Int
    sink = CL.fold (+) 0

    bench' (name, source) = bench name $ whnf (\i -> runIdentity $ source i $$ sink) 1000

    sources =
        [ ("sourceList", sourceList)
        , ("unfold", unfold)
        , ("enumFromTo", enumft)
        , ("yield", yielder)
        , ("raw", raw)
        , ("sourceState", state)

    test (name, source) = do
        let res = runIdentity $ source 1000 $$ sink
        putStrLn $ name ++ ": " ++ show res

sourceList is probably the approach most of us- myself included- would actually use in real life. It let's us take advantage of all of the list-processing functions and special syntax that Haskell already provides. unfold and enumFromTo are both new functions for 0.4.2 (in fact, I wrote them for the purpose of this comparison). They correspond very closely to their Data.List and Prelude counterparts.

yield is a new option we have starting with conduit 0.4. Due to the unified datatypes, Source has inherited a Monad instance. This allows us to fairly easily compose together different Sources, and the yield function provides the simplest of all Sources. In previous versions of conduit, we could have used Source's Monoid instance instead of do-notation.

raw goes directly against the datatypes. I find it interesting that the raw version isn't really much more complicated than yield or sourceState, though you do have to understand some of the extra fields on the constructors. Finally, we use sourceState. This is one of the oldest approaches, since this function has been available since the first release of conduit. I think that this function would compile and run perfectly on conduit 0.0.

The Criterion benchmarks are very informative. Thanks to Bryan's cool new report, let's look at the graph:

unfold, enumFromTo, and raw all perform equally well. sourceList comes in behind them: the need to allocate the extra list is the culprit. Behind that is yield. To see why, look at the difference between yielder and raw. They're structure almost identically. For the i > stop case, we have return () versus Done Nothing (). But in reality, those are the same thing! return is defined as Done Nothing.

The performance gap comes from the otherwise branch. If we fully expand the do-notation, we end up with:

yield i >>= (go $ i + 1)
==> HaveOutput (Done Nothing ()) (return ()) i >> (go $ i + 1)
==> HaveOutput (Done Nothing () >> (go $ i + 1)) (return ()) i
==> HaveOutput (go $ i + 1) (return ()) i

Which is precisely what raw says. However, without adding aggressive inlining to conduit, most of this transformation will occur at runtime, not compile time. Still, the performance gap is relatively minor, and in most real-world applications should be dwarfed by the actual computations being performed, so I think the yield approach definitely has merit.

What might be shocking is the abysmal performance of sourceState. It's a full 8 times slower than raw! There are two major contributing factors here:

  • Each step goes through a monadic bind. This is necessitated by the API of sourceState.
  • We have to unwrap the SourceStateResult type.

sourceState was great when it first came out. When conduit's internals were ugly and based on mutable variables, it provided a clean, simple approach to creating Sources. However, conduit has moved on: the internals are pure and easy to work with and we have alternatives like yield for high-level stuff. And performance wise, the types now distinguish between pure and impure actions. sourceState forces usage of an extra PipeM constructor at each step of output generation, which kills GHC's ability to optimize.

So our main takeaway should be: don't use sourceState. It's there for API compatibility with older versions, but is no longer the best approach to the problem. Similarly, we can improve upon sourceIO, but we have to be a bit careful here, since we have to ensure that all of our finalizers are called correctly. Let's take a look at a simple Char-based file source, comparing a sourceIO implementation to the raw constructors.

import Data.Conduit
import qualified Data.Conduit.List as CL
import Control.Monad.Trans.Resource
import System.IO
import Control.Monad.IO.Class (liftIO)
import Criterion.Main

sourceFileOld :: MonadResource m => FilePath -> Source m Char
sourceFileOld fp = sourceIO
    (openFile fp ReadMode)
    (\h -> liftIO $ do
        eof <- hIsEOF h
        if eof
            then return IOClosed
            else fmap IOOpen $ hGetChar h)

sourceFileNew :: MonadResource m => FilePath -> Source m Char
sourceFileNew fp = PipeM
    (allocate (openFile fp ReadMode) hClose >>= go)
    (return ())
    go (key, h) =
        self = PipeM pull close
        pull = do
            eof <- liftIO $ hIsEOF h
            if eof
                then do
                    release key
                    return $ Done Nothing ()
                else do
                    c <- liftIO $ hGetChar h
                    return $ HaveOutput self close c
        close = release key

main :: IO ()
main =
    defaultMain [bench "old" $ go sourceFileOld, bench "new" $ go sourceFileNew]
    go src = whnfIO $ runResourceT $ src "source-io.hs" $$ CL.sinkNull

The results are much closer here:

We're no longer getting the benefit of avoiding monadic binds, since by its very nature this function has to call IO actions constantly. In fact, I believe that the performance gap here doesn't warrant avoiding sourceIO in normal user code, though it's likely a good idea to look at optimizing the Data.Conduit.Binary functions. Perhaps even better is if we can get some combinators that make it easier to express this kind of control flow.

The story is much the same with Sinks and Conduits, so I won't bore you with too many details. Let's jump into the code first, and then explain what we want to notice.

import Criterion.Main
import Data.Conduit
import qualified Data.Conduit.List as CL
import qualified Data.List
import Data.Functor.Identity

main :: IO ()
main = defaultMain
    [ bench "mapOutput" $ flip whnf 2 $ \i -> runIdentity $ mapOutput (* i) source $$ sink
    , bench "map left" $ flip whnf 2 $ \i -> runIdentity $ source $= (* i) $$ sink
    , bench "map right" $ flip whnf 2 $ \i -> runIdentity $ source $$ (* i) =$ sink
    , bench "await-yield left" $ flip whnf 2 $ \i -> runIdentity $ source $= awaitYield i $$ sink
    , bench "await-yield right" $ flip whnf 2 $ \i -> runIdentity $ source $$ awaitYield i =$ sink
    source :: Monad m => Source m Int
    source = CL.sourceList [1..1000]

    sink :: Monad m => Sink Int m Int
    sink = CL.fold (+) 0

    awaitYield :: Monad m => Int -> Conduit Int m Int
    awaitYield i =
        self = do
            mx <- await
            case mx of
                Nothing -> return ()
                Just x -> do
                    yield $ x * i

There are five different ways presented to multiple each number in a stream by 2. is likely the most obvious choice, since it's a natural analogue to the list-based map function. But we have two different ways to use it: we can either left-fuse the source to the conduit, and then connect the new source to the sink, or right-fuse the conduit to the sink, and connect the source to the new sink.

We also have an awaitYield function, which uses the await and yield functions and leverages the Monad instance of Conduit. Like map, we have both a left and a right version.

We also have a mapOutput function. In that case, we're not actually using a Conduit at all. Instead, we're modifying the output values being produced by the source directly, without needing to pipe through an extra component. Let's see our benchmark results:

There are three things worth noticing:

  1. Like previously, the high-level approach (using await and yield) was slower than using the more highly optimized function from Data.Conduit.List.
  2. There no clear winner between left and right fusing.
  3. mapOutput is significantly faster than using a Conduit. The reason is that we're able to eliminate an entire extra Pipe in the pipeline.

mapOutput will not be an option in the general case. You're restricted in a number of ways:

  • It can only be applied to a Source, not a Sink.
  • You have to have transformations which produce one output for one input.
  • You can perform any monadic actions.

However, if your use case matches, mapOutput can be a very convenient optimization.


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