February 2007 - Haskell-Cafe

Summer of Code
by Bryan Burgers 15 Feb '07

15 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

5 9

Programming an Evaluation Context
by Klaus Ostermann 15 Feb '07

15 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

3 3

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.

2 3

factorising to prime numbers
by Dougal Stanton 14 Feb '07

14 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

5 4

Data.ByteString.Lazy.Char8 and finding substrings
by Magnus Therning 14 Feb '07

14 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

2 1

Calendar Dates before the epoch
by jim burton 14 Feb '07

14 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.

2 2

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

5 9

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) }

5 5

Re: [Haskell-cafe] How to solve this problem?It's quite easy in PHP.
by dons＠cse.unsw.edu.au 13 Feb '07

13 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

1 0

Suggestion for hackage
by Benjamin Franksen 13 Feb '07

13 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

2 1