[Haskell-cafe] Restricted file reading monad

[oleg at okmij.org]

George Pollard wrote:
> The structure of an ID3 tag goes something like this:
> Header:
> - total size of tag
> - other header info
> A series of frames, each with:
> - total size of frame
> - other header info
> - frame data
>
> Since the ID3 tag as a whole has size information, I need to pass that
> into the frame-reading functions to ensure that I never read past the
> end of the total header. This exact same requirement is present within
> the frame-reading function itself; it must read the frame data without
> reading past the end of the frame.

The structure of the chunk-encoded content is similar: first comes the
size, then come data of that size. We have to read exactly that amount
(and then check for the chunk trailer, CRLF). In a sense, we have a
data stream embedded inside another stream. The Iteratee IO framework 
was specifically designed to process these sorts of arbitrarily
nested and encoded data streams. In particular, the file

	http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs

contains the function

-- Read n elements from a stream and apply the given iteratee to the
-- stream of the read elements. Unless the stream is terminated early, we
-- read exactly n elements (even if the iteratee has accepted fewer).
-- This procedure shows a different way of composing two iteratees:
-- `vertical' rather than `horizontal'
stake :: Monad m =>
	 Int -> IterateeG el m a -> IterateeGM el m (IterateeG el m a)

The implementation is 11 lines long, counting the `where' line. I
believe the code is declarative rather than imperative. The function
stake (the analogue of List.take) is then used in

enum_chunk_decoded :: Monad m => Iteratee m a -> IterateeM m a

The end of the file IterateeM.hs has several complete tests.