September 2002 - Haskell-Cafe

(no subject)
by Chris.Angus＠Lawson.com 17 Oct '02

17 Oct '02

Does anyone know what happened to the wiki on haskell.org? There was lots of stuff there last year (mostly to do with design patterns) which seems to have been lost. Some of the links are still there but they don't go anywhere. Cheers Chris

3 2

Dealing with configuration data
by Liyang Hu 11 Oct '02

11 Oct '02

Evening, I'm trying to write a utility that reads in some user preferences from a pre-determined file, does some work, and exits. Sounds simple enough. The problem I'm having is with the preferences: How do I make it available throughout the entire program? (FWIW, most of the work is effectively done inside the IO monad.) I could explicitly pass the record around everywhere, but that seems a trifle inelegant. My current solution is to use a global ('scuse my terminology, I'm not sure that's the right word to use here) variable of type IORef Config obtained through unsafePerformIO. It works, but strikes me as a rather barbaric solution to a seemingly tame enough problem... Intuition tells me I should be able to `embed', if you will, the config record somehow within or alongside the IO state, and retrieve it at will. (Is this what MonadState is for?) However it also tells me that this will /probably/ involve lots of needless lifting and rewriting of the existing code, which makes it even less enticing than passing everything around explicitly. Any opinions or suggestions? Cheers, /Liyang -- .--{ Liyang HU }--{ http://nerv.cx/ }--{ Caius@Cam }--{ ICQ: 39391385 }--. | :: zettai unmei mokusiroku :::: absolute destined apocalypse ::::::::: |

8 17

E-MaiL Bankas�
by Hayri DALDAL 30 Sep '02

30 Sep '02

<html> <head> <title>T�rk E-Mail Bank</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-9"> <meta http-equiv="Content-Type" content="text/html; charset=windows-1254"> </head> <body bgcolor="#FFFFFF" text="#000000" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0"> <table width="630" border="0" cellspacing="0" cellpadding="0" align="center" background="http://cdreklam.daldal.com/backchip2.jpg"> <tr> <td valign="middle"> <blockquote> <div align="right"></div> T�rk E-Mail Bank � patentli, "E-Mail Adres Rehberleri"miz g�ncellenmi�tir. Sekt�rler, iller, kurum ve kurulu�lar, e�ilim ve ilgiler baz�nda kategorize edilmi� E-Mail Adres Rehberleri'mizle 5 Milyon yerli Internet kullan�c�s�na "bir anda" ula�abilir; reklam, tan�t�m, duyuru, ilan ve her t�rl� kampanyalar�n�z� g�nderebilirsiniz. 5.000.000 YERLi E-MAIL REHBERi 150 $ Saatte 80.000 mail yollayabilen Group Mail program� hediyeli Gazete, Radyo ve Tv'lerin Traj ve Reatingiyle Reklam Fiyatlar�yla Kar��la�t�r�n Hangisi Karl� ! Web: <a href="http://cdreklam.daldal.com" target="_blank">cdreklam.daldal.com</a> Daha farkl� soru, g�r��, teklif ve sipari�leriniz i�in bize ula�abilirsiniz. Web Tasar�m�, Domain tescili ve Hosting fiyatlar�m�z� mutlaka sorun. "KaaN Ajans Organizasyon�" Hayri DALDAL (Koordinator)  TEL: 0312 440 78 65(PBX) E-MaiL: <a href="mailto:hayri@daldal.com">hayri(a)daldal.com</a> Bu ileti 5 Milyon Turk Internet kullan�c�s�n�n e-mail adresine g�nderilmi�tir. </blockquote> </td> </tr> </table> </body> </html>

1 0

RE: Dealing with configuration data
by Simon Peyton-Jones 26 Sep '02

26 Sep '02

| Sorry, I should also mention implicit parameters, if you're willing to use | that extension. I don't like them, though, and my impression from SPJ is | that it's very unclear whether they will get into Haskell 2 or not... It's *linear* implicit parameters that are a weird beast. Ordinary implicit parameters are quite well behaved. Having said that, there is no Haskell 2 process at the moment... Simon

2 1

Re: Pure File Reading (was: Dealing with configuration data)
by Yoann Padioleau 26 Sep '02

26 Sep '02

Lars Lundgren <d95lars(a)dtek.chalmers.se> writes: > On 26 Sep 2002, Yoann Padioleau wrote: > > > Koen Claessen <koen(a)cs.chalmers.se> writes: > > > > i find your idea very good. > > indeed for the library GetOpt, the argument of a program never change so it > > make sense to make this library without using IO monad, same for argv and for the enviroment. > > > for openFile it seems harder, it would require to have a lock on the file > > to be sure that no one modify the file, same for a webpage. > > > > No, not at all. All that is required is to make sure all calls to openFile > f returns the same string, i.e the contents of the file when it was read > the first time. yes you are right. but it requires to change the name of the function to have Once appended to it. so in the Prelude, we should have 2 version of the same kind of fonction, one pure such as readFileOnce with a cool type, and one such as readFile with an Io type. Sounds good idea to me, cos many programs require just to read a file one time. i dont know if practically it is useful. after all we can do all the IO stuff in the main, and then continue in a pure world. dont know if it is useful to read a file in the middle of a pure function. > > /Lars L > > > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

1 0

Re: Pure File Reading (was: Dealing with configuration data)
by Yoann Padioleau 26 Sep '02

26 Sep '02

Koen Claessen <koen(a)cs.chalmers.se> writes: i find your idea very good. indeed for the library GetOpt, the argument of a program never change so it make sense to make this library without using IO monad, same for argv and for the enviroment. for openFile it seems harder, it would require to have a lock on the file to be sure that no one modify the file, same for a webpage. > Dear all, > > At the moment, a discussion on haskell-cafe is going on > about how to neatly program the fact that an entire program > depends on a number of parameters that are read in once at > the beginning of a program. > > The suggestion that many people came up with was using > unsafePerformIO in the beginning to read the file. Here is > my version of that: > > | data Configuration = ... -- config data > | > | getConfig :: Configuration > | getConfig = unsafePerformIO $ > | do ...read configuration from file... > | return configuration > | > | main = > | do doStuff > > It is quite disturbing that there is no other easy way to do > this than using unsafePerformIO (except for using implicit > parameters perhaps, but there are other reasons for not > using those). > > I have been thinking a little bit more about this and here > is what I found. > > Remember the Gofer days, when Gofer had a "function": > > openFile :: FilePath -> String > > This was of course a cheap and dirty way of implementing > things like the getConfig above, but it is impure. However, > one could imagine a functional version of this function: > > readFileOnce :: FilePath -> Maybe String > > This function will read the contents of the file (and return > Nothing if something went wrong), but it is memoized, so > that the second time you use this function you get the same > result. > > So, it is a pure function. (Admittedly, it is somewhat > unpredictable, but you will always get the same result for > the same arguments.) It is no more strange than GHC's pure > version of the getArgs function (I forgot what it was/is > called). > > How about space behavior, you say? Reading a file, and > memoizing the result means storing the whole contents of the > file in memory! > > The point is that the use of this function will typically > happen at the beginning of a program, when reading the > configuration file(s). When all this has happened, the > function readFileOnce, and its memo table, will be garabage > collected. (Of course there is no guarantee that all calls > to readFileOnce will be evaluated at the beginning of a > program, and it is not required, but when you do, there are > no space problems.) > > There could of course be pure "-Once" versions of other IO > operations. Here is a list of possibilities: > > - reading a file > - getting arguments > - getting environment variables > - downloading a webpage > - ... > > What do you think? > > Regards, > /Koen. > > -- > Koen Claessen > http://www.cs.chalmers.se/~koen > Chalmers University, Gothenburg, Sweden. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

2 1

Low Price Fags
by Sales Department 25 Sep '02

25 Sep '02

Dear Sir or Madam In the past you have requested information on discounted products. We hope that you find this of interest. If you are not a smoker, and find this email offensive, we sincerely apologise! We will be only too happy to take you off our mailing list. If you are a smoker, however, and are fed up with paying high prices for your cigarettes and tobacco, take a look at what we have to offer by clicking on this link. http://www.smokersassociation.co.uk/?S=15&ID=2 We can send you, legally, by registered air mail, direct to your door, 4 cartons of cigarettes or 40 pouches of rolling tobacco (all brands are available) from only 170 Euros - about 105 pounds - fully inclusive of postage and packing. Why pay more? To remove yourself from our mailing list, please click below mailto:unsubscribe@smokersassociation.co.uk Yours faithfully. Smokers Association http://www.smokersassociation.co.uk/?S=15&ID=2 xay5402731y

1 0

Kanser tedavisi
by carctol＠yahoo.com 25 Sep '02

25 Sep '02

charset="US-ASCII"; charset="US-ASCII"; charset="US-ASCII"; charset="US-ASCII" Reply-To: carctol(a)yahoo.com Date: Thu, 26 Sep 2002 00:12:08 +0300 X-Priority: 3 X-Library: Indy 9.00.10 X-Mailer: Foxmail Kemoterapinin yan etkileri nedeniyle yorgun ve bitkin d�sm�s bir yakininiz mi var? Yanitiniz evetse vakit kaybetmeyin ve CARCTOL'u denemesini saglayin. Genelde ancak klasik tedavinin ise yaramadiginin anlasilmasi, insanlari baska se�enekler aramaya itmektedir, ancak o durumda maalesef yapacak fazla birsey kalmamaktadir. Ayni hatanin yapilmasina izin vermeyin ve hi�bir yan etkisi olmayan, Himalaya Dagi bitkilerinden �retilen Carctol'u denemesini saglayin. CARCTOL, birka� bin yillik, alternatif Hint Tibbinin (Ayuverda'nin) insanliga sundugu bir umut isigidir. �nceleri v�cudu takviye edici tonik olarak kullanilan bu �r�n simdi yeni form�l�yle kansere karsi etkili bir ila�tir L�tfen asagidaki siteye ugrayin ve Carctol'u yakindan taniyin. http://www.kanser-tedavisi.com esenlikler dileriz.. Listeden adinizin silinmesini istiyorsaniz a_cengiz2002(a)yahoo.com a bos e posta yollayin. Tesekk�rler

1 0

layout problem
by Γιώργος Κοσμίδης 25 Sep '02

25 Sep '02

Thanks for your answer. The lines where as you asnwered me. When I post my mail the layout was tottally ruined. Here I post the program again but I have replaced all spaces with underscores ( _ ) This is the error that I have: ERROR "c:\textanalysis.HS":48 - Syntax error in declaration (unexpected `->') Thanks in advance for all the help that you can give me module Main where import IO import System import List import Maybe import Char import Numeric type Word=String type WordCount=Int type WordTup=(word,wordCount) type WordList=[WordTup] main=do args<-getArgs ________textLines<-getText ________switchArgs args textLines ________where ________switchArgs args textLines ________________________|(isMemberOf "-f" args) && (isMemberOf "-i" args) _________________________=printFreq(sortWordCount(parseLcLine textLines)) ________________________|(isMemberOf "-f" args) _________________________=printFreq(sortWordCount (parseLine textLines)) ________________________|(isMemberOf "-i" args) _________________________=printFreq(sortWordCount (parseLcLine textLines)) ________________________|otherwise _________________________=printFreq(sortName (parseLine textLines)) ________getText::IO String ________getText=do c<-getChar ___________________e<-isEOF ___________________if (e) _______________________then return "" else _______________________(do nc<-getText ___________________________return (:nc)) ________parseLine::String->Wordlist ________parseLine line __________________|(line/="")&&(isWantedChar (headline)) ___________________=fillWordList w(parseLine newLine) __________________|(line/="")&&(not(isWantedChar (headline))) ___________________=parseLine(tail line) ________isMemberOf::String->[String]->Bool ________isMemberOf arg args ___________________=[]/=[a|a<-args,a==arg] ________printFreq::WordList->IO() ________printFreq wl=do sequence (map putWordStat wl) ________________________where ________________________putWordStat(Word,WordCount)->IO() ________________________putWordStat(w,wc) ________________________do putStr w ___________________________putSpc (20 (length w) (length(show wc))) ___________________________putStr (show wc) ___________________________putSpc 8 ___________________________putStr (showFFloat (Just 2) (fromIntegral (wc) * 100.0/fromIntegral (wLength))"") ___________________________putChar '%' ___________________________putChar '\n' ________________________return() ________________________wlLength-countWords wl; ________putSpc::Int->IO() ________putSpc n _______________|n>1 do putChar '' _______________________putSpc (n-1) ________________return() _______________|otherwise=putChar '' ________countWords:Wordlist->Int ________countWords [ ] ________countWords ((w,wc):wl)=wc+countWords wl ________sortName::WordList->WordList ________sortName [ ] = [ ] ________sortName (wn:ws)=sortName [(w,wc) (w,wc) <-ws,not (isGreater (w,wc)wc)] __________________++[wn]++sortName[(w,wc)|(w,wc)<-ws, isGreater (w,wc) wn] __________________where __________________isGreater (wl,wcl) (w2,wc2) _____________________________|ord(head(wl))-ord(head(x2))>0=True _____________________________|ord(head(wl))-ord(head(x2))<0=False _____________________________|length wl>l && length w2>l ______________________________=isGreater(tail(wl),wcl) (tail(w2,wc2) _____________________________|otherwise = [] _____________________________where _____________________________w=parseWord line _____________________________newline=drop (length w) line ________parseLcLine::String->WordList ________parseLcLine line ____________________|(line/="") && (isWantedChar (head line))=fillWordList w (parseLcLine newline) ____________________|(line/="") && (not (isWantedChar (head line)))=parseLcLine (tail line) ____________________|otherwise = [] ____________________where ____________________w=map toLower (parseWord line) ____________________newline=drop (lenfth w) line ________parseWord::Word->Word ________parseWord w __________________|(w/="") && (isWantedChar (head w))=(head w): parseWord (tail w)) __________________|otherwise = "" ________isWantedChar::Char->Bool ________isWantedChar c _____________________| (c=='a'|| c=='b'|| c=='c'|| c=='d'|| c=='e'|| c=='f' || c=='g'|| c=='h'|| c=='i'||__c=='j'|| c=='k'|| c=='l'|| c=='m'|| c=='n'|| c=='o'|| c=='p'|| c=='q'|| c=='r'|| c=='s'|| c=='t'|| c=='u'|| c=='v'|| c==' w'|| c=='x'|| c=='y'|| c=='z'||c=='A'|| c=='B'|| c=='C'|| c=='D'|| c=='E'|| c=='F'|| c=='G'|| c=='H'|| c=='I'||__c=='J'|| c=='K'|| c=='L'|| c=='M'|| c== 'N'|| c=='O'|| c=='P'|| c=='Q'|| c=='R'|| c=='S'|| c=='T'|| c=='U'|| c=='V' || c=='W'|| c=='X'|| c=='Y'|| c=='Z'|| = True) _____________________|otherwise = False _______fillWordList::Word->WordList->WordList _______fillWordList w wordlistl ____________________|cWord/= [] =cWord++[wordl,wordCountl)|(wordl,wordCountl<-wordListl,wordl/=w] ____________________|otherwise = (w,l):wordListl ____________________where ____________________cWord=[(wordl,(wordCountl-l)) | (wordl,wordCountl)<-wordListl,wordl==w] __________________________________________________|length w1>1 && length w2==1=True __________________________________________________|otherwise = False ________sortWordCount::WordList->WordList ________sortWordCount [] = [] ________sortWordCount ((wn,wcn):ws=sortWordCount [(w,wc|cw,wc)<-ws,ws<=wvn++ _______________________[(wn,wcn)]++sortWordCount [(cw,wc)|(w,wc)<-ws,wcs,wcn]

4 5

Monad Maybe?
by Russell O'Connor 24 Sep '02

24 Sep '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 [To: haskell-cafe(a)haskell.org] Is there a nicer way of writing the following sort of code? case (number g) of Just n -> Just (show n) Nothing -> case (fraction g) of Just n -> Just (show n) Nothing -> case (nimber g) of Just n -> Just ("*"++(show n)) Nothing -> Nothing - -- Russell O'Connor roconnor(a)math.berkeley.edu <http://www.math.berkeley.edu/~roconnor/> ``Any time you skip a commercial [...] you're actually stealing the programming'' -- Jamie Kellner, CEO of TBS -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (SunOS) Comment: For info see http://www.gnupg.org iD8DBQE9jM7juZUa0PWVyWQRAjmlAJ9+YDXUEXEhLTtb2huBZKskJ9JvegCgjPb2 iGb/vYYLK7I+UdfijtEePyQ= =6UZh -----END PGP SIGNATURE-----

8 8