Edward's reply was quite good. I'll just try to fill in a few items he didn't address.
From: Maciej Piechotka
I read a lot about iteratee IO and it seemed very interesting (Unfortunately it lacks tutorial). Especially features like 'no input yet' in network programming (with lazy IO + parsec I run into problems as it tried to evaluate the first response character before sending output).
I decided first write a simple program and then attempt write a Stream implementation for parsec.
{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} import Data.Iteratee import Text.Parse
data Buffer a = Buffer instance Monad m => Stream (Buffer a) (IterateeG [] a m) a where uncons Buffer = IterateeG loop where loop (Chunk []) = return $! Cont (IterateeG loop) Nothing loop (Chunk (x:xs)) = return $! Done (Just (x, Buffer)) (Chunk xs) loop (EOF Nothing) = return $! Done Nothing (EOF Nothing) loop (EOF (Just e)) = return $! throwErr e
2. Is there any package which contains this stream definition?
I believe this Stream instance is incorrect. According to the parsec-3.0.1 docs, "A Stream instance is responsible for maintaining the 'position within the stream' in the stream state s. This is trivial unless you are using the monad in a non-trivial way." This is necessary for referential integrity. That is, "uncons s" needs to always evaluate to the same result for the same 's'. Your Stream instance doesn't preserve this. As an example,
testIter = let b = Buffer in uncons b >> uncons b
should be an iteratee that returns a "Maybe (t, buffer)" where t is the first element in the enumeration, but with your instance it will return the second. See http://inmachina.net/~jwlato/haskell/ParsecIteratee.hs for a valid Stream instance using iteratee. Also Gregory Collins recently posted an iteratee wrapper for Attoparsec to haskell-cafe. To my knowledge these are not yet in any packages, but hackage is vast.
3. Why Seek FileOffset is error message?
Version 3 of iteratee is somewhat experimental, one of the ideas on trial is that of resumable exceptions. This framework is perfectly suited to handle control messages as well, which is why Seek is included as an error message. I don't want to make a major release just to fix this, but both error handling and control messages will undergo a substantial cleanup in the next major version. Included in this will be a proper separation between control messages and true exceptions, most likely based upon the extensible-exceptions framework. Cheers, John