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Is there a nicer way to do this?
by Michael Feathers 07 Jul '08

07 Jul '08

segment :: Int -> [a] -> [[a]] segment 0 _ = [] segment _ [] = [] segment n x = (take n x) : segment n (drop n x) I did a version of this which used splitAt but I wasn't sure whether it was going to buy me anything re performance that would justify its ugliness. Michael

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[WARN] Bug fix release of pureMD5 (Was: pureMD5)
by Thomas M. DuBuisson 06 Jul '08

06 Jul '08

Cafe, Daniel Larsson noticed a correctness issue with the pureMD5 package. This issue would affect you if you built the value incrementally via the 'updateMD5' function (vs just using 'md5') and didn't provide 512 bit long bytestrings (an MD5 block of operation). As you can probably tell, I didn't invest enough into the non-performance aspects of pureMD5. Faced with actual users ;-), I have released version 0.2.0 which has the bug fix, a new API (type prevention from re-finalizing a digest), and a reasonable set of quickchecks (covering Show / Binary instances, known answer and incremented hashing). Oh, also the module name has changed to place it inline with 'Crypto' package naming while not colliding. Sorry if this causes anyone headaches. Daniel, Good catch, I hope this didn't consume much of your time. Thanks a bunch! TomMD On Fri, 2008-06-13 at 04:14 +0200, Daniel Larsson wrote: > Hi Thomas, > > > I was fiddling around with your pureMD5 package, and encountered some > problems with using the md5Update/md5Finalize sequence. I tried to > calculate the md5 of some scattered data, but it kept returning the > wrong values. It seems that each md5Update must supply an exact > blockSize number of bits, since the mdLeftOver part isn't taken into > account in subsequent calls to md5Update. > > > I wrote a small patch, and a simple QuickCheck property, to support > calculating md5 of scattered data, attached to this mail. Hopefully I > didn't mess up something... > > > -- > Daniel

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I/O system brokenness with named pipes
by John Goerzen 06 Jul '08

06 Jul '08

So I have a need to write data to a POSIX named pipe (aka FIFO). Long story involving a command that doesn't have an option to read data from stdin, but can from a named pipe. I have created the named pipe from Haskell no problem. But I can't use writeFile to write data to it. Worse, it returns: *** Exception: /tmp/bakroller.zD0xHj/fifo: openFile: does not exist (No such device or address) which is completely false, as it *does* exist, and I can cat to it as expected. The call should block until everything is read on the remote end. I thought maybe writeFile is being weird, so I tried: openFile "/tmp/bakroller.zD0xHj/fifo" WriteMode Same thing. There is no logical reason I can see for this behavior. In fact, something must be going to *extra* effort to avoid writing to a named pipe, and I can't work out why. Named pipes are a standard, useful part of a system and shouldn't be ignored like this. Interestingly, readFile works fine on a named pipe. What's going on here? Am I going to have to resort to the System.Posix interface just to be able to write to a named pipe? -- John

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eyes again please .... on my POSIX aio code
by Galchin, Vasili 05 Jul '08

05 Jul '08

Hello, I have modified my aioError and aioReturn to hopefully be more in line with the actual aio_error and aio_return signatures, respectively. In the first implementation I had suffered a serious brain f**t ... oh well ..... The implementations below are still somewhat preliminary. Not seemingly rocket science .... I am still not getting a correct count from aioReturn .. should be 20 but I get 0! I have a C program which behaves properly giving 20 for aio_return! grrrrrr! aioReturn :: AIOCB -> IO (AIOCB, ByteCount) aioReturn aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb -- DO A PEEK HERE!!!!! -- count <- throwErrnoIfMinus1 "aioReturn" (c_aio_return p_aiocb) count <- (c_aio_return p_aiocb) putStrLn ("count -> " ++ (show count)) aiocb <- peek p_aiocb putStrLn "aioReturn" aiocb1 <- peek p_aiocb dumpAIOCB aiocb1 return (aiocb, fromIntegral count) foreign import ccall safe "aio.h aio_return" c_aio_return :: Ptr AIOCB -> IO CInt aioError :: AIOCB -> IO (Errno) aioError aiocb = do allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do poke p_aiocb aiocb --throwErrnoIfMinus1 "aioError" (c_aio_error p_aiocb) errno <- (c_aio_error p_aiocb) putStrLn ("errno -> " ++ (show errno)) putStrLn "aioError" aiocb1 <- peek p_aiocb dumpAIOCB aiocb1 return (Errno errno) foreign import ccall safe "aio.h aio_error" c_aio_error :: Ptr AIOCB -> IO CInt I added an internal helper function dumpAIOCB to print out the aiocb at points for sanity checking!! dumpAIOCB :: AIOCB -> IO () dumpAIOCB (AIOCB aioFd aioLioOpcode aioReqPrio aioOffset aioBuf aioBytes aioSigevent) = do putStrLn "aiocb dump" putStrLn ("fd => " ++ (show aioFd)) putStrLn ("opcode => " ++ (show aioLioOpcode)) putStrLn ("prio => " ++ (show aioReqPrio)) putStrLn ("offset => " ++ (show aioOffset)) -- aioBuf putStrLn ("nbytes => " ++ (show aioBytes) ++ "\n") -- aioSigevent Kind regards, Vasili

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nested parsing?
by Evan Laforge 05 Jul '08

05 Jul '08

Here's something I occasionally want to do with parsec: msg = do header body <- manyTill anyToken footer vals <- sub_parse parse_body (preprocess body) ... By coincidence I notice that perhaps the "manyTill stack overflow" thread could use something like this too. So I wrote: sub_parse :: Parsec.GenParser subtok st a -> [subtok] -> Parsec.GenParser tok st a sub_parse parser toks = do st <- Parsec.getState pos <- Parsec.getPosition case Parsec.runParser (Parsec.setPosition pos >> parser) st "" toks of Left err -> fail (show err) Right val -> return val -- oh yes, and I'd set the state to the final sub parse state too In this particular case, I found another solution that didn't need sub-parsing, so I haven't actually tested this much. In particular parsec doesn't let me throw a "raw" exception so I have to use "fail" and the error msgs look ugly. In addition, if 'preprocess' adds or removes tokens from 'body', I have to do something awkward like pass tokens around as (pos, tok). But come to think of it, I've done this pattern before, where I invoke a sub-monad and (mostly) splice it in with the current one, in one case to allow the submonad to be monomorphic on the monad type while calling monad remains polymorphic, in another case so I can run the submonad from dynamically loaded code which must be monomorphic but splice its results into the polymorphic calling monad... this is similar to the first one I guess. It seems somewhat similar to what catchError is doing.

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parsec manyTill stack overflow
by Badea Daniel 04 Jul '08

04 Jul '08

I'm trying to parse a large file looking for instructions on each line and for a section end marker but Parsec's manyTill function causes stack overflow, as you can see in the following example (I'm using ghci 6.8.3): > parse (many anyChar) "" ['a'|x<-[1..1024*64]] It almost immediately starts printing "aaaaaaaaaaa...." and runs to completion. > parse (manyTill anyChar eof) "" ['a'|x<-[1..1024*1024]] *** Exception: stack overflow I guess this happens because manyTill recursively accumulates output from the first parser and returns only when it hits the 'end' parser. Is it possible to write a version of 'manyTill' that works like 'many' returning output from 'anyChar' as soon as it advances through the list of tokens? Thanks, Daniel

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Question about abstraction
by Bas van Dijk 04 Jul '08

04 Jul '08

Dear list, I have I question about the following code I was playing with: (you can past the following right into your editor) ---------------------------------------------------------------------- import Data.Foldable (Foldable, foldMap) import Data.Monoid (mempty, mappend) import Data.Traversable (Traversable, traverse) import Control.Applicative (pure, (<$>), (<*>)) -- I was playing with the following tree-like datastructure (my plan -- is to make some kind of kd-tree but that's not important now): data T a = L | N C2 a (T a) (T a) (T a) (T a) type C2 = (Float, Float) -- A fold always comes in handy: foldT :: b -> (C2 -> a -> b -> b -> b -> b -> b) -> T a -> b foldT e _ L = e foldT e n (N c x tl tr bl br) = n c x (foldT e n tl) (foldT e n tr) (foldT e n bl) (foldT e n br) instance Functor T where fmap f = foldT L (\p -> N p . f) -- Now I defined the following instances: instance Foldable T where foldMap f = foldT mempty $ \_ x tl tr bl br -> f x `mappend` tl `mappend` tr `mappend` bl `mappend` br instance Traversable T where traverse f = foldT (pure L) $ \p x tl tr bl br -> N p <$> f x <*> tl <*> tr <*> bl <*> br ---------------------------------------------------------------------- -- If you look at the previous two functions you see a similar pattern: -- they both combine an initial value: 'f x' and 'N p <$> f x' respectively -- with the childs using a combining function: 'mappend' and '<*>' respectively. -- My question is: can I abstract from that? -- It looks like I can using a function like: combineWith :: b -> (b -> a -> b) -> a -> a -> a -> a -> b n `combineWith` f = \tl tr bl br -> n `f` tl `f` tr `f` bl `f` br -- Now 'foldMap' becomes: instance Foldable T where foldMap f = foldT mempty $ \_ x -> f x `combineWith` mappend -- But 'traverse' won't typecheck: instance Traversable T where traverse f = foldT (pure L) $ \p x -> (N p <$> f x) `combineWith` (<*>) -- Is it possible to make 'combineWith' more general so that the -- previous typechecks (maybe using arbitrary-rank polymorphism but I -- don't see how)? ---------------------------------------------------------------------- Thanks, Bas van Dijk

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Question concerning datatype "Either"
by phy51km4n 04 Jul '08

04 Jul '08

There is a exercise using datatype "Either" I'm a bit confused about... The following datatypes are given: data Either a b = Left a | Right b data Tuple a b c d = One a | Two a b | Three a b c | Four a b c d Now the exercise: "Based on our definition of Tuple, write a function which takes a Tuple and returns either the value (if it’s a one-tuple), a Haskell-pair (i.e., (’a’,5)) if it’s a two-tuple, a Haskell-triple if it’s a three-tuple or a Haskell-quadruple if it’s a four-tuple. You will need to use the Either type to represent this." Why does that not work? : fromTuple (One a) = a fromTuple (Two a b) = (a, b) fromTuple (Three a b c) = (a, b, c) fromTuple (Four a b c d) = (a, b, c, d) Why is this correct? : fromTuple (One a ) = Left (Left a) fromTuple (Two a b ) = Left (Right (a,b)) fromTuple (Three a b c ) = Right (Left (a,b,c)) fromTuple (Four a b c d) = Right (Right (a,b,c,d)) Why does this combination of Rights and Lefts work and how does it work?? -- View this message in context: http://www.nabble.com/Question-concerning-datatype-%22Either%22-tp18264739p… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Alternatives to convoluted record syntax
by Dougal Stanton 04 Jul '08

04 Jul '08

Hi Haskellers, This is a style question, as my program works fine but looks really ugly and can be confusing to maintain. I take options from the command line using GetOpt and hand back this structure for later use. > data Opts = Opts > { query :: Query > , queryLimit :: Maybe Limit > , disabledOnly :: Bool > } deriving Show Here's a snippet from the parser for one option (others omitted for clarity): > options :: [OptDescr (Opts -> Opts)] > options = > [ Option "b" ["bus"] (ReqArg busNum "NUM") "Bus number" > , ... > ] > where busNum n os = let b = (query os) { queryBusNumber = Just n } > in if isBusId n then os { query = b } else os Variations on that ugliness are repeated four times for other fields. Is there an alternative way to change the value of nested fields? Thanks, Dougal. -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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Integer = infinite precision integer? How?
by leledumbo 04 Jul '08

04 Jul '08

Does anyone have an explanation how Haskell implement this? Or a pointer to a article describing this? -- View this message in context: http://www.nabble.com/Integer-%3D-infinite-precision-integer--How--tp182738… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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