[Haskell-cafe] letting go of file handles and Data.Binary

Bryan O'Sullivan bos at serpentine.com
Sun Apr 20 17:16:14 EDT 2008

Ben wrote:

> this throws a permission denied exception, presumably because the file
> is still open when the removeFile is called.

Yes.  The file handle opened by encodeFile won't be closed until its
finalizer is run.  There is no guarantee that the finalizer will be run
immediately.  In fact, you can usually rely on the finalizer *not*
running immediately, because the GC has to kick in, notice that the
handle is dead, and hand it to the finalizer thread.  It's actually
possible that your finalizer could not run at all.

> i've grovelled the
> source of Data.Bytestring.Lazy and all but i can't seem to understand
> the "right" way to make this work.

Don't use encodeFile at all.  Instead, write your own:

import Data.Binary (Binary, encode)
import Control.Exception (bracket)
import qualified Data.ByteString.Lazy as L
import System.IO (IOMode(..), hClose, openFile)

encodeFile :: Binary a => FilePath -> a -> IO ()
encodeFile path = bracket (openFile path WriteMode) hClose .
                  flip L.hPut .

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