June 2009 - Haskell-Cafe

Conversion from string to array
by John Ky 06 Jun '09

06 Jun '09

Hi Haskell Cafe, I'm trying to send stuff over UDP. To do that, I've written a test program that sends strings across. That was fine, but I wanted to send binary data too. So I tried that only to find I am having difficulty putting together some binary data. For examples take the fromHex function in the following code. It is supposed to convert from a Hexadecimal string to a list of bytes, but I am having trouble coercing the integer types to the size I want. Is this the right way to do it? Cheers, -John > import Data.Bits > import Data.Char > import Data.Word > import System.Environment > import XStream.Tsmr.Client > data CmdLineOptions = CmdLineOptions > { optionHelp :: Bool > , optionVersion :: Bool > , optionPort :: String > , optionMessage :: String > , optionInvalids :: [String] > } > deriving (Eq, Show) > initCmdLineOptions = CmdLineOptions > { optionHelp = False > , optionVersion = False > , optionPort = "1234" > , optionMessage = "" > } > parseArgs :: [String] -> CmdLineOptions > parseArgs [] = initCmdLineOptions > parseArgs ("--port":port:xs) = (parseArgs xs) { optionPort = port } > parseArgs ("--help":xs) = (parseArgs xs) { optionHelp = True } > parseArgs ("--version":xs) = (parseArgs xs) { optionVersion = True } > parseArgs (('-':opt):xs) = let option = (parseArgs xs) in > option { optionInvalids = ('-':opt):optionInvalids option } > parseArgs (message:xs) = (parseArgs xs) { optionMessage = message } > printUsage = do > putStrLn "Usage: udp-server.lhs [options] <message>" > putStrLn "" > putStrLn "Options:" > putStrLn " --help Get help information." > putStrLn " --vesion Get version information." > putStrLn " --port n The port number to listen on." > putStrLn "" > putStrLn "Message:" > putStrLn " The message to send." > putStrLn "" > printVersion = do > putStrLn "Version." > fromHex :: String -> [Word8] > fromHex [] = [] > fromHex (u:l:xs) = (hexU .|. hexL):fromHex xs > where > hexU = (fromInteger $ hexValue u) :: Word8 > hexL = (fromInteger $ hexValue l) :: Int > hexValue c > | '0' <= c && c <= '9' = ord c - ord '0' > | 'a' <= c && c <= 'z' = ord c - ord 'a' + 10 > run port message = do > h <- openlog "localhost" port "udp-client.lhs" > syslog h (fromHex message) > main = do > args <- getArgs > let options = parseArgs args > let port = optionPort options > let message = optionMessage options > if optionHelp options > then printUsage > else if optionVersion options > then printVersion > else do > putStrLn ("Starting UDP listener on port: " ++ port) > run port message

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Re: [Haskell-cafe] Record initialise question (RESOLVED)
by John Ky 06 Jun '09

06 Jun '09

On Fri, Jun 5, 2009 at 8:05 PM, Martijn van Steenbergen < martijn(a)van.steenbergen.nl> wrote: > Hi John, > > John Ky wrote: > >> > full = do >> > let myOrder = init -- [1] >> > { item = Just init >> > { itemId = "Something" >> > } >> > , operation = Just Buy >> > } >> > putStrLn $ show myOrder >> > return () >> >> Where initOrder and initItem are both replaced with just 'init' and the >> compiler works out from the context (ie. the type of the field) what the >> types are supposed to be and therefore the actual init function to call? >> > > Sure! This is one of the simplest forms of overloading. Here's a small > example: > > data Circle = Circle { center :: (Float, Float), radius :: Float } >> data WrapCircle = WrapCircle { circle :: Circle } >> >> class Ini a where >> ini :: a >> >> instance Ini Circle where >> ini = Circle { center = (0, 0), radius = 1 } >> >> exampleCircle :: WrapCircle >> exampleCircle = WrapCircle { circle = ini { radius = 2 } } >> > > Hope this helps! > > Martijn. >

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A small puzzle: inTwain as function of foldr
by Martijn van Steenbergen 06 Jun '09

06 Jun '09

Bonjour café, A small puzzle: Consider the function inTwain that splits a list of even length evenly into two sublists: > inTwain "Hello world!" ("Hello ","world!") Is it possible to implement inTwain such that the recursion is done by one of the standard list folds? Is there a general way to tell if a problem can be expressed as a fold? Thank you, Martijn.

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Re: Non Empty List?
by Tillmann Rendel 05 Jun '09

05 Jun '09

Hi, please write to the whole list, not just me. There are a lot of people around who can help you. MH wrote: > Rendel do you mind to explain to me how Container a = Many a > (Container [a]) prevents user from creating an empty list? > I did try the following: > let a = Many "string" > a :: Container [Char] -> Container [Char] > let b = Many [] > b :: forall a. Container [a] -> Container [a] > > but I can not understand what is the difference. You cannot prevent users from creating an empty list, because lists are already defined in the Haskell prelude, and you cannot change that definition. But you can define your own datatype Container which prevents users from creating empty containers. Your datatype data Container a = Many a (Container [a]) is quite complicated, because you have (Container a) on the left hand side of the equation, but (Container [a]) on the right hand side. Is that on purpose? Anyway, your Container has the property that a (Container a) is never empty, because it always contains at least one value of type a, namely the first parameter of Many. To see that, we can write a function which extracts that element: getContainerElement :: Container a -> a getContainerElement (Many element something) = element It is not possible to write such a function for ordinary lists without calling error or entering a nonterminating loop: getListElement :: [a] -> a getListElement (element : something) = element getListElement [] = error "no element around :(" So ordinary lists can be empty, but your Containers can't. Tillmann

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Is it possible to do type-level arithmetic without UndeciableInstances?
by Ryan Ingram 05 Jun '09

05 Jun '09

> {-# LANGUAGE TypeFamilies #-} > module PeanoArith > data Z = Z > data S n = S n So, this type family works just fine: > type family Plus a b > type instance Plus Z b = b > type instance Plus (S a) b = S (Plus a b) As does this one: > type family Minus a b > type instance Minus a Z = Z > type instance Minus Z (S b) = Z > type instance Minus (S a) (S b) = Minus a b But this one doesn't work without UndecidableInstances: > type family Times a b > type instance Times Z b = Z > type instance Times (S a) b = Plus b (Times a b) I tried several different implementations for Times but I was unable to come up with one that passed the type family termination checker. Is there a way to do so? Some examples of attempts: > -- (a+1)*(b+1) = ab + a + b + 1 > type family Times1 a b > type instance Times1 Z Z = Z > type instance Times1 (S n) Z = Z > type instance Times1 Z (S n) = Z > type instance Times1 (S a) (S b) = S (Plus (Times1 a b) (Plus a b)) > -- inline Plus > type family Times2_H a b acc > type Times2 a b = Times2_H a b Z > type instance Times2_H a b (S acc) = S (Times2_H a b acc) > type instance Times2_H Z b Z = Z > type instance Times2_H (S a) b Z = Times2_H a b b Do you have any other ideas? Is (a*b) too big for the termination checker to ever accept? -- ryan

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Fast code question
by Bartosz Wójcik 05 Jun '09

05 Jun '09

Hi Folks, I had to transform Packed Decimal file into csv format (does anybody here know what this Mainframe format is?). Because of the file size I could not do this on mainframe directly. So I've created simply program using ByteString. Generally I'm happy with my solution: pgm processes arroud 2MB/s on my pc, so my 3GB file was transformed in reasonable 30 min time. My question is: how to do this faster? {code} module Main where import qualified Data.ByteString.Lazy as B main = B.getContents >>= myPrint . myConcat . B.unpack myConcat = myConcat' 0 myConcat' :: (Integral a) => Integer -> [a] -> [Integer] myConcat' _ [] = [] myConcat' acc (x:xs) = case x `mod` 16 of 12 -> (10*acc + (restOf . fromIntegral) x) : myConcat' 0 xs 13 -> ((-10)*acc + (restOf . fromIntegral) x) : myConcat' 0 xs 15 -> (10*acc + (restOf . fromIntegral) x) : myConcat' 0 xs 10 -> fail $ show acc 11 -> fail $ show acc 14 -> fail $ show acc v -> myConcat' (100*acc + (numberOf . fromIntegral) x) xs where restOf n = (n - 12) `div` 16 numberOf n = n - 6 * (n `div` 16) myPrint [] = return () myPrint xs = mapM_ myShow (take 14 xs) >> putStrLn "" >> myPrint (drop 14 xs) myShow x = (putStr . show) x >> putStr ";" {code} I knew that csv output had to be 14 fields per line. Best, Bartek

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Re: [Haskell-cafe] Fast code question
by Bartosz Wójcik 05 Jun '09

05 Jun '09

Packed Decimal downloaded on pc is just a stream of bytes without any comma. I was supposed to reformat data. If I undersdand bytestring-csv library, it parses csv format data. Thanks for the hint. I'll investigate next time when I have to deal with huge files. Bartek On Thursday 04 June 2009 23:21:21 you wrote: > Can you use the bytestring csv parser (or convert it into a pretty > printer?) > > bartek: > > Hi Folks, > > > > I had to transform Packed Decimal file into csv format (does anybody here > > know what this Mainframe format is?). Because of the file size I could > > not do this on mainframe directly. So I've created simply program using > > ByteString. Generally I'm happy with my solution: pgm processes arroud > > 2MB/s on my pc, so my 3GB file was transformed in reasonable 30 min time. > > > > My question is: how to do this faster? > > > > {code} > > module Main > > where > > > > import qualified Data.ByteString.Lazy as B > > > > main = B.getContents >>= myPrint . myConcat . B.unpack > > ^^^^^^^^^^^^^^^^^^^^^ > That looks bad. -------------------------------------------------------

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web musing
by Conor McBride 05 Jun '09

05 Jun '09

Comrades I'm in a perplexing situation and I'd like to appeal to the sages. I've never written anything other than static HTML in my life, and I'd like to make a wee web service: I've heard some abbreviations, but I don't really know what they mean. I've got a function (possibly the identity, possibly const "", who knows?) assistant :: String -> String and I want to make a webpage with an edit box and a submit button. If I press the submit button with the edit box containing string s, I'd like the page to reload with the edit box reset to (assistant s). Will I need to ask systems support to let me install some haskelly sort of web server? Looks likely, I suppose. In general, what's an easy way to put a web front end on functionality implemented in Haskell? Hoping this isn't a hard question Conor

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specialization in type classes
by Cetin Sert 05 Jun '09

05 Jun '09

module IOStream where import System.IO import System.IO.Unsafe class Out a where out :: a → String instance Show a ⇒ Out a where out = show instance Out String where {-# SPECIALISE out :: String → String #-} out = id instance Out Char where {-# SPECIALISE out :: Char → String #-} out = \x → [x] infixl 0 <<, ≪ (≪), (<<) :: Out a ⇒ IO Handle → a → IO Handle (<<)= (≪) h ≪ a = do s ← h hPutStr s $ out a return s cout, cin, cerr :: IO Handle cout = return stdout cin = return stdin cerr = return stderr endl :: String endl = "\n" --- cetin@unique:~/lab/c/linking/demo$ ghci -fglasgow-exts iostream.hs GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Ok, modules loaded: IOStream. Prelude IOStream> cout << 22 << False 22False{handle: <stdout>} Prelude IOStream> cout << 22 << False << endl <interactive>:1:0: Overlapping instances for Out String arising from a use of `<<' at <interactive>:1:0-26 Matching instances: instance (Show a) => Out a -- Defined in IOStream instance Out String -- Defined in IOStream In the expression: cout << 22 << False << endl In the definition of `it': it = cout << 22 << False << endl o________________O how can I specialise a type class function?

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Non Empty List?
by Günther Schmidt 05 Jun '09

05 Jun '09

Hi, I need to design a container data structure that by design cannot be empty and can hold n elements. Something like a non-empty list. I started with: data Container a = Single a | Many a [a] but the problem above is that the data structure would allow to construct a Many 5 [] :: Container Int. I can't figure out how to get this right. :( Please help. Günther

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