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Looking for documentation on Control.Parallel.Strategies
by Jefferson Heard 19 Feb '07

19 Feb '07

Is there anything that documents this further than the Haddock documentation available from Haskell.org or the GHC pages? I've gotten some basic parallelism to work using parMap and using >|| and >|, but I had a fold and a map that I could logically compute at the same time. I found this example for quicksort using GPH: import Strategies main = print (quicksort ([999,998..0]::[Int])) quicksort :: (Ord a, NFData a) => [a] -> [a] quicksort [] = [] quicksort [x] = [x] quicksort (x:xs) = (lo ++ (x:hi)) `using` strategy where lo = quicksort [ y | y <- xs, y < x] hi = quicksort [ y | y <- xs, y >= x] strategy result = rnf lo `par` rnf hi `par` rnf result Is the syntax the same for GHC using Control.Parallel.Strategies? If so, does the following execute the two computations from the where clause in parallel? import Control.Parallel import Control.Parallel.Strategies stats bigList summation mean = (stddev, proportions) `using` strategy where stddev = sqrt (fold ((+) . (^2) . (-mean)) 0 bigList) proportions = map (/summation) bigList strategy result = rnf stddev >|| rnf proportions >|| rnf result Thanks! --Jeff

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Application Level server state in Haskell Web server
by Pieter Laeremans 18 Feb '07

18 Feb '07

Hello, I'm trying to write a simple module for the haskell web server (hws- cgi). And I would like to write a simple module that maintains some kind of state for a session. But I'm I write I cannot do this in pure Haskell ? Without adopting the sources of the Haskell web server ? I'll examplify to make it more concrete : The requestHandler function a module has to implement has this signature. requestHandler :: ServerState -> ServerRequest -> IO (Maybe Response) Let 's assume I have this implementation requestHandler st _ = return $ Just $ mkRequest where mkRequest = okResponse (serverConfig st) mkBody hs True mkBody = HereItIs " This is a test" hs = mkHeaders [contentTypeHeader "text/html"] And I would like the response to include, for example, a number indicating the number of calls that has been handled by the module. I would concider using an Mvar but I can't "remember" the mvar between requests. Am I right to assume that the interface of the requestHandler method has to be adapted ? Or that serverstate has to be adopted so that it can act as a datastore ? kind regards, Pieter

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Re: [Haskell-cafe] How to solve this problem?It's quite easy in PHP.
by Henk-Jan van Tuyl 18 Feb '07

18 Feb '07

On Fri, 16 Feb 2007 22:44:25 +0100, David House <dmhouse(a)gmail.com> wrote: > On 16/02/07, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: >> Yet simpler: >> > formatTableItems (a,b) = a : " = " ++ (show b) ++ "\n" > > You can drop the parentheses around 'show b'; infix function > application always binds tighter than pretty much everything else. I missed that one; of course I knew it, but you could say it's in my passive vocabulary. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ -- Using Opera's revolutionary e-mail client: https://secure.bmtmicro.com/opera/buy-opera.html?AID=789433

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LugRadio talk
by Dougal Stanton 17 Feb '07

17 Feb '07

So which one of you wonderful people volunteered to give a talk on Haskell at the coming LugRadio Live? I'm afraid I didn't catch the name, though it might have been a Magnus something? It might be enough to make me go if I knew there would be some funky Haskelling going on. Cheers, D. -- Dougal Stanton

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Newbie Q: GHCi: Where “List” module is imported from?
by Dmitri O.Kondratiev 17 Feb '07

17 Feb '07

Going through "Haskell. The Craft of Functional Programming" book , in section 16.8 I found a Set module example. Module declarations starts with: import List hiding (union) "Set" module here is built with list and uses among other things list comparison functions such as (==) and (<=). For example: eqSet :: Eq a => Set a -> Set a -> Bool eqSet (SetI xs) (SetI ys) = (xs == ys) Q1: Where "List" module is imported from? GHC "Base" package contains "Data.List" module, not just "List" module. Besides, "Data.List" does not have (<=) function. Q2: Any tutorial on using GHC libraries out there? Thanks! Dima

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How to solve this problem?It's quite easy in PHP.
by keepbal 17 Feb '07

17 Feb '07

For example, ----------------------------------- <?php //test.php require ("tiny.php");//Tiny is a small template engine. $tn=new Tiny(); $arr=new Array(); $arr['a']='1'; $arr['b']='2'; $arr['c']='3'; $tn->set('arr',$arr); $tn->show('_test.php'); ?> ----------------------------------- <?php require('_header.php');?> <?php foreach($arr as $key => $val){ echo "$key = $val <br>"; } ?> <?php require('_footer.php');?> ----------------------------------- a = 1 b = 2 c = 3 -----------------------------------

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new member
by P. R. Stanley 17 Feb '07

17 Feb '07

Hi Paul here - new member - checking to see my account is fully functional. Does the list know of another list where I can post questions on functional programming and the related mathematics? I'm assuming Haskell Cafe is mainly for discussions on Haskell programming and not so much the underlying theory. Any help would be appreciated. Cheers Paul

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Strange memory consumption problems in something that should be tail-recursive
by Jefferson Heard 16 Feb '07

16 Feb '07

Hi, I am running the following code against a 210 MB file in an attempt to determine whether I should use alex or whether, since my needs are very performance oriented, I should write a lexer of my own. I thought that everything I'd written here was tail-recursive, but after compiling this with GHC 2.4.6, and running it, I eat up 2GB of RAM in less than a second. So far, I have tried token and character oriented Parsec parsers and alex and alex is winning by a factor of 2. I would like to be able to tokenize the entirety of a 1TB collection in less than 36 hours on my current machine, which is where alex has gotten me so far. Thanks in advance! -- Jeff --- module Main where import qualified FileReader import qualified Data.Set as Set punct = foldl (flip Set.insert) Set.empty "<,>.?/:;\"'{[}]|\\_-+=) (*&^%$##@!~`" stripTagOrComment [] = [] stripTagOrComment ('>':rest) = rest stripTagOrCOmment (c:rest) = stripTagOrComment rest pass1 :: String -> String -> String pass1 left [] = left pass1 left ('<':right) = pass1 left (stripTagOrComment right) pass1 left (' ':right) = pass1 left right pass1 left (c:right) | Set.member c punct = pass1 (' ':c:' ':left) right | otherwise = pass1 (c:left) right pass2 :: [String] -> String -> Char -> String -> [String] pass2 left word ' ' [] = word:left pass2 left word c [] = (c:word):left pass2 left word ' ' (' ':right) = pass2 left word ' ' right pass2 left word ' ' (c:right) = pass2 (word:left) "" c right pass2 left word l (c:right) = pass2 left (l:word) c right tokenize = (pass2 [] "" ' ') . (pass1 []) main = do file <- do FileReader.trecReadFile "trecfile" print (tokenize (head (tail file))) -- print (length (map (runParser tokenizeDoc [] "") file))

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State of OOP in Haskell
by Alexy Khrabrov 16 Feb '07

16 Feb '07

...In the tradition of the "letters of an ignorant newbie"... What's the consensus on the OOP in Haskell *now*? There're some libraries such as OOHaskell, O'Haskell, and Haskell~98's own qualified type system with inheritance. If I have GHC, which way to do anything OOP-like is considered "right" today? Cheers, Alexy

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SoC: Port Haddock to use GHC
by Nicolas Frisby 16 Feb '07

16 Feb '07

I am very ready for a Haddock that can swallow infix typenames. In dons's recent overview of the last SoC, a darcs repo for Waern's project was listed. I jumped at the link but couldn't find much documentation (i.e. the README file is GHC's, not the SoC project's). Some Google queries followed with no success. My questions for Waern or anyone else who knows where to look: 1) Where to look for more complete documentation? 2) Is the implementation integrated into GHC yet? GHC head? Separate repo? 3) What Haddock features are supported as of yet? 4) Is it still actively developed? Thanks for any hints, Nick

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Lisp on xkcd.com, very funny!
by Magnus Therning 16 Feb '07

16 Feb '07

I think the readers of this list will appreciate this http://xkcd.com/comics/lisp.jpg /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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Summer of code and spam
by Magnus Therning 16 Feb '07

16 Feb '07

There seems to be a serious problem with spam on Haskell's SoC page: http://tinyurl.com/fl2dw Or maybe that's a general problem for hackage? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus

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Unicode Strings and ICU for Haskell
by Brian Smith 16 Feb '07

16 Feb '07

Has anybody written a wrapper for ICU (International Components for Unicode) for Haskell (GHC and/or Hugs)? I am processing Thai text and I want to use the ICU word-breaking, collation, encoding conversion, and transliteration services from my Parsec-based parsers. Also, what is the plan for Unicode string processing for Haskell? What is the standard module for high-performance, Unicode-capable strings? Thanks, Brian [1] http://icu.sourceforge.net/ [2] http://www-306.ibm.com/software/globalization/icu/index.jsp

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Summer of Code
by Bryan Burgers 16 Feb '07

16 Feb '07

Hello, Yes, I realize it's mid-February right now and the summer is still months away, but it's probably not too early to think about the future. I am wondering if there are any Summer of Code projects that I would be able to do for the Haskell community. I will be graduating from my undergrad program this semester and am hoping to go on to graduate school in the fall, and I think a good Haskell project would be the perfect way to spend my transitional summer. I looked at http://hackage.haskell.org/trac/summer-of-code/query?status=new&status=assi… to see if there was anything that people already had in mind, but that page looks old and unkempt (judging by the abundance of advertisements) so I am not sure which of those are still available or needed. So, are there any projects that will need a student for the summer? If this is not the place to ask, where should I be asking? Bryan Burgers

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Programming an Evaluation Context
by Klaus Ostermann 16 Feb '07

16 Feb '07

Hi there, in structural operational semantics, an evaluation context is often used to decompose an expression into a redex and its context. In a formal semantics on paper, an expression can just be "pattern matched" over the grammar of an evaluation context. If one wants to implement such a semantics in the form of an interpreter, I could not come up with a similarly nice solution. I have declared two separate data types (one for expressions, and one for evaluation contexts) and explicit functions to convert an expression into a (evaluation context, redex) pair. For example, I could have data Expr = Val Int | Plus Expr Expr and data Ctx = Hole | CPlusl Ctx Expr | CPlusr Int Expr Are there any tricks to mimick more closely what is going on in the formal semantics? Klaus

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Where can I find XmlRpc package?
by keepbal 15 Feb '07

15 Feb '07

http://www.cs.chalmers.se/~bringert/darcs/blob/Makefile BlobXmlRpc: GHCFLAGS += -package XmlRpc I can't find XmlRpc,so I use haxr instead,but it doesn't work.

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factorising to prime numbers
by Dougal Stanton 15 Feb '07

15 Feb '07

Hi folks, I recently read in my copy of Concrete Mathematics the relationship between prime factors powers and lcm/gcd functions. So I decided to reimplement gcd and lcm the long way, for no other reason than because I could. If you look at the definition of 'powers' you'll note it's infinite. So there's no easy way to take the product of this list, if I don't know how many items to take from it. Is there a better way to turn an integer N and a list of primes [p1,p2,p3,...] into powers [c1,c2,c3,...] such that N = product [p1^c1, p2^c2, p3^c3, ...] If I'm missing something really obvious I'll be very grateful. I can't really work out what kind of structure it should be. A map? fold? D. -- Concrete Mathematics -- Graham, Knuth & Patashnuk module Concrete where import Data.List -- the sieve of eratosthenes is a fairly simple way -- to create a list of prime numbers primes = let primes' (n:ns) = n : primes' (filter (\v -> v `mod` n /= 0) ns) in primes' [2..] -- how many of the prime p are in the unique factorisation -- of the integer n? f 0 _ = 0 f n p | n `mod` p == 0 = 1 + f (n `div` p) p | otherwise = 0 powers n = map (f n) primes --gcd :: Integer -> Integer -> Integer --gcd = f . map (uncurry min) -- Dougal Stanton

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Data.ByteString.Lazy.Char8 and finding substrings
by Magnus Therning 15 Feb '07

15 Feb '07

I'm curious, why doesn't Data.ByteString.Lazy.Char8 have the functions for searching for substrings that Data.ByteString.Char8 has (isPrefixOf, isSuffixOf, isSubstringOf, findSubstring and findSubstrings)? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus

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Calendar Dates before the epoch
by jim burton 15 Feb '07

15 Feb '07

It seems that CalendarTime is for dates since the epoch...what do I use to handle dates before that? Sorry if this is an FAQ, I looked on the wiki and tried to find MissingH since I thought it might be in there, but don't know where to find it. I also found this from 2003 - http://www.arcknowledge.com/gmane.comp.lang.haskell.libraries/2003-11/msg00… - is the code in a library somewhere? Thanks. -- View this message in context: http://www.nabble.com/Calendar-Dates-before-the-epoch-tf3221685.html#a89477… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Network.CGI.Compat.pwrapper
by Gracjan Polak 14 Feb '07

14 Feb '07

Hi, I wanted to setup really simple http server, found Network.CGI.Compat.pwrapper and decided it suits my needs. Code: module Main where import Network.CGI import Text.XHtml import Network doit vars = do return (body (toHtml (show vars))) main = withSocketsDo (pwrapper (PortNumber 7777) doit) Pointng any browser to http://127.0.0.1:7777 does not render the page. It seems the response headers are broken. How do I report this bug (trac? something else?). We might want to either fix it, or just get rid of it, as nobody seems to notice the problem :) $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.6 Tested under WinXP and MacOSX 10.4.9. Another question is: how do I do equivalent functionality without pwrapper? -- Gracjan

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Difference between Lazy ByteStrings and Strings in alex
by Jefferson Heard 14 Feb '07

14 Feb '07

It was suggested that I might derive some performance benefit from using lazy bytestrings in my tokenizer instead of regular strings. Here's the code that I've tried. Note that I've hacked the "basic" wrapper code in the Lazy version, so the code should be all but the same. The only thing I had to do out of the ordinary was write my own 'take' function instead of using the substring function provided by Data.Lazy.ByteString.Char8. The take function I used was derived from the one GHC uses in GHC.List and produces about the same code. The non-lazy version runs in 38 seconds on a 211MB file versus the lazy versions 41 seconds. That of course doesn't seem like that much, and in the non-lazy case, I have to break the input up into multiple files, whereas I don't have to in the lazy version -- this does not take any extra time. The seconds do add up to a couple of hours for me, though once I'm done, and so I'd like to understand why, when the consensus was that Data.ByteString.Lazy might give me better performance in the end, it doesn't do so here. I am running GHC 2.6 now, and am using -O3 as my optimization parameter. I'm profiling the code now, but was wondering if there was any insight... -- Jeff Non-lazy version { module Main where import qualified FileReader } %wrapper "basic" $letter = [a-zA-Z] $digit = 0-9 $alphanum = [a-zA-Z0-9] $punct = [\! \@ \# \$ \% \^ \& \*  \_ \- \+ \= \{ \[ \} \] \\ \| \; \: \' \" \, \. \? \/ \` \~] $dec = \. $posneg = [\- \+] @date1 = jan($punct|uary)?\ $digit{1,2}(\,\ $digit{2,4})? | feb($punct|ruary)?\ $digit{1,2}(\,\ $digit{2,4})? | mar($punct|ch)?\ $digit{1,2}(\,\ $digit{2,4})? | apr($punct|il)?\ $digit{1,2}(\,\ $digit{2,4})? | may?\ $digit{1,2}(\,\ $digit{2,4})? | jun($punct|e)?\ $digit{1,2}(\,\ $digit{2,4})? | jul($punct|y)?\ $digit{1,2}(\,\ $digit{2,4})? | aug($punct|ust)?\ $digit{1,2}(\,\ $digit{2,4})? | sep($punct|tember)?\ $digit{1,2}(\,\ $digit{2,4})? | sept($punct)?\ $digit{1,2}(\,\ $digit{2,4})? | oct($punct|ober)?\ $digit{1,2}(\,\ $digit{2,4})? | nov($punct|ember)?\ $digit{1,2}(\,\ $digit{2,4})? | dec($punct|ember)?\ $digit{1,2}(\,\ $digit{2,4})? @date2 = $digit{1,2} $punct $digit{1,2} $punct $digit{2,4} @time = $digit{1,2} \: $digit{2} (am|pm)? @word = $alphanum+ @number = $posneg? $digit+ | $posneg? $digit+ $dec $digit+ | $posneg? $digit+ (\,$digit{3})+ | $posneg? $digit? (\,$digit{3})+ $dec $digit+ $white = [\t\r\n\v\f\ ] @doc = \< DOC \> @tag = \< $alphanum+ \> | \<\/ $alphanum+ \> tokens :- @doc { \s -> "" } @tag ; $white+ ; @time { \s -> s } @number { \s -> s } @word { \s -> s } $punct ; . ; { printCount c [] = print c printCount c (l:ls) = if l == "" then printCount (c+1) ls else printCount c ls main = do file <- readFile "trecfile1" printCount 0 (alexScanTokens file) } -- ------------------------------------------------------------------------------------------------------------ Version depending on ByteString.Lazy -- note that the grammar is the same, so it has been omitted -- ------------------------------------------------------------------------------------------------------------ ... grammar ... { type AlexInput = (Char, -- previous char B.ByteString) -- current input string takebytes :: Int -> B.ByteString -> String takebytes (0) _ = "" takebytes n s = c : takebytes (n-1) cs where c = B.index s 0 cs = B.drop 1 s alexGetChar :: AlexInput -> Maybe (Char,AlexInput) alexGetChar (_, bytestring) | bytestring == B.empty = Nothing | otherwise = Just (c , (c,cs)) where c = B.index bytestring 0 cs = B.drop 1 bytestring alexInputPrevChar :: AlexInput -> Char alexInputPrevChar (c,_) = c alexScanTokens :: B.ByteString -> [String] alexScanTokens str = go ('\n',str) where go inp@(_,str) = case alexScan inp 0 of AlexToken inp' len act -> act (takebytes len str) : go inp' AlexSkip inp' len -> go inp' AlexEOF -> [] AlexError _ -> error "lexical error" printCount :: Int -> [String] -> IO () printCount c [] = print c printCount c (l:ls) = if l == "" then printCount (c+1) ls else printCount c ls main = do file <- B.readFile "trecfile1" printCount 0 (alexScanTokens file) }

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Re: [Haskell-cafe] How to solve this problem?It's quite easy in PHP.
by dons＠cse.unsw.edu.au 14 Feb '07

14 Feb '07

keepbal: > > The code and template are separated in the PHP example,so > designers can design with out too much PHP knowledge.This is > actually what I want to solve. I'd use one of the Html/XML pretty printing libraries then, xhtml: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhtml-3000.0.1 haxml: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml-1.13.2 hxt: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hxt-7.1 All available on hackage.haskell.org. There's few others too, here, http://www.haskell.org/haskellwiki/Libraries_and_tools/Web_programming It should be perfectly possible to write clean code, with the presentation separated from the content. -- Don

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Suggestion for hackage
by Benjamin Franksen 14 Feb '07

14 Feb '07

Hi It would be a nice feature if one could look online at the documentation of a package, i.e. w/o downloading and building the package first. Fr instance, haddock generated API docs can give you a much better idea what you can expect from a library package than the mere package description. Cheers Ben

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Optimization fun
by Creighton Hogg 13 Feb '07

13 Feb '07

Hello Haskell-ers, So a friend and I were thinking about making code faster in Haskell, and I was wondering if there was a way to improve the following method of generating the list of all prime numbers. It takes about 13 seconds to run, meanwhile my friend's C version took 0.1. I'd love to learn a bit more about how to optimize Haskell code. Cheers, Creighton Hogg -- Naive way to calculate prime numbers, testing each new n to see if it has prime factors less than sqrt(n). import Data.List primes = 2:(foldr (\x y -> if isPrime x then x:y else y) [] [3..]) where isPrime x = foldl' (\z y -> z && (if x `mod` y == 0 then False else True)) True (take (floor $ sqrt $ fromIntegral x) primes)

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SIP & SDP parsers
by Neil Davies 13 Feb '07

13 Feb '07

Hi Has anyone out there done any work on parsers for SIP (Session Initiation Protocol) and/or SDP (Session Description Protocol)? Thought that I would ask before I embarked on it myself. Neil

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[Haskell-cafe] Foldr tutorial, Inspired by Getting a Fix from a Fold
by Chris Moline 13 Feb '07

13 Feb '07

I hope the following doesn't come across as condescending... The recent issue of The Monad Reader has generated some excitement, mostly to do with the time travel article. I, however, would like to discuss a simpler solution to implementing dropWhile with foldr, which is discussed in the first article of TMR. When I was taught about folds, I was told that foldr starts at the end of the list and then builds up the result from there. But this turns out to be an unhelpful way of thinking. It's better to say foldr starts at the beginning of the list, just like foldl. But how does this work? To see, we'll start by implementing a simpler function than dropWhile, one which determines whether we should cons an element onto a list or just return the list. dropHead p x xs = if p x then xs else x:xs Note that I've funnily given a separate arg to the head of the list and to its tail. That's not a typo and later we will see why. Now let's apply dropHead to each element of the list [1, 2, 3, 4, 5]: map (dropHead p) [1, 2, 3, 4 ,5] => [dropHead p 1, dropHead p 2, dropHead p 3, dropHead p 4, dropHead p 5] Now we need to figure out where to get the second argument for each of the dropHead applications. I know! If a call to dropHead comes before another call to dropHead, we'll pass the result of the second call as the second arg of the first. And if there are no calls after a call to dropHead, we'll pass our base case as the second arg to that call. Let's have a look: threadResults basec [] = basec threadResults basec [f:fs] = f $ threadResults basec fs threadResults [] [dropHead p 1 ...] = dropHead p 1 $ dropHead p 2 $ dropHead p 3 $ dropHead p 4 $ dropHead p 5 [] Let's see it in action! First let's let our predicate determine whether a value is less than three. dropHead (< 3) 5 [] = if (< 3) 5 then [] else 5:[] => [5] dropHead (< 3) 4 [5] = if (< 3) 4 then [5] else 4:[5] => [4, 5] dropHead (< 3) 3 [4, 5] = if (< 3) 3 then [4, 5] else 3:[4, 5] => [3, 4, 5] dropHead (< 3) 2 [3, 4, 5] = if (< 3) 2 then [3, 4, 5] else 2:[3, 4, 5] => [3, 4, 5] dropHead (< 3) 1 [3, 4, 5] = if (< 3) 1 then [3, 4, 5] else 1:[3, 4, 5] => [3, 4, 5] => [3, 4, 5] Do you now see how foldr works? It starts at the beginning of the list and applies the function you gave it to the first element of the list. Then to get the second argument of the first call, it applies your function to the second element of the list and passes the result of that to the first call. In other words, it /recurs/ on the next element of the list. When we finally get to the end, the base case is passed to the final call of your function and it returns, and all the previous calls return building their results from the calls after. So in other words to implement dropWhile with foldr merely requires: dropHead p x xs = if p x then xs else x:xs dropWhile p l = foldr (dropHead p) [] l Or even simpler: dropWhile p = foldr (\x l' -> if p x then l' else x:l') [] take 3 $ dropWhile (< 5) [1..] => [5, 6, 7] No abtruse machinery required! __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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ANNOUNCE: urlcheck 0.1, (smp) parallel link checker
by dons＠cse.unsw.edu.au 13 Feb '07

13 Feb '07

This little tool has been kicking around on my harddrive for a month or two now, so time to release! I'm pleased to announce the first release of urlcheck, an parallel link checker, written in Haskell. Frustrated with the resources and time consumed by 'linkchecker', when preparing the weekly news, I coded up a lightweight, smp-capable replacement in Haskell. urlcheck pings urls found in the input file, checking they aren't 404s. It uses Haskell threads to run queries concurrently, and can transparently utilise multiple cores if you have them. Usage: $ urlcheck urlcheck.html Found 0 broken links. Checked 10 links (10 unique) in 1 file. Search time: 5 secs Get it from Hackage! http://hackage.haskell.org/cgi-bin/hackage-scripts/package/urlcheck-0.1 -- Don

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Even better Eratosthenes sieve and lucky numbers
by oleg＠pobox.com 13 Feb '07

13 Feb '07

We further simplify the previously posted genuine sieve algorithm and generalize it to the finding of lucky numbers. We observe that we only need to store marks _signifying_ the integers, but never the integers themselves. Thus we arrive at the algorithm that is distinguished from all previously posted by: (i) doing no operations on composite numbers (ii) using neither multiplication nor division nor the remainder operation (iii) using neither general addition nor the comparison. The algorithm only relies on the successor, predecessor and zero comparison. The predecessor can be easily eliminated. Thus the algorithm can be used with Church and Peano numerals, or members of Elliptic rings, where zero comparison and successor take constant time but other arithmetic operations are more involved. > -- repl_every_n n l replaces every (n+1)-th element in a list (_:l) > -- with False > repl_every_n :: Int -> [Bool] -> [Bool] > repl_every_n n l = repl_every_n' n l > where repl_every_n' 0 (_:t) = False: repl_every_n n t > repl_every_n' i (h:t) = h: repl_every_n' (pred i) t > > primes = 2:(loop 3 (repeat True)) > where loop n (False:t) = loop (succ (succ n)) t > loop n (_:t) = n:(loop (succ (succ n)) (repl_every_n (pred n) t)) > > main = putStrLn $ "Last 10 primes less than 10000: " ++ > show (take 10 $ reverse $ takeWhile (< 10000) primes) Last 10 primes less than 10000: [9973,9967,9949,9941,9931,9929,9923,9907,9901,9887] The algorithm easily generalizes to lucky numbers http://mathworld.wolfram.com/LuckyNumber.html http://www.research.att.com/~njas/sequences/A000959 > -- Consider the series of odd numbers starting with k=1; the k-th number > -- is 2k-1. > -- If we start the elimination from the very beginning of the series (k=1), > -- we start with the phase (2k-2) and eliminate with the step 2k-2. > -- If we start at the number k+1, the phase becomes (2k-2-k) = (k-2). > -- Thus the starting phase gets incremented by 1 as we move up the series. > -- However, already eliminated numbers don't count, so as we skip > -- over eliminated numbers, we increment the phase by 2 > lucky = 1:(loop 3 0 (repeat True)) > where loop n i (False:t) = loop (succ (succ n)) (succ (succ i)) t > loop n i (_:t) = n:(loop (succ (succ n)) (succ i) > (repl_every_np i (pred n) t)) > > -- repl_every_np i n eliminates (marks as False) every (n+1)-th number > -- starting with the phase i. > -- Already eliminated numbers don't count. > repl_every_np :: Int -> Int -> [Bool] -> [Bool] > repl_every_np i n (False:t) = False: repl_every_np i n t > repl_every_np 0 n (_:t) = False: repl_every_np n n t > repl_every_np i n (_:t) = True: repl_every_np (pred i) n t *Main> take 10 lucky [1,3,7,9,13,15,21,25,31,33] *Main> takeWhile (<100) lucky [1,3,7,9,13,15,21,25,31,33,37,43,49,51,63,67,69,73,75,79,87,93,99]

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IO is not a monad
by Yitzchak Gale 13 Feb '07

13 Feb '07

<troll> Prelude> let f .! g = ((.) $! f) $! g Prelude> let f = undefined :: Int -> IO Int Prelude> f `seq` 42 *** Exception: Prelude.undefined Prelude> ((>>= f) . return) `seq` 42 42 Prelude> ((>>= f) .! return) `seq` 42 42 </troll> Regards, Yitz

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Parsec and Java
by Joel Reymont 13 Feb '07

13 Feb '07

Folks, Is there a Java parser implemented using Parsec? Thanks, Joel -- http://wagerlabs.com/

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pythags
by phiroc＠free.fr 13 Feb '07

13 Feb '07

Hello, the "Advanced Monads" page in the Haskell Wikibook (http://en.wikibooks.org/wiki/Haskell/Advanced_monads) contains the following example of a List Monad pythags = do x <- [1..] y <- [x..] z <- [y..] guard (x^2 + y^2 == z^2) return (x, y, z) However, whenever you load that function definition into Hugs or GHCi, you get a message saying that "guard" is an undefined variable. Does anyone know why? Thanks. phiroc

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Re: Re[2]: [Haskell-cafe] FFI basics
by Yitzchak Gale 12 Feb '07

12 Feb '07

Bulat Ziganshin wrote: > examples of lifting C functions into Haskell world: > > mysin :: Double -> Double > mysin = realToFrac . c_mysin . realToFrac > > -- c_mysin :: CDouble -> CDouble > > rnd :: Int -> IO Int > rnd x = do r <- c_rnd (fromIntegral x) > return (fromIntegral r) > > -- c_rnd :: CInt -> IO CInt OK, got it. I'll put that in. Thanks, Yitz

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Genuine Eratosthenes sieve [Was: Optimization fun]
by oleg＠pobox.com 12 Feb '07

12 Feb '07

It has been already remarked that any algorithm of finding prime numbers that uses division or `mod` operations cannot be called (Eratosthenes) sieve. The insight of Eratosthenes is finding primes without resorting to division or multiplication. In his time, doing either of those operations was quite expensive (especially given the way numbers were written in Ancient Greece). Thus Eratosthenes devised one of the early examples of trading space for time. These days it's the other way around: it is far easier for a modern CPU to divide numbers than to access memory. So, the original algorithm is of historical interest it seems. Still, in the interest of purity, here it is, in Haskell. As the original Eratosthenes sieve, this algorithm uses only successor and predecessor operations. -- repl_every_n n l replaces every (n+1)-th element in a list (_:l) with 0 {- The following leaks memory! repl_every_n :: Int -> [Int] -> [Int] repl_every_n n l = case splitAt n l of (l1,_:l2) -> l1 ++ 0:(repl_every_n n l2) -} -- But this seems to run in constant space... repl_every_n :: Int -> [Int] -> [Int] repl_every_n n l = repl_every_n' n l where repl_every_n' 0 (_:t) = 0: repl_every_n n t repl_every_n' i (h:t) = h: repl_every_n' (pred i) t primes = 2:(loop [3,5..]) where loop l = case dropWhile (==0) l of (h:t) -> h:(loop (repl_every_n (pred h) t)) main = putStrLn $ "Last 10 primes less than 100000: " ++ show (take 10 $ reverse $ takeWhile (< 100000) primes) {- Last 10 primes less than 1000: [997,991,983,977,971,967,953,947,941,937] -} {- Last 10 primes less than 10000: [9973,9967,9949,9941,9931,9929,9923,9907,9901,9887] -} {- Last 10 primes less than 100000: [99991,99989,99971,99961,99929,99923,99907,99901,99881,99877] -}

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Re: [Haskell-cafe] Lambada and connecting Haskell to a Weblogic server
by Joel Reymont 11 Feb '07

11 Feb '07

Yep, don't have access to the Weblogic server. I'm re-evaluating my options, though, since I'm lazy by nature. On Feb 11, 2007, at 12:30 PM, Neil Bartlett wrote: > Joel, > > Implementing Java RMI in Haskell sounds like a nightmare. Why not > use HTTP? You could easily write a wrapper Servlet that speaks XML > or JSON over HTTP, and deploy that to the Weblogic server. Unless > you don't have permission to deploy anything to that server for > whatever reason. -- http://wagerlabs.com/

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Re: [Haskell-cafe] What's wrong with "cgi-undecidable"?
by Matthew Brecknell 11 Feb '07

11 Feb '07

<keepbal(a)gmail.com> said: > Then another problem,after I unregistered cgi-2006.9.6,the > fastcgi-2006.10.9could't work well with > cgi-1.0 . You might need fastcgi-1.0: http://www.cs.chalmers.se/~bringert/darcs/haskell-fastcgi > Actually,I was trying my best to install hope: > http://www.cs.chalmers.se/~bringert/darcs/hope and lasted for three > days, > it's really too hard. Would I be right to think that your problems are not specifically related to the cgi/fastcgi/hope code-base, but more general problems diagnosing package version conflicts? The Haskell community has been working hard to improve various aspects of the user experience. Perhaps if you describe the kinds of issues you had (and any solutions you've found), others on the list might be motivated to provide a better experience for future users.

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Beginner Question
by vishy anand 11 Feb '07

11 Feb '07

I have just started on my journey in learning Haskell.I have started off reading wikibook,then will read yet another tutorial on haskell.Please guide me if I am on right track thks

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Fwd: [Haskell-cafe] Optimization fun
by Peter Berry 11 Feb '07

11 Feb '07

Gah! Gmail has really broken defaults for posting to lists. On 10/02/07, Creighton Hogg <wchogg(a)gmail.com> wrote: > Hello Haskell-ers, > So a friend and I were thinking about making code faster in Haskell, and I > was wondering if there was a way to improve the following method of > generating the list of all prime numbers. It takes about 13 seconds to run, > meanwhile my friend's C version took 0.1. I'd love to learn a bit more > about how to optimize Haskell code. Which subproblem takes 13 seconds? (Surely generating a list of all primes will take an infinite amount of time, since there are infinitely many of them?) -- Peter Berry <pwberry(a)gmail.com> Please avoid sending me Word or PowerPoint attachments. See http://www.gnu.org/philosophy/no-word-attachments.html

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Re: [Haskell-cafe] Haskell soltion ofr I/O: is it monads or uniqueness types, after all?
by Stefan O'Rear 11 Feb '07

11 Feb '07

Gah. For the first time ever, I seem to have accidentally done a reply to sender. (IE Bulat please ignore this message) On Sat, Feb 10, 2007 at 07:25:19PM +0300, Bulat Ziganshin wrote: > Hello haskell-cafe, > > just another interesting discussion in russian forum raised such idea: > > we all say that monads are the haskell way to do i/o. is it true? may > be, uniqueness types, just like in Clean and Mercury, are real way, and > monads are only the way to write programs that use uniqueness types > easier? > > so, IO monad is like any other monad - it simplifies writing of > complex code, but by itself it don't solve any problems. all code that > can be written with monads can also be written using ordinal function > calls. we know it for IO monad too - in ghc, we can use low-level > representation of IO type and write imperative code without use of > any monad operators Just because Jhc, GHC, and Yhc (any others? any non-others?) all use RealWorld# tokens, doesn't mean it's the only way to implement IO. Note that unsafePerformIO will need to be a primitive... data Process = Getc (Char -Process) | Putc Char (() -Process) | forall a. NewIORef a (IORef a -Process) | forall a. ReadIORef (IORef a) (a -Process) | forall a. WriteIORef (IORef a) a (() -Process) | Exit {- IORef is specifically mentioned because it is the canonical example of something IO can do that 1.2-style streams can't -} type IO = Cont Process -- using Cont monad's monad getChar = Cont $ Getc putChar = Cont . Putc newIORef = Cont . NewIORef readIORef = Cont . ReadIORef writeIORef a b = Cont $ WriteIORef a b exit = Cont $ \_ -Exit {- in runtime, forgive my rusty C -} int main() { init_runtime_foo(); thunk* tree = hseval("runCont main Exit") while(1) { whnf(tree); switch (tree->tag) { case _Getc: char ch = getchar(); thunk* hch = hs_wrapchar(ch); tree = hsmkapp( tree->payload(0) , hch ); continue; ... case _Exit: return 0; } } } of course, this doesn't refute the non-primitive-ness of IO, but it does refute the uniqueness types part.

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Haskell soltion ofr I/O: is it monads or uniqueness types, after all?
by Bulat Ziganshin 11 Feb '07

11 Feb '07

Hello haskell-cafe, just another interesting discussion in russian forum raised such idea: we all say that monads are the haskell way to do i/o. is it true? may be, uniqueness types, just like in Clean and Mercury, are real way, and monads are only the way to write programs that use uniqueness types easier? so, IO monad is like any other monad - it simplifies writing of complex code, but by itself it don't solve any problems. all code that can be written with monads can also be written using ordinal function calls. we know it for IO monad too - in ghc, we can use low-level representation of IO type and write imperative code without use of any monad operators -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Embedding ghc
by Joel Reymont 10 Feb '07

10 Feb '07

Has anyone tried embedding ghc into their app? How big are the resulting binaries? Thanks, Joel -- http://wagerlabs.com/

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Query
by vishy anand 10 Feb '07

10 Feb '07

hi i am going through yaht tutorial and exercise 4.6 and 4.7..i understood 4.6,but not 4.7 in which fromTuple (One a ) = Left (Left a ) and fromTuple (Two a b ) = Left (Right (a,b) ) function r written..why use Either type..cant i just say fromTuple (Two a b )=(a,b)

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What's wrong with "cgi-undecidable"?
by keepbal 10 Feb '07

10 Feb '07

http://darcs.haskell.org/packages/cgi-undecidable/ There only 3 or 4 lines of source code, but it can't be installed. The ghc version is 6.6 . Undecidable.hs:23:0: Duplicate instance declarations: instance [overlap ok] (MonadTrans t, MonadCGI m, Monad (t m)) => MonadCGI (t m) -- Defined at Undecidable.hs:23:0 instance [overlap ok] (MonadTrans t, MonadCGI m, Monad (t m)) => MonadCGI (t m) -- Defined in Network.CGI.Monad

2 1

Random numbers without IO
by Dougal Stanton 10 Feb '07

10 Feb '07

Well, nobody likes tainting their beautiful pure code with IO, so I rewrote the Random module to take advantage of the latest research [1]: > module Random where > > getRandom = 4 Cheers, D. [1] <http://xkcd.com/c221.html> -- Dougal Stanton

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learning Haskell
by Bulat Ziganshin 10 Feb '07

10 Feb '07

Hello, i'd a discussion in russian programmer's forum (rsdn.ru) about haskell and learning it. one interesting point was that imperative programmers imagine that multi-threading programming should require special knowledges. they was really amazed when i showed the following program that creates thread and interacts with it - without locks, semaphores, synchronized access and all other old stuff -- Thread, that cyclically read commands from channel, and executes them graphThread c = do cmd <- getChan c cmd graphThread c -- Creates graphThread and sends a few commands to it for execution main = do c <- newChan forkOS $ graphThread c putChan c (print "Hi") tid <- forkOS $ putChan c (print "Hi from other thread") putChan c (print "Bye") waitThread tid i think that some example like it would be appropriate in haskell book for *novices* - just because it's easy to do in Haskell. so, i vote for adding Multi-threading chapter to such books (and it may be especially important nowadays when everyone are crazy about multi-cores :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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weak references & caches
by Thomas Conway 09 Feb '07

09 Feb '07

Hi All, I'm hacking some (external) B-Tree code, and amongst the numerous interesting problems I've come up against[*], is to do with managing which pages/nodes are in memory and which are not. We use TVars to point from one (in-memory version of a) page to another, so we have a type like the following: data Node = Leaf [(Key,Value)] | Node NodePtr [(Key,NodePtr)] type NodePtr = TVar (Maybe Address, Maybe Node) type Address = Integer Addresses are just page numbers or file offsets, or similar. NodePtr is the interesting bit - the following table summarizes what the combination of values mean: explain (Just addr, Nothing) = "An external node we have not read in yet" explain (Nothing, Just node) = "A new node for which we have not yet allocated a page" explain (Just addr, Just node) = "A node we've read in" explain (Nothing, Nothing) = "Emit nasal daemons" Now, we want a cache to hang on to nodes that we've read in to hold them in memory, to save repeatedly reading them off disk unnecessarily. But more than that, we want to make sure that when we create a NodePtr pointing to a particular Address, that all references to this particular address go through the same NodePtr, otherwise in a concurrent situation we could well end up with two copies of the page in memory, both recieving updates leading to havoc. At the same time, we'd like to be able to evict pages according to some policy (the Oracle hot-list cold-list cache is attractive, if implemented carefully). So one scheme that comes to mind is to keep a table mapping addresses to (Weak NodePtr). When we ask for a node, first we consult the cache (if we have one) and get the NodePtr right back. If the addressed node is not in the cache then we check the table to see if there is/was a known NodePtr for that address. If there is none then we can allocate a new one and stick it in the table, and if it is there then we return it. This is also nice because we can attach finalizers to the (Weak NodePtr)s to write them out if dirty. So the question is, is there a simpler scheme (though this one isn't tooooo bad), and more importantly, is this likely to be horribly slow? How efficient is dereferencing a weak reference, and what is the likely impact on performance of having 10^3-10^5 weak references around? cheers, Tom [*]Film at 11. I'm planning to write it all up, maybe for a conference, maybe just for the Monad Reader, or something.

1 1

Portable implementation of unsafeCoerce
by Alfonso Acosta 09 Feb '07

09 Feb '07

Hi all, Reading the sources of Lava (a Haskell Hardware Description DSL) I run into this definition ... unsafeCoerce :: a -> b unsafeCoerce a = unsafePerformIO $ do writeIORef ref a readIORef ref where ref = unsafePerformIO $ do newIORef undefined Is this actually equivalent to the infamous unsafeCoerce# ? Thanks in advance, Alfonso Acosta

3 2

Monolithic module tool
by Chad Scherrer 09 Feb '07

09 Feb '07

Maybe a year or so ago, I came across a tool for Haskell that takes a collection of modules, does some name-mangling, and gives a single module that only needs to export main. There were wonderful reductions in the size of the resulting executable, and potentially more optimizations available to GHC. Anyway, I can't find the link now, and I'm wondering if anyone knows of it. Thanks! Chad

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still wrestling with io
by Martin DeMello 09 Feb '07

09 Feb '07

Code here: http://zem.novylen.net/anagrid.html I've got an instance of IO appearing unexpectedly and I can't figure out where from. It throws up the following error: $ ghc --make test.hs Chasing modules from: test.hs Compiling Main ( test.hs, test.o ) test.hs:38:15: Couldn't match `StaticText ()' against `IO (StaticText ())' Expected type: StaticText () Inferred type: IO (StaticText ()) In the application `staticText p [text := (labelText a b)]' In the definition of `textOf': textOf p a b = staticText p [text := (labelText a b)] martin

3 2

How did you stumble on Haskell?
by Alexy Khrabrov 08 Feb '07

08 Feb '07

How do people stumble on Haskell? I've taught ML at UPenn, and many of my colleagues in Amazon are in SeaFunc -- switching from C funk to func funk. I've got Hudak's book a while ago, but didn't have time/excuse to delve into it until recently. Then the most fantastic chain if events triggered it: -- finally switched to Intel Mac -- got Parallels -- got a recent Linux, openSUSE 10.2, to stick into Parallels -- decided finally to try Gentoo -- found equery slow, came across Adelie/FQuery as "fast equery" -- emerge adelie To my surprise, saw it emerge GHC! Realized Adelie is a Haskell way to hack Portage. Dug all things Haskell! I also used Darcs for a long time. It probably takes several pro et contra factors to push away from boring things and to be inducted into interesting FP things. Again, UPenn focus on ML was invaluable, and Ullman's book on ML superb -- and thin! What's folks most interesting ways to arrive at FP? Cheers, Alexy

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currying vs partial application
by Steve Downey 08 Feb '07

08 Feb '07

I think I finally have it. Partial application is taking a function of N parameters, binding a value to one of them, and turning it into a function of N-1 parameters. Currying is where ask is that a function that takes two ints and returns an int, or is that a function that takes one int and returns a function that takes an int and returns an int, and the answer is yes. Currying implies partial application, but not all partial application is currying.

1 0