There are co-routine packages on Hackage, but I have not had much luck with them. The point here, really, is to show you how it all works.

What’s a co-routine?

A co-routine (called a ‘generator’ in Python) is where you create two interleaved flows of control on a single thread. Unlike threads, co-routines switch co-operatively using a ‘yield’ operation. (This is quite a good trick in GUI programming for implementing complex workflow that spans multiple GUI events, since most GUI libraries require everything to be on one thread.)

The example I’m presenting here works in this way: A caller executes a CoroutineT monad transformer, which adds the ‘yield’ operation to the underlying monad (which can be anything). From the caller’s point of view, the ‘yield’ looks like the monad has returned, but with a continuation. In the callee, ‘yield’ appears to block until the caller executes the continuation. In addition, we add the ability to pass a value in both directions.

So we’ve inverted the flow of control in the callee. Continuation passing style (CPS) can also do this, but co-routines are better than CPS because 1. it’s a bit neater, and 2. it allows for recursion.

One application of co-routines is to separate I/O from logic. By way of example I am going to implement an expert system for identifying fruit. I try not to use contrived examples, and as you can see, this time I have completely failed.

In this example, the CoroutineT sits on top of the identity monad, so it’s pure, but the approach works just the same on top of IO or anything else. This example is not deeply nested, but this approach happily supports any level of recursion or nesting.

So here’s our expert system logic. We’ll define CoroutineT shortly. You can read ‘yield’ as ‘askUser’:

type Question = String
data Answer = Y | N deriving Eq

type Expert a = CoroutineT Answer Question Identity a

data Fruit
    = Apple
    | Kiwifruit
    | Banana
    | Orange
    | Lemon
    deriving Show

identifyFruit :: Expert Fruit
identifyFruit = do
    yellow <- yield "Is it yellow?"
    if yellow == Y then do
        long <- yield "Is it long?"
        if long == Y then
            return Banana
          else
            return Lemon
      else do
        orange <- yield "Is it orange?"
        if orange == Y then
           return Orange
         else do
           fuzzy <- yield "Is it fuzzy?"
           if fuzzy == Y then
               return Kiwifruit
             else
               return Apple

Our ‘Expert’ type above…

type Expert a = CoroutineT Answer Question Identity a

…specifies the type we are sending into our co-routine (Answer) and the type we are getting out of it (Question) as viewed from the caller.

Now we just need a main program to drive it. Because the I/O is separated out, we can later replace this with a nice touch-screen GUI for the seriously fruit-impaired.

main :: IO ()
main = do
    putStrLn $ "Expert system for identifying fruit"
    run identifyFruit
  where
    run :: Expert Fruit -> IO ()
    run exp = handle $ runIdentity $ runCoroutineT exp

    handle (Yield q cont) = do
        putStrLn q
        l <- getLine
        case map toLower l of
            "y"   -> run $ cont Y
            "yes" -> run $ cont Y
            "n"   -> run $ cont N
            "no"  -> run $ cont N
            _   -> putStrLn "Please answer 'yes' or 'no'" >> handle (Yield q cont)

    handle (Result fruit) = do
        putStrLn $ "The fruit you have is: "++show fruit

When we run our co-routine, it returns with one of these two events:

• Yield, which happens when yield is executed. It gives us the output value (Question) and the continuation, which when passed the input value (Answer) gives us the same ‘Expert’ type we started with.
• Result, which happens when the co-routine has finished executing.

So how does CoroutineT work?

We’ll start with the types:

data Result i o m a = Yield o (i -> CoroutineT i o m a) | Result a

-- | Co-routine monad transformer
--
--   * i = input value returned by yield
--
--   * o = output value, passed to yield
--
--   * m = next monad in stack
--
--   * a = monad return value
data CoroutineT i o m a = CoroutineT {
        runCoroutineT :: m (Result i o m a)
    }

Hopefully that’s pretty straightforward. ‘yield’ is defined like this:

-- | Suspend processing, returning a @o@ value and a continuation to the caller
yield :: Monad m => o -> CoroutineT i o m i
yield o = CoroutineT $ return $ Yield o (\i -> CoroutineT $ return $ Result i)

The key point here is that the continuation does nothing except return the value, which is what we want it to do when we run a monad that contains only a yield.

Most of the magic is in the definition of >>=, thus:

instance Monad m => Monad (CoroutineT i o m) where
    return a = CoroutineT $ return $ Result a
    f >>= g = CoroutineT $ do
        res1 <- runCoroutineT f
        case res1 of
            Yield o c -> return $ Yield o (\i -> c i >>= g)
            Result a  -> runCoroutineT (g a)
    -- Pass fail to next monad in the stack
    fail err = CoroutineT $ fail err

A typical monad would normally execute f then pass its result to g and execute that, and this is in fact exactly what we do in the Result case. Ho hum.

But there’s no law that says you have to execute g. This is Haskell so we can do whatever we like. g is just a plain old closure representing the continuation.

So what we do in the Yield case is take the continuation that executing f gave us, and bind that to the continuation g, then bail out of the monad (in the same way ErrorT does when it gets an error), handing our constructed continuation to the caller. So we end up with a closure that represents the entire execution state of the monad, and it doesn’t matter how deeply nested we are. It just puts the continuation together in the right way as we unravel everything on our way back to the caller.

Here’s the code in downloadable form:

• expert.hs
• Coroutine.hs

This code is released in the public domain.

Stephen Blackheath, Manawatu, New Zealand

Day 2 of AusHac2010 is coming to an end, and we’ve made a lot of progress:

• Bernie Pope has been making great progress with a new MPI binding for Haskell

• Ben Lippmeier, Erik de Castro Lopo and Ben Sinclair have been busily hacking on DDC, with 13 commits today alone

• Stephen Blackheath has been working on some code using the Accelerate library that rasterises triangles for use in a commercial computer game.

• Hamish Mackenzie, Jens Petersen and Matthew Sellers have been working on better Yi integration for Leksah, working on using Yi’s current configuration file, and improving “launch experience”, focusing on eliminating the requirement of creating an initial workspace file.

• Lang Hames has been using his experience with LLVM from working at Apple as an intern to improve various low level problems in LLVM. His work should help resolve some of the problems the LLVM backend to GHC has, but should also be very beneficial to many other LLVM users. While doing this, he’s written a very nice tool that illustrates register liveness, with further work focusing on colouring the HTML output to show register pressure. The LLVM guys seem quite excited about this work, which is great.

• Mark Wotton and Sohum Banerjea have been trying to extend Hubris, the Haskell-Ruby bridge, to work with polymorphic functions. Their heads are quite sore from all the head banging. Raphael Speyer has been working on an install script to make installation much easier for users.. but only if you use Ubuntu so far.

• Ivan Miljenovic has been prematurely optimising his containers library, before finalising the API. This library is designed to let library writers leave the choice of which container data structure to output to the library consumer as well as making it easier to change which data structure you want to use in your code, with minimal code change. See his blog post for more details.

• Trevor McDonell has been working on the CUDA backend to Accelerate, adding support for efficient nested tuple types, and other bug fixes. Sean Lee has been helping out with testing of this code, along with Manuel Chakravarty.

With one more full day to go, I think we’ll be getting a lot of awesome work done tomorrow!

Erik de Castro Lopo did a lot of work on Ben Lippmeier’s DDC compiler for his Disciple language.

There was some initial work on the Accelerate library for accelerated array computations in Haskell, using various backends. Most of the current work is aiming at making the CUDA backend usable, after which more backends will likely be added, such as an LLVM backend, and possibly an OpenCL backend as well.

Due to the restricted time yesterday, not all that much work was started, but day 2 (see my next post!) has been much more productive.

It’s rather unfashionable on #haskell, but I like XML.  So, 18 months ago, I took over the hexpat package from Evan Martin.  It was going to be a small project – a simple XML parser binding to Expat.  The fastest Haskell XML parser alive.  Or so I thought.

It’s become a passion, a way of life.  It’s XML parsing in Haskell the way I think it should be done.  The best as well as the fastest.  (I like to think big.)

I’ve finally finished adding all the features that I and a number of contributors wanted, and I would now like to announce that hexpat is going beta.  I want to make this package really, really good, so please help by testing and critiquing.  I want to stabilize hexpat, but hexpat-iteratee will be unstable for a while yet.

The future is chunky

The cherry on top of the hexpat galaxy is the still experimental hexpat-iteratee based on Oleg Kiselyov’s iteratee, which is a bit of a hot ticket these days.  It provides lazy XML parsing without the practical issues and philosophical dodginess inherent in Haskell’s lazy I/O through functions like hGetContents.

hexpat-iteratee allows for effectful XML processing done in a functional way, and the magic behind this is Yair Chuchem’s humbly named List package.  It is “merely” a generalization of lists, and I think it deserves to be a common piece of infrastructure.

The example project is a chunked XML-over-TCP movie database lookup server.  Every home should have one.  So, let’s start like all good blogs do, with imports:

{-# LANGUAGE OverloadedStrings #-}
import Control.Concurrent
import Control.Exception
import Control.Monad
import Control.Monad.IO.Class
import Control.Monad.ListT
import qualified Data.ByteString as B
import qualified Data.ByteString.Unsafe as B (unsafeUseAsCStringLen)
import Data.Iteratee
import Data.Iteratee.IO.Fd
import Data.Iteratee.WrappedByteString
import Data.List.Class as List
import Data.Maybe
import Data.Text (Text)
import qualified Data.Text as T
import Network
import System.IO
import System.Posix.IO (handleToFd, fdWriteBuf, closeFd)
import System.Posix.Types (Fd)
import Text.XML.Expat.Chunked
import qualified Text.XML.Expat.Chunked as Tree
import Text.XML.Expat.Format
import Foreign.Ptr

main :: IO ()
main = do
    let port = 6333
    putStrLn $ "listening on port "++show port
    ls <- listenOn $ PortNumber port
    forever $ do
        (h, _, _) <- accept ls
        forkIO $ handleToFd h >>= \fd -> do
            iter <- parse defaultParserOptions (session (fdPutStrBS fd))
            result <- enumFd fd iter >>= run
            print result
          `finally`
            closeFd fd


fdPutStrBS :: Fd -> B.ByteString -> IO ()
fdPutStrBS fd bs = B.unsafeUseAsCStringLen bs $ \(buf, len) ->
        writeFully (castPtr buf) (fromIntegral len)
  where
    writeFully _ len | len == 0 = return ()
    writeFully buf len = do
        written <- fdWriteBuf fd buf len
        if written < 0
            then fail "write failed"
            else writeFully (buf `plusPtr` fromIntegral written) (len - written)

The handler is a co-routine.  When it runs out of input data, it gets suspended, and control returns to enumFd.

session :: (B.ByteString -> IO ())  -- ^ Write output data to socket
        -> ListOf (UNode IO Text)   -- ^ Input XML document
        -> XMLT IO ()
session writeOut inputXML = do
    let outputXML = formatG $ indent 2 $ Element "server" [] (processRoot inputXML)
    execute $ liftIO . writeOut =<< outputXML
    return ()

execute here makes all the IO actually happen. It iterates over a List of monadic actions and sequences them. This translates into a sequence of writes of data blocks to the socket.  The elements in the list are monadic, so execute also must execute those in order to extract each output ByteString.

In this way, even though processRoot is pure at the top level, it can contain effectful computations.

processRoot :: ListOf (UNode IO Text) -> ListOf (UNode IO Text)
processRoot root = do
    Element _ _ children <- genericTake 1 root
    child <- children
    extractElements child
  where
    extractElements :: UNode IO Text -> ListOf (UNode IO Text)
    extractElements elt | isElement elt = processCommand elt `cons` mzero
    extractElements _                   = mzero

The root of the input document is actually given as a List containing one item – the top-level XML tag.  The reason why we do this is so that we have to ask for it to be pulled.  If it were just passed as a UNode IO Text type, we would have to calculate it before the handler was called, and the handler wouldn’t get a chance to do output before it requests input.

The function is implemented using List’s Monad instance, which behaves exactly like a list monad.  The reason for genericTake 1 root is so we stop processing after the root node and don’t wait for a node that will never come.  I need to fix this in hexpat-iteratee.

`cons` is the generalized list cons operator like : and  `mzero` corresponds to [].

processCommand :: UNode IO Text -> UNode IO Text
processCommand elt@(Element "title" _ _) = Element "title" [] $ joinL $ do
    txt <- textContentM elt
    return $ search txt
processCommand (Element cmd _ _) = Element "unknown" [("command", cmd)] mzero

joinL is a bit of List magic that lets you drop down into the underlying monad, which in this case is XMLT IO a.  joinL’s type is :: ItemM l (l a) -> l a where ItemM l is a type function giving the list’s monad.  So, the stuff after joinL resolves to a type of :: XMLT IO (ListOf (UNode IO Text)).

search :: Text -> ListOf (UNode IO Text)
search key = joinL $ do
    iter <- liftIO $ parse defaultParserOptions $ \root -> do
        let l = do
                elt@(Element _ _ children) <- genericTake 1 root
                movie <- List.filter isElement children
                return movie
        execute l
        return l
    eMovies <- liftIO $ fileDriver iter "movies.xml"
    case eMovies of
        Left err -> fail $ "failed to read 'movies.xml': "++show err
        Right movies -> return $ List.filter matches movies
  where
    matches elt = key `T.isInfixOf` fromMaybe "" (getAttribute elt "title")

So, I build and run the server, and here is the result, using Unix’s nc command as my client.  I typed this:

<a>
<title>of the</title>

<?xml version="1.0" encoding="UTF-8"?>
<server>
 <title>
   <movie id="dvzrwfvryd" disc="41" title="War of the Worlds (2005)"
        director="Steven Spielberg" genre="Sci Fi Thriller" rating="6"
        description="Tom Cruise alert" imdbID="tt0407304"/>
   <movie id="xxvjgxpokp" disc="44" title="Shaun of the Dead"
        director="Edgar Wright" genre="Comedy Horror" rating="8"
        description="British send-up zombie movie" imdbID="tt0365748"/>
   <movie id="duvcjsygqi" disc="104" title="March of the Penguins (La Marche de l&apos;empereur)"
        director="Luc Jacquet" genre="Documentary" description="" imdbID="tt0428803"/>
   <movie id="dawcezoiro" disc="109" title="Pirates of the Caribbean: Dead Man&apos;s Chest"
        director="Gore Verbinski" genre="Action/Comedy" rating="7" description="" imdbID="tt0383574"/>
 </title>

And the session can process more commands interactively.

And pickled

I should also mention my related hexpat-pickle package which is a shameless rip-off of the picklers from Uwe Schmidt’s excellent hxt package.  I find it a very practical and quick way to bang out XML picklers.  (It doesn’t work with hexpat-iteratee yet.)

Bye bye

Here’s the code in downloadable form.  Make sure you use the monads-fd and transformers packages instead of mtl.  Also hexpat-iteratee and text.

I hope you found this interesting.  I hope the XML haters of #haskell will be miraculously transformed into XML tolerators, and I hope you’ll help me improve hexpat. – Stephen Blackheath, Manawatu, New Zealand

• The LLVM backend to GHC
• the Accelerate, a Haskell EDSL for regular array computations, using various backends (CUDA, OpenCL, LLVM etc.)
• Hubris, the Haskell-Ruby binding
• Leksah, the Haskell IDE written in Haskell
• MPI bindings

then please put your name down on the sign up page.

This should be a great opportunity for Aussie (and non aussie!) haskell hackers to come and meet all those people you know from Planet Haskell and #haskell, and give something back to the community, while having a great time.

Hope to see you there, – Alex Mason

Hi, thanks for writing this stuff. I think it could be pretty cool, but it could benefit from more precise reading and implementation of the spec.

For example, bencoded integers can be negative.

Also, my alarms go off whenever I see ‘read’. In ‘getBString’, you pass ‘read count’ to ‘getByteString’, which expects Int. But check, e.g., ‘read (show $ 2^64-1) :: Int’ in ghci. So if the torrent data is malformed, you could end up passing a negative length to ‘getByteString’. Maybe it knows how to deal with that, but it’s not something you should rely on.

You also have to decide what to do about dictionaries you read whose keys aren’t in order, etc.

Basically, please be more precise, especially if you put this on Hackage. This stuff is supposed to be industrial strength. Thanks.

The first problem, not handling negative integers was pretty trivial to fix, all I needed to do was check to see if there was a ‘-’ char out the front, and if not, just get all the digits, and then read them:

-- | Parses a BInt
getBInt :: Get BCode
getBInt = BInt . read <$> getWrapped 'i' 'e' intP
    where intP = ((:) <$> char '-' <*> getDigits) <|> getDigits

Brian also pointed out something I also wasn’t particularly happy with, the use of read to read in an Int64. This should under normal circumstances be more than large enough to read any bytestring that should be in any bencoded data (.torrent files are usually less than 1-200KB), so we should never have run into a problem here, but it’s still good to make sure we can be ‘industrial strength’:

-- | Parses a BString
getBString :: Get BCode
getBString = do
    count <- getDigits
    BString <$> ( char ':' *> getStr (read count :: Integer))
    where maxInt = fromIntegral (maxBound :: Int) :: Integer
          getStr n | n >= 0 = B.concat <$> (sequence $ getStr' n)
                   | otherwise = fail $ "read a negative length string, length: " ++ show n
          getStr' n | n > maxInt = getByteString maxBound : getStr' (n-maxInt)
                    | otherwise = [getByteString . fromIntegral $ n]

Here you can see we’re now using an Integer as the read value, and taking chunks of maxBound :: Int bytes, until there are less than that many bytes left to fetch.

I’ve decided to ignore the problem with dictionaries with out of order elements, I can see this being something others may have overlooked in their implementations, and it’s entirely possible that other implementations do not put the keys in the right order. Our implementation does, but can easily handle malformed implementations. I see this is a bonus, and I hope others do too (I feel the code is more robust, and that’s always good).

I hope this has made some difference to the code, and what people think of it.

Until next time,

– Axman

I’ve been playing around with cereal lately in jlouis’ haskell-torrent project, rewriting the various binary parsing and producing parts of the program (the torrent file parser, and the wire protocol parser). I though it would be nice to share some of the code used for these, to demonstrate how easy cereal makes it to do such things.

To begin with, I’ll show you the part that decodes and encodes torrent files (if needed in the future). Torrent files are encoded using a very simple encoding, known as bencoding, which consists of four major primitives: Integral numbers, Strings of bytes, Arrays of bencoded objects, and Dictionaries of String, bencoded object pairs. This is very nicely represented using this datatype:

-- | BCode represents the structure of a bencoded file
data BCode = BInt Integer                       -- ^ An integer
           | BString B.ByteString               -- ^ A string of bytes
           | BArray [BCode]                     -- ^ An array
           | BDict (M.Map B.ByteString BCode)   -- ^ A key, value map
  deriving Show

the specification for bencoded data goes something like this:

Integers are encoded as the ASCII character for ‘i’ as a byte, followed by the digits of the integral value, terminated by the ASCII byte for ‘e’.

Eg: the number ‘42’ would be encoded as “i42e”

Strings are encoded as the digits of their length, followed by a colon (‘:’), then the bytes of the string. these strings are really just byte sequences, and probably shouldn’t be treated as having an encoding (as jlouis and I found out when I tried to test the current code on GHC 6.12.1, with the BString type using Strings, instead of ByteStrings, and finding out that the simple test contained byte sequences that could not be represented as Strings).

Eg: the string “hello” would become “5:hello”, “hello world” would become “11:hello world”

Arrays are encoded as ASCII ‘l’ (for list I believe), followed by any number of bencoded objects, terminated by an ASCII ‘e’. (This is where using binary became difficult, as you had to explicitly check whether you had reached the terminating ‘e’ using lookAhead when parsing before attempting to parse another bencoded object, du the the lack of actual failure handling)

Eg: ["Hello", 123] would become “l5:helloi123ee”. Notice how we’ve used the previous definitions for integral numbers, and strings.

Dictionaries are encoded as an ASCII ‘d’, followed by the String, object pairs, followed by an ASCII ‘e’.

Eg: fromList [("test",123),("arr",[1,2,"hello"])] would become “d4:testi123e3:arrli1ei2e5:helloee”.

It looks a bit of a mess, but it is quite efficient.

Encoding

When writing my Serialize instance (Cereal’s version of the Binary class) for the BCode type, I decided it would be much easier to write the put methods first. This turned out to be rather straight forward, once I’d written a few helper functions.

toW8 :: Char -> Word8
toW8 = fromIntegral . ord


fromW8 :: Word8 -> Char
fromW8 = chr . fromIntegral

toBS :: String -> B.ByteString
toBS = B.pack . map toW8

fromBS :: B.ByteString -> String
fromBS = map fromW8 . B.unpack

-- | Put an element, wrapped by two characters
wrap :: Char -> Char -> Put -> Put
wrap a b m = do
    putWord8 (toW8 a)
    m
    putWord8 (toW8 b)

-- | Put something as it is shown using @show@
putShow :: Show a => a -> Put
putShow x = mapM_ put (show x)

With these in hand, I set to work implementing the put function. The Integer and Array functions were straight forward:

instance Serialize BCode where
     put (BInt i)     = wrap 'i' 'e' $ putShow i
     put (BArray arr) = wrap 'l' 'e' . mapM_ put $ arr
 

The Dictionary and String implementations weren’t too bad either:

    put (BDict mp)   = wrap 'd' 'e' dict
                      where dict = mapM_ encPair . M.toList $ mp
                            encPair (k, v) = put (BString k) >> put v
     put (BString s)  = do
                          putShow (B.length s)
                          putWord8 (toW8 ':')
                          putByteString s

As you can see, the code is quite clear, and matches the specification quite well.

Decoding

Parsing the data was the next step. this proved a little more difficult, but with my recent (shallow) experience with Parsec, I knew what was needed.

I decided to start by writing some useful combinators (this is a lie, I wrote them when needed, but lying makes the post flow better >_>). These included the following:

-- | Get a Char. Only works with single byte characters
 getCharG :: Get Char
 getCharG = fromW8 <$> getWord8


-- | Parse a given character
 char :: Char -> Get ()
 char c = do
     x <- getCharG
     if x == c
         then return ()
         else fail $ "Expected char: '" ++ c:"' got: '" ++ [fromW8 x,'\'']

-- | Get something wrapped in two Chars
 getWrapped :: Char -> Char -> Get a -> Get a
 getWrapped a b p = char a > p < char b
                  -- The same as char a >> p >>= \x -> char b >> return x

-- | Parse zero or items using a given parser
 many :: Get a -> Get [a]
 many p = many1 p mplus return []

-- | Parse one or more items using a given parser
 many1 :: Get a -> Get [a]
 many1 p = (:) <$> p <*> many p

-- | Returns a character if it is a digit, fails otherwise. uses isDigit.
 digit :: Get Char
 digit = do
     x <- getCharG
     if isDigit x
         then return x
         else fail $ "Expected digit, got: " ++ show x

-- | Get one or more digit characters
 getDigits :: Get String
 getDigits = many1 digit

My favourite two definitions here are many and many1, which nicely show the use of Alternative: they are mutually recursive, with many1 being the only one of the two to actually do and parsing, while many checks to see if many1 failed to parse one object using the parser p. It’s really quite beautiful, and makes the code that follows a hell of a lot nicer to write. This is where the love mentioned in the title comes in by the way.

With these in hand, I could now go ahead and write the actual parsers for various BCode types. Parsing BInts and BArrays is dead simple now:

-- | Parses a BInt
 getBInt :: Get BCode
 getBInt = BInt . read <$> getWrapped 'i' 'e' getDigits


-- | Parses a BArray
 getBArray :: Get BCode
 getBArray = BArray <$> getWrapped 'l' 'e' (many get)

As as side note, I’ve now come to see just what the folks on #haskell were on about when they said Applicative is nice. I think I’ve fallen in love (yet again!).

BStrings were a little more difficult, but not hard, given what I’ve just written:

-- | Parses a BString
 getBString :: Get BCode
 getBString = do
     count <- getDigits
     BString <$> ( char ':' *> getByteString (read count))

Here we get as many digits as we can, followed by a colon, and then take the number of bytes the digits specified. Finally, we have the BDict definition, which also is quite nice, if slightly annoying with its use of pattern matching (don’t get me wrong, i love pattern matching, but it’s the only place it’s used in the parser )

-- | Parses a BDict
 getBDict :: Get BCode
 getBDict = BDict . M.fromList <$> getWrapped 'd' 'e' (many getPairs)
     where getPairs = do
             (BString s) <- getBString
             x <- get
             return (s,x)

Putting it all together, we finally have a definition for the get function in the Serialize class.

    get = getBInt <|> getBArray <|> getBDict <|> getBString
 

Please, I implore you, do let me know what you think of this all, I’m always interested in seeing what others think of my code, and ways to improve it.

Until next time,

— Axman

After trying to compile the code using GHC 6.12.1, I noticed that a rather strange dependency: HCodecs. Investigating further, I realised that all that was being used was its fairly nice binary builder and parser, and was being used in the parsing and construction of bencoded files. This seemed to me to be the domain of one of my favourite packages, binary. I also noticed that when trying to parse the sample torrent file jlouis had provided, there was binary data that could not be represented in a String type, because there were byte sequences that could not be parses into Char’s.

So I went off to try and implement the BCode module using Data.Binary. The encoding was really simple and straight forward, but parsing was another matter. Binary has no real error handeling, which meant that decoding things like lists where you keep adding elements to the list until you reach an ‘e’, was made quite verbose due to this fact. After implementing a (semi) working module, I went in search of a more parsec like interface to binary parsing. This is when someone on #Haskell informed me of cereal, a package based on binary, which added an Alternative instance, and an Either String a return type for better error reporting.

I fell in love. This was exactly what I was after, I could create my own combinators like many and many1, with the same ease I could with parsec. Using cereal, I was able to make a really clear, concise parser, which should also be quite efficient.

I will post some of this code at a later time, when i’m not writing on my iPhone, and show you just what I mean. I’m also considering releasing the BCode module as a package of it’s own, if anyone else feels it might me useful to them. Either that, or I might write up a module of useful combinators for use with cereal.

Until next time, happy new year and I hope this year treats you all better than the last one did.

– Al

The HTML type is now

data HTML a = Doctype String String String (HTML a)
            | Content a
            | Text String
            | Node Tag Attrs [HTML a]
            | NoContent Tag Attrs
            | Comment String
            deriving Show

instead of

data HTML a = Doctype String String String [HTML a]
            | Content a
            | Text String
            | Node Tag Attrs [HTML a]
            | NoContent Tag Attrs
            | Comment String
            deriving Show

I’ve also added some more helper functions:

addAttrs :: Attrs -> HTML a -> HTML a
addAttrs attrs (Node tag attrs' rest) = Node tag (attrs' ++ attrs) rest
addAttrs attrs (NoContent tag attrs') = NoContent tag (attrs'++attrs)
addAttrs _ x = x


toTable :: [[String]] -> HTML a
toTable xss = table . map (tr . map (\x -> td [text x])) $ xss

toTable' :: Show a => [[a]] -> HTML b
toTable' = toTable . map (map show)

And updated my example:

example :: HTML ()
example =
    doctype "html PUBLIC"
            "-//W3C//DTD XHTML 1.0 Transitional//EN"
            "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" $
        xhtml [
            header [
                title "My random page",
                meta "name" "Alex"
            ],
            body [
                comment "This is a comment",
                para [
                    text "The hr tag defines a horizontal rule:"
                ],
                hr,
                para [
                    text "This is a paragraph"
                ],
                hr,
                para [
                    text "This is a paragraph"
                ],
                addAttrs [("border","1px")] . toTable' $ [[0..n] | n <- [0..3]]
            ]
        ]

Which produces this html:

    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
       "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
        <head >
            <title >
                My random page
            </title>
            <meta name="name" content="Alex"/>
        </head>
        <body >
            <!-- This is a comment -->
            <p >
                The hr tag defines a horizontal rule:
            </p>
            <hr />
            <p >
                This is a paragraph
            </p>
            <hr />
            <p >
                This is a paragraph
            </p>
            <table border="1px">
                <tr >
                    <td >
                        0
                    </td>
                </tr>
                <tr >
                    <td >
                        0
                    </td>
                    <td >
                        1
                    </td>
                </tr>
                <tr >
                    <td >
                        0
                    </td>
                    <td >
                        1
                    </td>
                    <td >
                        2
                    </td>
                </tr>
                <tr >
                    <td >
                        0
                    </td>
                    <td >
                        1
                    </td>
                    <td >
                        2
                    </td>
                    <td >
                        3
                    </td>
                </tr>
            </table>
        </body>
    </html>

The main type behind the package is:

data HTML a = Doctype String String String [HTML a]
            | Content a
            | Text String
            | Node Tag Attrs [HTML a]
            | NoContent Tag Attrs
            | Comment String
            deriving Show

where

type Attr = (String, String)
type Attrs = [Attr]
data Tag = Html
         | ARef
         | BlockQuote
         | Body
         ...
         | Title
         | TR
         | UL

I’ve also provided helper functions to make this an actual DSL (I guess?):

example :: HTML ()
example = 
    doctype "html PUBLIC" "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [
        xhtml [
            header [
                title "My random page",
                meta "name" "Alex"
            ],
            body [
                comment "This is a comment",
                para [
                    text "The hr tag defines a horizontal rule:"
                ],
                hr,
                para [
                    text "This is a paragraph"
                ],
                hr,
                para [
                    text "This is a paragraph"
                ],
                table [
                    tr [
                        td [
                            text "data"
                        ],
                        td [
                            text "more data"
                        ]
                    ],
                    tr [
                        td [
                            text "more data"
                        ]
                    ]
                ]
            ]
        ]
    ]

produces:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
    <head >
        <title >
            My random page
        </title>
        <meta name="name" content="Alex"/>
    </head>
    <body >
        <!-- This is a comment -->
        <p >
            The hr tag defines a horizontal rule:
        </p>
        <hr />
        <p >
            This is a paragraph
        </p>
        <hr />
        <p >
            This is a paragraph
        </p>
        <table >
            <tr >
                <td >
                    data
                </td>
                <td >
                    more data
                </td>
            </tr>
            <tr >
                <td >
                    more data
                </td>
            </tr>
        </table>
    </body>
</html>

I don’t claim that anyone should use this (there’s a good chance I won’t be releasing it, unless someone really wants me to), but I’m still quite proud of it.