ANN: MissingH 0.7.2 with GZip

John Goerzen jgoerzen at
Sun Dec 5 14:41:24 EST 2004

On 2004-12-05, David Roundy <droundy at> wrote:
> On Sat, Dec 04, 2004 at 05:42:30PM -0600, John Goerzen wrote:
> The FileArchive.GZip module is interesting, but I'm not sure I care for the
> interface.  Wouldn't it be better for
> decompress :: String -> (String, Maybe GZipError)
> to be
> decompress :: [Word8] -> Either GZipError [Word8]

The reason it doesn't use Either GZipError String is this:

There are two places where there could be a problem.  First, when
reading the header (not a GZip file, etc.)  Secondly, if the CRC-32 of
the extracted data did not match the stored CRC-32 value.  To validate
this part, we must first inflate all the data and calculate the CRC-32
along the way.

If the return type is an Either, then this will force the system to
buffer the entire file in memory and check the CRC-32 before returning
anything.  In other words, the laziness is lost.

As for [Word8] vs. String, I'm not sure what the advantages of using a
Word8 instead of a Char are.  I presume a slight performance benefit?
The String seemed to be handy, given hGetContents and hPutStr.

> And I'm not sure how hDecompress works.  Could we use this with a pipe to
> read a gzipped file, or does it have to be able to synchronously write the
> entire file to the output handle?

Yes, you could do that.

> What I'd really like would be
> openGZippedFile :: IOMode -> FilePath -> IO Handle

Yes, me too.  Though from what I can tell, that is impossible, or at
least it's impossible to do in Haskell.

> And of course, speed is an issue, so an optional zlib backend would be
> double-nice (or a really well-optimized decompressor).  For now, I guess
> I'll stick with my tried and true (and butt ugly) zlib ffi interface, which
> uses threads and pipes to implement a gzOpenFile.

Ahh.  I suppose one could do that, but that sounds nasty.  But you're
right, I would like to see good Haskell bindings for both zlib and the
bzip2 library.

> Actually, I see that compression is not implemented either...

I haven't found any Deflate implementation for Haskell yet.

-- John

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