- Haskell-Cafe - Haskell.org

IO and syntax
by Jyrinx 13 Mar '02

13 Mar '02

I just got back from a high-school programming competition where the only allowed languages were C[++] and Java (*grmbl*). To see just how cool Haskell is, I'm redoing some of the problems in this more enlightened language :-) Anyway, that explains the rather strict requirements for my program's interface. At one point, the user must enter pairs of numbers like so: 1 4 2 4 3 2 4 1 4 3 0 0 (where two zeros terminate the input). Here is my latest attempt at a function to do this part of the problem (and it's the easy part ...) --- snip --- -- type Player = Int readIncompatList :: IO [(Player, Player)] readIncompatList = readIL' [] where readIL' ans = do a <- readDec b <- readDec if a == 0 && b == 0 then ans else readIL' ((a, b):ans) --- snip --- GHC complains with the following: --- snip --- prob13.hs:37: Couldn't match `String -> t' against `[t1]' Expected type: String -> t Inferred type: [t1] Probable cause: `(:)' is applied to too many arguments in the call ((:) (a, b) ans) In the first argument of `readIL'', namely `((a, b) : ans)' --- snip --- Of course, this "probable cause" is ridiculous at face value ... anyway, my code makes as much sense to me as it can at the moment ... how should I get it to make sense to GHC? (I couldn't find any examples on something as mundane as console I/O; funny how people are so fixated on the real stuff :-) ) Thanks! Jyrinx jyrinx_list(a)mindspring.com

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by Christine Hall 12 Mar '02

12 Mar '02

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RE: Hiring Haskell programmers
by Konst Sushenko 12 Mar '02

12 Mar '02

I have always been wondering what exactly does "quickly learn Haskell" mean? Quickly learn Haskell syntax? Can one learn how to paint quickly? konst > -----Original Message----- > From: Eray Ozkural [mailto:erayo@cs.bilkent.edu.tr] > Sent: Monday, March 11, 2002 4:24 PM > To: Mark Carroll; haskell-cafe(a)haskell.org > Subject: Re: Hiring Haskell programmers > > > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On Tuesday 12 March 2002 01:17, Mark Carroll wrote: > > How easy is it to hire reasonable Haskell programmers? Of > course, this may > > mean, hiring people with the aptitude and interest to quickly learn > > Haskell. Has anyone any experience of this that they can share? > > > > I know it's easy. Try me. ;) > > Sincerely, > > - -- > Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> > Comp. Sci. Dept., Bilkent University, Ankara > www: http://www.cs.bilkent.edu.tr/~erayo > GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F > 7C07 AE16 874D 539C > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.0.6 (GNU/Linux) > Comment: For info see http://www.gnupg.org > > iD8DBQE8jUqcfAeuFodNU5wRAt1/AKCaN881FMXqzpx+xh1EpnFa/b6k9ACeNuPX > aYrkL3CfiF2C6uBzH+3chPs= > =U7Xm > -----END PGP SIGNATURE----- > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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RE: Hiring Haskell programmers
by Ashley Yakeley 12 Mar '02

12 Mar '02

At 2002-03-11 17:05, Mark Carroll wrote: >When I first learned Standard ML, after years of imperative >programming, my brain almost hurt for the first few weeks. For me the difference is that in imperative you tell the computer what to do, whereas in FP you tell the computer what things are. -- Ashley Yakeley, Seattle WA

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Conversion/porting to "mainstream" languages
by Mark Carroll 12 Mar '02

12 Mar '02

One criticism I've received of the suggestion that we use Haskell in our business is that some particularly large clients will demand code in some 'standard' language that they know they can deal with, especially if they end up stuck with the software and we stop supporting it. How are others dealing with this? How big a project would it be to translate, say, from Haskell to C or Java? I'm guessing that you'd borrow a lot of the type-inferencing, etc. from the current compilers - how easy is their code to get to grips with? I suppose that at least something to translate Haskell to Java bytecode would be useful someday, and then you could 'decompile' back to Java, perhaps, but as we wouldn't be doing graphical stuff with Haskell anyway then C might be best. Anyhow, any thoughts or comments on this difficulty would be of interest. -- Mark

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Re: Conversion/porting to "mainstream" languages
by Ashley Yakeley 12 Mar '02

12 Mar '02

At 2002-03-11 15:16, Mark Carroll wrote: >One criticism I've received of the suggestion that we use Haskell in our >business is that some particularly large clients will demand code in some >'standard' language that they know they can deal with, especially if they >end up stuck with the software and we stop supporting it. That's very reasonable, actually. >How are others dealing with this? How big a project would it be to >translate, say, from Haskell to C or Java? Huge. IMO, you'd probably need to rewrite everything. On the plus side, Haskell is typically excellent for exploring any problem domain. So you might end up with better designed and more maintainable Java or C code than you would if you started from scratch. Or not. -- Ashley Yakeley, Seattle WA

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Re: Hiring Haskell programmers
by Ashley Yakeley 12 Mar '02

12 Mar '02

At 2002-03-11 15:17, Mark Carroll wrote: >How easy is it to hire reasonable Haskell programmers? If you're in the Seattle area, there's this one... >Of course, this may >mean, hiring people with the aptitude and interest to quickly learn >Haskell. My best guess is if they already know ML or some other FP language, learning Haskell should not be hard. Otherwise I really don't think you can learn Haskell all that quickly. Mind you, I don't think there are all that many ML programmers out there either. -- Ashley Yakeley, Seattle WA

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informacion
by ceci 11 Mar '02

11 Mar '02

Soy estudiande universitaria de la Universidad Nacional de Cordoba ( Argentina) estoy haciendo la carrera Licenciatura en Ciencias de la Computaciòn y en una de las materias (Introducciòn a los Algoritmos) nos dieron como bibliografia para poder consultar su pagina. Yo quisiera saber como puedo conseguir su libro, asi lo puedo tener para poder estudiar Desde ya muchas gracias. Saluda muy atte. Ma.Lorena Soto

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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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