Following up on my last question, my next issue is also probably a very
straight ahead example of pipes, but I've managed to get tangled up going
back and forth in the packages' documentation.
I've got a file whose first 4 bytes give the offset into the file of a
series of binary data elements (called MDHs in my case). Given a Handle to
the start of such a file, I want to:
1. read the first Word32 in the file, to retrieve the offset;
2. skip the Handle to that offset; and
3. turn the rest of the file into a Producer MDH IO ()
Given that the file I'm reading may be large, I want to make sure this
process is going to run in constant memory. I thought I could use
pipes-attoparsec, but I couldn't get straight whether it would need to read
the whole file before it could produce anything (as I understand is
normally the case with attoparsec).
So far I have the following, which isn't complete, but at least does the
skip and converts the remaining file to a ByteString producer.
handleToMDHs :: Handle -> IO (Either P.DecodingError (P.Producer P.
ByteString IO ()))
handleToMDHs h = do
hLen <- P.evalStateT (P.decodeGet getWord32le) (PB.fromHandle h)
case (hLen :: Either P.DecodingError Word32) of
Left err -> return $ Left err
Right len -> fmap Right (skipAndProceed h len)
where
skipAndProceed :: Handle -> Word32 -> IO (P.Producer P.ByteString IO ())
skipAndProceed handle l = do
(hSeek handle AbsoluteSeek) (fromIntegral l)
return $ PB.fromHandle handle
My MDH type is an instance of Binary, so there is a get method available.
I'm wondering:
a) What's the right way to turn this into a Producer of MDHs instead of a
Producer of ByteStrings while operating in constant memory?
b) Is there a more elegant way to deal with error handling here? I'm not
even dealing with possible failure in hSeek, and I already think this looks
pretty messy. I'm not wedded to my function type being
handleToMDHs :: Handle -> IO (Either P.DecodingError (P.Producer MDH IO ()))
I just am not sure how else to express the possibility of failure in this
kind of operation.
Thanks,
Dylan
--
You received this message because you are subscribed to the Google Groups
"Haskell Pipes" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to haskell-pipes+unsubscr...@googlegroups.com.
To post to this group, send email to haskell-pipes@googlegroups.com.
