conduit 1.0.13 and ZipSource

February 4, 2014

GravatarMichael Snoyman

A few months ago, Petr Pudlák kicked off the conduit-extra package with the addition of ZipSink. As a refresher, ZipSink provides an alternative Applicative instance for Sink which allows multiple Sinks to consume the same stream.

I've just release version 1.0.13 of conduit which promotes ZipSink from conduit-extra into conduit itself. This abstraction has proven to be generally useful, and I hope others enjoy it as well. If you want a more in-depth review of it, please see the original blog post. The only change since then is renaming broadcast to sequenceSinks.

Along with this change, version 1.0.13 adds a new, similar concept: ZipSource and sequenceSources. The idea here is to combine together multiple streams, instead of sequencing one stream after another.

As a simple motivating example, let's say we have some files on the filesystem, where each file contains a list of student test scores. We want to combine together the test scores for all students for each of the tests. The following program does the job:

import           Control.Monad.Trans.Class (lift)
import qualified Data.ByteString.Char8     as S8
import           Data.Conduit
import qualified Data.Conduit.Binary       as CB
import qualified Data.Conduit.List         as CL
import qualified Data.Map                  as Map

type Name = String

people :: [Name]
people = words "alice bob charlie"

files :: Map.Map Name FilePath
files = Map.fromList $ map (\name -> (name, name ++ ".txt")) people

scores :: MonadResource m => FilePath -> Source m Int
scores fp
    = CB.sourceFile fp
   $= CB.lines
   $= CL.map (read . S8.unpack)

sources :: MonadResource m => Map.Map Name (Source m Int)
sources = fmap scores files

sources' :: MonadResource m => Source m (Map.Map Name Int)
sources' = sequenceSources sources

main :: IO ()
main = runResourceT $ sources' $$ CL.mapM_ (lift . print)

The important bit is the definition of sources'. We use sequenceSources to combine together each of the individual test scores into a single test score map. With some basic input files, the output looks like:

fromList [("alice",1),("bob",3),("charlie",2)]
fromList [("alice",2),("bob",2),("charlie",2)]
fromList [("alice",3),("bob",1),("charlie",2)]

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