Haskell general serialization

Question

I get this error when I try to deserialize an object.

Amy64.hs: GetException "too few bytes\nFrom:\tdemandInput\n\n" 

An output file is created and looks like this:

 00000000 1f 8b 08 00 00 00 00 00 04 03 63 60 00 03 56 a7 |..........c`..V.| 00000010 d2 f4 e2 4a 00 be b2 38 41 0d 00 00 00 |...J...8A....| 

I suspect I'm misusing new generics. Here is my code. By the way, if I delete lines 9, 22 and 23, I get the same result. And if I delete the gzip / ungzip stuff, I get the same result.

 {-# LANGUAGE DeriveGeneric #-} import Data.Conduit (runResourceT, ($$), (=$)) import Data.Conduit.Binary (sinkFile, sourceFile) import Data.Conduit.Cereal (sinkGet, sourcePut) import Data.Conduit.Zlib (gzip, ungzip) import qualified Data.Serialize as DS (Serialize, get, put) import GHC.Generics (Generic) import Data.Serialize.Derive (derivePut, deriveGet) -- line 9 readBug :: FilePath -> IO (Either String Bug) readBug f = runResourceT $ sourceFile f $$ ungzip =$ sinkGet (DS.get) writeBug :: FilePath -> Bug -> IO () writeBug ft = runResourceT $ sourcePut (DS.put t) $$ gzip =$ sinkFile f data Bug = Bug String deriving (Show, Generic) instance DS.Serialize Bug where put = derivePut -- line 22 get = deriveGet -- line 23 main = do let a = Bug "Bugsy" writeBug "x.dat" a (Right b) <- readBug "x.dat" :: IO (Either String Bug) print b 

Answer 1

It turned out that my mistake had nothing to do with generics. A bad type signature in the readBug function caused an attempt to read the Either String Bug instead of a Bug .

 {-# LANGUAGE DeriveGeneric #-} import Data.Conduit (runResourceT, ($$), (=$)) import Data.Conduit.Binary (sinkFile, sourceFile) import Data.Conduit.Cereal (sinkGet, sourcePut) import Data.Conduit.Zlib (gzip, ungzip) import qualified Data.Serialize as DS (Serialize, get, put) import GHC.Generics (Generic) import Data.Serialize.Derive (derivePut, deriveGet) -- line 9 readBug :: FilePath -> IO Bug readBug f = runResourceT $ sourceFile f $$ ungzip =$ sinkGet DS.get writeBug :: FilePath -> Bug -> IO () writeBug ft = runResourceT $ sourcePut (DS.put t) $$ gzip =$ sinkFile f data Bug = Bug String deriving (Show, Generic) instance DS.Serialize Bug where put = derivePut -- line 22 get = deriveGet -- line 23 main :: IO () main = do let a = Bug "Bugsy" writeBug "x.dat" a b <- readBug "x.dat" :: IO Bug print b