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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. ____**

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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. ____**

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25 Sep '02

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by carctol＠yahoo.com 25 Sep '02

25 Sep '02

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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]

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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-----

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RE: Monad Maybe?
by Simon Marlow 24 Sep '02

24 Sep '02

> I know this has been written about way too much, but I was > wondering what > people thought about using 'liftM f' as opposed to '>>= > return . f'. I > would probably have written Andrew's code using liftM, but I > don't know if > one is necessarily better than the other. Does anyone have strong > thoughts on this? so, using liftM: (number g >>= return . show) becomes (liftM show (number g)) or (show `liftM` number g) but it's important not to get too carried away with abstractions - this example requires a bit of a trawl around the library documentation for someone not familiar with liftM. Personally, unless I was writing fragments like this a lot, I'd just write it as (do r <- number g; return (show r)) Each to his own I suppose. Cheers, Simon

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Re: Problems with IO
by ?????? ????? ????? ?????? 24 Sep '02

24 Sep '02

Now I understand that it's just a lack of knowledge... Sorry for interrupring... By the way, can you recommend a good manual which I can find on the web? -- Zhbanov Pavel

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Problems with IO
by Жбанов Павел Геннад ьевич 24 Sep '02

24 Sep '02

I'm having a problem with writing a function dealing with I/O. Maybe it's just a lack of experience or simple Haskell knowledge because I'm just a beginer. The problem: I want to write a function that converts an IO String into String lala :: IO String -> String Is it possible? If yes, how? -- Жбанов Павел

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question regarding ordering of function definitions
by David Roundy 23 Sep '02

23 Sep '02

Hello all! I've just started learning (and using haskell), and am also using the literate comment mode (latex style), and just ran into a problem that seems like it ought to have a simple solution, which is that it seems that haskell requires you to define all the variations of a function sequentially, which is raining on my literate programming parade. Here is what I'd like to do: \begin{code} data D = A String | B String instance Show D where show = showD instance Read D where readsPrec _ = readsD readD s = (readsA s) ++ (readsB s) \end{code} A is formatted like ``A string''. \begin{code} showD (A s) = "A " ++ s readsA s = [(A thestr, r) | ("A", x) <- mylex s, (thestr, r) <- mylex x] \end{code} B is formatted like ``B string''. \begin{code} showD (B s) = "B " ++ s readsB s = [(B thestr, r) | ("B", x) <- mylex s, (thestr, r) <- mylex x] \end{code} The problem I have is that ghc complains because I've split up the definition of showD with readsA defined in between. This surprised me, since it seemed that the code should still be unambiguous. Is there some nice way around this, or do I have to define a separate function for each constructor, and then have a list in one place (as I do with the readsD, in that case because pattern matching won't give me what I want), like: showD (A s) = showA (A s) showD (B s) = showB (B s) Of course, in my actual example, there are more than two constructors, and the parsing is considerably more complex, which is why I'd like to put the analagous showing and parsing right next to eachother along with a description of what the formatting is intended to be. -- David Roundy http://civet.berkeley.edu/droundy/

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