[Haskell-cafe] Brainstorming on how to parse IMAP

[Malcolm Wallace]

> 1) Ideally I could parse stuff lazily.  I have tried this with FTP and
> it is more complex than it seems at first, due to making sure you
> never, never, never consume too much data.

PolyParse has lazy variants of its combinators, which would probably be
of use here.

Software:
    http://hackage.haskell.org/cgi-bin/hackage-scripts/package/polyparse
Paper:
    http://www-users.cs.york.ac.uk/~malcolm/partialparse.html

> 2) Avoiding Strings wherever possible.

PolyParse is completely independent of token type.  There is a
rudimentary module implementing the combinators on top of ByteString.
(Not currently exposed in the package interface, BTW.) I'm sure it would
not be difficult to build on top of LazyByteString also.  Patches
gratefully accepted.

> 3) Avoiding complex buffering schemes where I have to manually buffer
> data packets.

Provided the parser is lazy enough, the buffering would be done at the
level of Lazy ByteString (but I don't know whether LBS is suitable for
network-reading or not).

> Thoughts and ideas?

> A283 SEARCH {4} {21}
> TEXTstring not in mailbox

Assuming the first line can be read strictly, and the remainder should
be lazy, the parser might look something like this:

  searchResult = do n <- qnumber
                    word "SEARCH"
                    extents <- many1 (braces number)
                    return SearchResult n `apply`
                           (mapAF (\m-> exactly m char) extents)
    where
      -- embed map in an applicative functor
      mapAF :: (a -> Parser b) -> [a] -> Parser [b]
      mapAF p []     = return []
      mapAF p (x:xs) = return (:) `apply` p x `apply` mapAF p xs

Regards,
    Malcolm