[Haskell-cafe] Iteratee-based IO

oleg at okmij.org oleg at okmij.org
Sat Sep 20 03:42:36 EDT 2008

brian wrote:
> I want to use Parsec to parse NNTP data coming to me from a handle I
> get from connectTo.
> One unworkable approach I tried is to get a lazy String from the
> handle with hGetContents.

It seems there is another approach, which is neither unsafe nor
imperative. It relies neither on lazy IO nor on Handles. The input
data can come from a file or from an embedded (e.g., chunk-encoded or
encrypted) stream; the depth of embedding is arbitrary. The approach
is naturally incremental. It permits IO interleaving without any
unsafe operations. The approach is algebraic and declarative. The
approach is the topic of the DEFUN08 talk in the morning of
September 27. The code is already available 

The file
describes the other files in that directory. The running example is
reading lines (terminated by CR, LF or CRLF) from a file descriptor
and then from the chunk-encoded body. The main example illustrates
multiplexing across two file descriptors and the full IO
interleaving. The same line parser is used to process data from the
file descriptor stream and from the embedded chunk-encoded stream,
which is incrementally decoded.

The whole code is Haskell98. It is not optimized at all and has no
GHC-specific pragmas and options. The code has been used for the Wc
program demonstrated yesterday.
Perhaps the code answers the questions posed yesterday by
Don. Hopefully one can see several composition modes for the iteratees
and enumerators; enumerators are just iteratee transformers and
compose as such.

Incidentally, the operator ==<< is flipped >>==. Just like =<< (which
is flipped >>=), it is like a `call-by-value application'. When a 
call-by-value language evaluates the application (f e), the argument e
and all of its effects are executed first. Because of this analogy,
I'm tempted to rename <<== into something like $. or .$ (or
perhaps <$>, although the latter is taken). 

More information about the Haskell-Cafe mailing list