- Haskell-Cafe - Haskell.org

date sorting II
by Michael Ruth 11 Mar '02

11 Mar '02

I wrote to the group earlier this week regarding a haskell script to sort dates from input. Someone said: Why don't you write this up and send it to the list so that we can look at code rather than English text? So Here's the code... It is my first Haskell program so please forgive me if I broke any rules. I have been trying to debug it but keep running into walls, any and all help will be appreciated. import IO -- not really sure how to use globals variables. --myfile :: IO Handle myList :: [String] myList = [] --myDates :: [[String]] -- start is the main flow of the program. Each output will be used by -- the input of the next line or so I tried anyway. This was done so -- at the end I could compose the functions f1 . f2 . f3 ... start:: IO() start = do { putStr "Enter a filename: "; theFile <- getLine; myfile <- openFile theFile ReadMode; myList <- getInput myfile; myDates <- stripSpaces myList; myDates <- qSort myDates; myList <- formatOutput myDates; writeFile "output.txt" (getOutput (myList)); } -- get input was designed to suck in the input strings line by line -- into a list until there are no more lines -- Not at all sure how to do this I wallowed through -- example after example. getInput :: Handle -> [String] getInput myfile = while (do res <- hIsEOF myfile return (not res)) (do line <- hGetLine myfile return (line:myList)) -- Strip spaces turns the input strings into a list of strings -- so its [day, month, year] each of these being Int's -- this is done for easier sorting later on (or to make it possible) stripSpaces :: [String] -> [[String]] stripSpaces x = map words x -- The standard basic quicksort to be done using multidimensional -- lists. Never really got to this point yet, although I may -- rearrange this function to compare strings of the form... -- yyyy mm dd which should sort correctly then rearrange them -- once done for printing. qSort :: [[String]] -> [[String]] qSort [[]] = [[]] qSort ([x]:[xs]) = qSort elts_lt_x ++ [[x]] ++ qSort elts_greq_x where elts_lt_x = [[y] | [y] <- [xs], [y] < [x]] elts_greq_x = [[y] | [y] <- [xs], [y] >= [x]] -- this function puts each of the lists back into string form -- to format for output formatOutput:: [[String]] -> [String] formatOutput [n] = map unwords [n] -- This getoutput f(x) is designed to create the string to output to -- the file getOutput:: [String] -> String getOutput [] = [] getOutput (x:xs) = show x ++ "\n" -- -- this "while" function allowed me to write the above, it was in an -- example. while :: IO Bool -> IO () -> IO () while test action = do res <- test if res then do action while test action else return () The end. I am extremely lost and any and all help would be greatly appreciated. Michael _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.msn.com

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MonadCont, MonadFix
by Ashley Yakeley 09 Mar '02

09 Mar '02

Are there any useful monads that are instances of both MonadCont and MonadFix? I can't make the two meet. Perhaps this is because continuations have no fixed point, or something. Very annoying. -- Ashley Yakeley, Seattle WA

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Debugging Haskell
by Jerry, JiJie 09 Mar '02

09 Mar '02

Good day everyone, I was fiddling around with this tiny echo client/server haskell program from 'The Great Language Shootout' site (http://www.bagley.org/~doug/shootout/) and got stuck. The code (attached) has been reformatted with minimal API tweak (mkPortNumber, writeSocket, readSocket) to please my ghc-5.02.2, and all what I get is something stuck forever after the first iteration: $ ./echo 3 Client wrote: Hello there sailor Server recv: Hello there sailor Server read: Hello there sailor Server wrote: Hello there sailor After adding all these print statement, I still don't have a clue what's jammed there. Hope someone here can shred some light. BTW, I'd also like to take this chance to ask how to debug a haskell program in general? Regards, Jerry

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Question: Database access in Haskell
by yang dianne 07 Mar '02

07 Mar '02

Dear Sir/Madam, I am considering implement a system using Haskell. I need to use database to manage data there. How can I access the data records in database from Haskell? Thanks for your advice! Best Regards, Dianne __________________________________________________ Do You Yahoo!? Try FREE Yahoo! Mail - the world's greatest free email! http://mail.yahoo.com/

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Rectification Hugs bug
by Rijk J. C. van Haaften 07 Mar '02

07 Mar '02

Dear all, After intensive investication of several people here at Utrecht University, these are the results 1. The very latest Hugs version doesn't have the bug 2. All before-december-2001 versions don't have the bug I were using a version downloaded some weeks ago. After installing the current distribution, the problems disappeared Therefore, I suspect there have been one or more bug-fix updates after December 2001, fixing this problem. However, I couldn't find that documented on the Hugs site. Can anyone confirm such an update? Where can I find last- minute distribution information? Thanks, Rijk-Jan

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Bug in Hugs Dec 2001: type inferencer incorrect!
by Rijk J. C. van Haaften 07 Mar '02

07 Mar '02

Hello everybody, Although I didn't manage to reproduce the bug with a minimal example, I think it is still important enough to tell you (and especially the Hugs writers and maintainers). Yesterday evening, I tried to add some correct (!) code (by-hand-verifyable; by GHC accepted; just using plain Haskell 98) to one of the projects I'm working on. Hugs however said this: Type checking ERROR "TypeInferDecls.hs":102 - Type error in application *** Expression : checkedUnify (snd newTyEnv) inferredHpPtr explicitHpPtr (\t1 t2 ue -> ([l],ExplicitError n t1 t2 ue)) *** Term : checkedUnify *** Type : NonGenerics -> HpPtr -> HpPtr -> ErrorFunction -> TI Error () *** Does not match : a -> b -> c -> d -> e *** Because : types do not match I've read this ten times, checked my code looking whether I were doing strange things, but I can only conclude: This is, even without a minimal example, clearly a bug: the types do match. Moreover, the bug is reported about a completely different part of the file (relative to the location I edited), in code almost unrelated to the code I added (about line 300). I have explicit types everywhere in that file, so if it were wrong, the error should be reported about line 300. So the reporting-place is wrong (but I don't expect Hugs to report good error-placement). Though I didn't manage to write a minimal example, I hope our Hugs experts are able to find the bug soon. As a last point: the file is rather big: 726 lines of code, 27KB. Rijk-Jan van Haaften

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Re: [xml-dev] ANNOUNCE: HXML 0.2, XML parser for Haskell
by postmaster＠bol.net.in 06 Mar '02

06 Mar '02

Mail not delivered to darpeshp(a)bol.net.in. Quota exceeded. Try sending later.

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ANNOUNCE: HXML 0.2, XML parser for Haskell
by Joe English 06 Mar '02

06 Mar '02

Announcing HXML version 0.2, a non-validating XML parser written in Haskell. It is designed for space-efficiency, taking advantage of lazy evaluation to reduce memory requirements. HXML is available at: <URL: http://www.flightlab.com/~joe/hxml > The current version is 0.2, and is pre-beta quality. HXML has been tested with GHC 5.02, NHC 1.10, and various recent versions of Hugs 98. Changes in version 0.2: + New Arrow-based combinator library + Added support for CDATA sections + New function parseDocument recognizes (and ignores) the document prolog (XML and DOCTYPE declarations) + Several data structures and public functions have been renamed + Space fault in comment parsing fixed Please contact Joe English <jenglish(a)flightlab.com> with any questions, comments, or bug reports. --Joe English jenglish(a)flightlab.com

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Re: Haskell-Cafe digest, Vol 1 #342 - 1 msg
by Zhe Fu 06 Mar '02

06 Mar '02

I want to read a file's content into a string. content :: FilePath -> String content f = readFile f But when there is a type error: Type error in explicitly typed binding *** Term : content *** Type : FilePath -> IO String *** Does not match : FilePath -> String How can I solve it? Thanks. Zhe

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Re: how to read a file's content to a string?
by Ashley Yakeley 05 Mar '02

05 Mar '02

At 2002-03-05 14:43, Zhe Fu wrote: >content :: FilePath -> String >content f = readFile f You have the wrong type signature for 'content'. This should solve it: content :: FilePath -> IO String content f = readFile f Of course, you'll need to change whatever uses 'content'. -- Ashley Yakeley, Seattle WA

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