On 3/27/11 11:38 AM, John A. De Goes wrote:
Enumeratees solve some use cases but not others. Let's say you want to incrementally compress a 2 GB file. If you use an enumeratee to do this, your "transformer" iteratee has to do IO. I'd prefer an abstraction to incrementally and purely produce the output from a stream of input.
I don't see why? In pseudocode we could have, enumRead2GBFile :: FilePath -> Enumerator IO ByteString enumRead2GBFile file iter0 = do fd <- open file let loop iter = do mline <- read fd case mline of Nothing -> return iter Just line -> do iter' <- feed iter line if isDone iter' then return iter' else loop iter' iterF <- loop iter0 close fd return iterF compress :: Monad m => Enumeratee m ByteString ByteString compress = go state0 where go state = do chunk <- get let (state',hash) = compressify state chunk put hash go state' compressify :: Foo -> ByteString -> (Foo,ByteString) it's just a pipeline like function composition or shell pipes. There's no reason intermediate points of the pipeline have do anything impure. -- Live well, ~wren