[Haskell-cafe] Problems with iteratees

[oleg at okmij.org]

I agree with Maciej Wos' diagnosis and solution. I'd like to point out
that the Tiff library has to do a similar processing: use an iteratee
to read bytes from the outer stream and to produce elements for the
inner stream (e.g., signed 32-bit integers). In your version of the
Iteratee library, please look at Data/Iteratee/Codecs/Tiff.hs and
search for the following fragment:

  -- Read the array of long integers
  -- of 1 element: the offset field contains the value
  read_value typ e' 1 | typ == TT_long || typ == TT_slong = ...

  -- of n elements
  read_value typ e' count | typ == TT_long || typ == TT_slong = do
      offset <- endianRead4 e'
      return . Just . TEN_INT $ \iter_int -> return $ do
            Iter.seek (fromIntegral offset)
            let iter = convStream
                         (liftM (either (const Nothing) (Just . (:[]) . conv_long typ)) (checkErr (endianRead4 e')))
                         iter_int
            Iter.joinI $ Iter.joinI $ Iter.takeR (4*count) iter


Of relevance is the line that contains convStream. Please notice
(const Nothing) that Maciej was talking about.

The newer version has a somewhat more convenient sequence_stream
function, which creates a nested stream using the supplied iteratee to
read from the outer stream and produce an element for the inner stream.
The function is particularly useful if you read the inner stream all
the way, until it is exhausted. Here is how the above read_value
clause looks now

			-- of n elements
  read_value typ count | typ == TT_long || typ == TT_slong = do 
    offset <- endian_read4
    let converter = endian_read4 >>= return . conv_long typ
    return . Just . TEN_INT $ \iter_int -> do
            seek_stream (fromIntegral offset)
            runI =<< takeR (4*count) (sequence_stream converter iter_int)