[Haskell-cafe] two problems with Data.Binary and Data.ByteString

Tim Newsham newsham at lava.net
Tue Aug 12 20:13:26 EDT 2008

I have a program that read in and populated a large data structure and
then saved it out with Data.Binary and Data.ByteString.Lazy.Char8:

    saveState db = B.writeFile stateFile =<<
        encode <$> atomically (readTVar db)

when I go to read this in later I get a stack overflow:

loadState db = do
     d <- decode <$> B.readFile stateFile
     atomically $ writeTVar db d

   Stack space overflow: current size 8388608 bytes.
   Use `+RTS -Ksize' to increase it.

or from ghci:

     d <- liftM decode
          "savedState.bin") :: IO InstrsDb

     fromList *** Exception: stack overflow

The data type I'm storing is a Map (of maps):

    type DailyDb = M.Map Date Daily
    type InstrsDb = M.Map String DailyDb

What's going on here?  Why is the system capable of building and saving
the data but not in reading and umarhsalling it?  What is the proper way
to track down where the exception is happening?  Any debugging tips?

I also noticed another issue while testing.  If my program loads
the data at startup by calling loadState then all later calls to
saveState give an error:

   Log: savedState.bin: openFile: resource busy (file is locked)

this does not occur if the program wasnt loaded.  My best guess here
is that B.readFile isnt completing and closing the file for some
reason.  Is there a good way to force this?

Tim Newsham

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