[Haskell-cafe] reading file content with conduit

2 Nov 2018


      Hello,
I have a strange behaviour with my code when reading data from a file saved by sinkFile.
I only see the first record of the file, although the file seems to contain several records.

Any hints ?

-- built in a sandbox with GHC 8.2.2, base 4.10.1.0, binary 0.8.5.1, bytestring 0.8.10.2, conduit 1.3.1
{-# LANGUAGE DeriveGeneric #-}
import qualified Data.ByteString as DB
import qualified Data.ByteString.Lazy as DBL
import qualified Data.Binary as DBI
import GHC.Generics (Generic)
import Conduit
import Data.Int (Int64)

data MySubRec = MySubRec { sr1 :: Float,
                           sr2 :: Float }
             deriving (Generic, Show)

data MyRec = MyRec { r1 :: Int64,
                     r2 :: String,
                     r3 :: [MySubRec],
                     r4 :: [MySubRec]
                     }
             deriving (Generic, Show)

instance DBI.Binary MySubRec
instance DBI.Binary MyRec

es = MySubRec { sr1 =1.0, sr2 =1000.5 }
myList = repeat es
e1 = MyRec{ r1=1, r2="e1", r3=take 2 myList, r4=take 1 myList}
e2 = MyRec{ r1=2, r2="e2", r3=take 2 myList, r4=take 1 myList}
myData = concat $ repeat [e1,e2]

dataToBs :: Monad m =>
            ConduitT MyRec DB.ByteString m ()
dataToBs = do
                            d <- await
                            case d of
                             Just bs -> do
                                yield $ DBL.toStrict $ DBI.encode bs
                                dataToBs
                             _ -> return ()
bsToData :: Monad m =>
            ConduitT DB.ByteString MyRec m ()
bsToData = do
                            d <- await
                            case d of
                             Just bs -> do
                                yield $ DBI.decode $ DBL.fromStrict bs
                                bsToData
                             _ -> return ()
main = do
        runConduitRes $ yieldMany (take 10 myData) .| dataToBs .|  sinkFile "/tmp/res.bin"
        runConduitRes $ sourceFile "/tmp/res.bin" .| bsToData .| mapM_C (liftIO . putStrLn . show)

[Haskell-cafe] reading file content with conduit

Fabien R