- Haskell-Cafe - Haskell.org

Throwback of inferred types
by Jon Harrop 21 Jan '08

21 Jan '08

Is it possible to get throwback of inferred types into Emacs or an IDE for Haskell? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/products/?e

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Invitation for MathematiKa '08 : projectEuler format
by anshuman＠students.iiit.ac.in 21 Jan '08

21 Jan '08

Hello Math Enthusiast, IIIT-Hyderabad, India cordially invites you to be a part of Mathematika '08, an online competition that emphasizes on fusion of mathematics and computing. Where: http://felicity.iiit.ac.in/~math When: 4th February, 2008 This Online Math Contest will consist of 10-12 problems made randomly from different domains of existing Math. All problem statements will be visible to the contestant from the start of contest spanning over a 10 hour period. The solution to each problem would be an integer or a floating point number which is to be submitted in real-time. One Minute Rule: The problems are designed in a way that support 1 minute rule which means that an efficient implementation will allow a solution to be obtained on a modestly powered computer in less than one minute. Also, make sure to bring your appetite for math, programming and fun. There will be plenty of all three to go around. Regards Mathematika Team http://felicity.iiit.ac.in/~math

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bug in all about monads tutorial
by Peter Hercek 21 Jan '08

21 Jan '08

Hi, About 3 weeks ago I reported this bug to Jeff Newbern. But I got no response - maybe I got filtered out as spam :) Since it was not fixed I'm trying once more here. Maybe there is somebody here who has access to the web site http://www.haskell.org/all_about_monads and cares enough to fix it. On page http://www.haskell.org/all_about_monads/html/writermonad.html there is "listens" defined like this: listens f m = do (a,w) <- m; return (a,f w) ... but it should be like this: listens f m = do (a,w) <- listen m; return (a,f w) ... or maybe a less strict option (as ghc libs have it): listens f m = do ~(a,w) <- listen m; return (a,f w) Peter.

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Re: [Haskell-cafe] First go at reactive programming
by Steve Lihn 21 Jan '08

21 Jan '08

This fixed the second example. Thanks. > > I think handleConnection should be > > handleConnection :: RequestHandler -> Handle -> IO () > > handleConnection r h = > handleToRequest h >>= responseSend h . runRequestHandler r > > > Levi > > >

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CPP and INLINE pragmas
by Adam Langley 21 Jan '08

21 Jan '08

Since CPP mode removes newlines in the out macro expansion. It appears to be impossible to have a macro expand to a function with an INLINE pragma since it appears to need to be in its own line. For example: #define GETHOSTWORD(name, m, type) \ {-# INLINE name #-} \ name :: m type ; \ name = getPtr (sizeOf (undefined :: type)) \ Does work (since you can't follow the INLINE pragma with anything else on the same line. Likewise: #define GETHOSTWORD(name, m, type) \ name :: m type ; \ name = getPtr (sizeOf (undefined :: type)) \ {-# INLINE name #-} Also doesn't work since the INLINE needs to start at the beginning of a line. Has anyone a workaround for this, or a way to get the preprocessor to output a newline? Cheers AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

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ANN: A triple of new packages for talking to the outside world
by Adam Langley 20 Jan '08

20 Jan '08

Over the Xmas break I made some headway on writing an RPC package since many of the ideas that I want to play with involve such a thing as a basic building block. However, some might find some of the spin-off packages that I wrote along the way useful. * binary-strict: This is mostly a cut-n-paste job from the excellent binary package which provides Data.Binary.Strict.Get - a monad which is a drop in replacement for Get, but which parses strict ByteStrings and returns an Either, thus one can parse binary data without triggering an exception on failure. * codec-libevent: This parses the "tagging" IDL files introduced in libevent[1] 1.4.0-beta and later and can generate the Haskell code for them. These are platform-independent "structs" (in the C sense) and libevent provides the complementary C infrastructure for them. You should be able to understand almost everything these is to know about them from a quick example: struct msg { string from_name = 1; string to_name = 2; optional struct[kill] attack = 3; array struct[run] run = 4; } [1] http://monkey.org/~provos/libevent/ * control-timeout: This module provides simple timeouts - e.g. actions which occur after a given number of seconds and which can be canceled. It does it in a not-totally-stupid fashion so that you don't need to worry about setting hundreds of them. AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

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bytea decoding
by Bryan Green 20 Jan '08

20 Jan '08

Does anyone know of a library that will handle bytea encodings from postgres? The bytea field that I need to access contains a jpg file. I want to retrieve it from the database and write it out for an image display program. bytea: Bytea octets are also escaped in the output. In general, each "non-printable" octet is converted into its equivalent three-digit octal value and preceded by one backslash. Most "printable" octets are represented by their standard representation in the client character set. The octet with decimal value 92 (backslash) has a special alternative output representation. Details are in Table 8-8<http://www.postgresql.org/docs/7.4/interactive/datatype-binary.html#DATATYP…>. *Table 8-8. bytea Output Escaped Octets* Decimal Octet ValueDescriptionEscaped Output Representation ExampleOutput Result92backslash\\SELECT '\\134'::bytea;\\ 0 to 31 and 127 to 255 "non-printable" octets\*xxx* (octal value) SELECT '\\001'::bytea;\00132 to 126"printable" octetsclient character set representation SELECT '\\176'::bytea;~ Depending on the front end to PostgreSQL you use, you may have additional work to do in terms of escaping and unescaping bytea strings. For example, you may also have to escape line feeds and carriage returns if your interface automatically translates these. So, here is part of the file from the database: \377\330\377\340\000\020JFIF\000\001\001\000\000\001\000\001\000\000\377\333\000C\000\012\007\007\010\007\006\012\010\010\010\013\012\012\013\016\030\020\016\015\015\016\035\025\026\021\030#\037%$"\037"!&+7/&)4)!"0A149;>>>%.DIC<H7=>;\377 I may have to write a converter myself but I don't like re-inventing the wheel if I don't need to do so. Any pointers would be greatly appreciated. Bryan Green

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Need help with Parsec
by Nicu Ionita 20 Jan '08

20 Jan '08

Hi, I'm playing since a few hours with Parsec and trying to write a small html (fragment) parser, but I'm stuck in a point which I really can't understand. The problem seem to be either in "parseProperCont" or in "closing" (see code below). It looks like closing does not work (but it is a very simple function!) or (also hard to believe) function "try" from Parsec has some problems. Anyway I get this answer: Prelude ParseSHtml> pf parseHtmlFrg "ptest.txt" Left "ptest.txt" (line 5, column 2): unexpected "/" expecting element name when I'm parsing this file: <div id="normtext"> one line with break<br /> another line <br /><br /> Mail: <a href="mailto:user@dom.at">user(a)dom.at</a> </div> with this code (sorry for the longer mail): import Text.ParserCombinators.Parsec hiding (label) import Text.XHtml.Strict -- Helper function: parse a string up to one of the given chars upTo :: [Char] -> Parser [Char] upTo ds = many1 (noneOf ds) parseHtmlFrg :: Parser Html parseHtmlFrg = do many space choice [parseElem, parseText] <?> "html fragment" parseElem :: Parser Html parseElem = do en <- parseElTag many1 space (ats, cnt) <- restElem en return $ compElem en cnt ! ats <?> "html element" -- Compose a html element from tag name and content compElem en cnt = if isNoHtml cnt then itag en else tag en cnt parseElTag :: Parser String parseElTag = do char '<' en <- elemName return en <?> "element tag" elemName :: Parser String elemName = many1 lower <?> "element name" restElem :: String -> Parser ([HtmlAttr], Html) restElem nm = do ats <- parseAttList ht <- (restElNoCont <|> restElCont nm) return (ats, ht) <?> ("> or /> to close the tag " ++ nm) -- Rest element with no content restElNoCont = do char '/' char '>' return noHtml <?> "/>" -- Rest element with content restElCont nm = do char '>' many space els <- parseProperCont nm return $ concatHtml els <?> "element with content" -- Parse closing tag or proper content(s) parseProperCont :: String -> Parser [Html] parseProperCont nm = try (do closing nm return [] ) <|> (do h <- parseHtmlFrg hs <- parseProperCont nm return (h:hs) ) -- <|> return [] <?> "proper element content" closing nm = do char '<' char '/' nm1 <- elemName char '>' if nm1 == nm then return () else fail $ nm ++ ", encountered " ++ nm1 <?> ("closing of " ++ nm) -- Parse a html attribute parseAttr :: Parser HtmlAttr parseAttr = do at <- many1 lower char '=' va <- parseQuote many space return $ strAttr at va <?> "Attribut" parseAttList = many1 parseAttr <|> return [] <?> "attribute list" -- Parse a quoted string parseQuote :: Parser String parseQuote = do char '"' cs <- upTo ['"'] char '"' return cs -- Parse a text element parseText :: Parser Html parseText = do s <- upTo "<" return (stringToHtml s) <?> "some text" -- For tests: pf p file = parseFromFile p file Nicu

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Type issues for a foldable trie in Haskell
by Brad Larsen 20 Jan '08

20 Jan '08

Hello there, I have written a trie in Haskell generalized to Eq a => [a] rather than simply String. I want to make this type an instance of Foldable, but I've run into a type dilemma. My datatypes look like this: data TrieElem a = Elem a | Start | End deriving (Read, Show, Eq, Ord) data Trie a = Trie {label :: TrieElem a ,children :: [Trie a]} deriving (Read, Show, Eq, Ord) The signature of Data.Foldable.foldr is (Data.Foldable.Foldable t) => (a -> b -> b) -> b -> t a -> b. However, I want the functions in Foldable to operate on the _list type_ that Trie stores rather than the _elements_ of that list type---Trie a stores lists of type a. For example, a Trie storing strings would have type Trie Char, and I want Trie Char to be Foldable, but where the functions operate on String rather than Char. So, with the datatype definitions of Trie and TrieElem as they are above, to define a fold function that operates the way I want would have signature ([a] -> b -> b) -> b -> Trie a -> b, which is no good for making Trie a an instance of Foldable. Hopefully this doesn't just seem like rambling :-). How might I rewrite my datatypes to do what I want, preferably without using ghc extensions? Thanks! Brad Larsen

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Basic binary IO
by Jamie Love 20 Jan '08

20 Jan '08

Hello all, I'm wondering if anyone has a reference to any binary IO and data conversion tutorials. I'm playing around with generating a BMP file in haskell, and am a little stuck on the "best" way to go about the simple task of creating the BMP header. The header is "BM" + 4 bytes for file size + 4 bytes reserved + 4 bytes for offset where data begins. I have the basis starting off at: bmpHeader = B.pack $ [ 0x42, 0x4D ] ++ [0 , 0, 0, 0] ++ [0 , 0, 0, 0] ++ [14 :: Int32] (where B is Data.ByteString) I'm wondering how I can: 1/ convert a 32 bit number (Int32, Char32) to 4 Char8 elements 2/ rotate bits/bytes in a 32 bit Char32 (or Int32) so they are explicitly little-endian (I work on a mac powerbook, and it is big-endian) 3/ convert an Integer or Int type to an Int32 type Any pointers or suggestions would be helpful. Thanks -- Jamie Love Senior Consultant Aviarc Australia Mobile: +61 400 548 048 ------------------------------------------------------------ This message has been scanned for viruses and dangerous content by MailScanner and is believed to be clean.

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