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Evolution of a Haskell Programmer
by Dmitry Astapov 01 Nov '01

01 Nov '01

It's a pity that such a link is not mentioned in "Haskell Bookshelf" :) http://www.willamette.edu/~fruehr/haskell/evolution.html -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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HaXml, emeory usage and segmentation fault
by Dmitry Astapov 01 Nov '01

01 Nov '01

I have Hugs version February 2001, HaXml version 1.02 and this program: > module Main where > import XmlLib > > main = processXmlWith (invoices `o` tag "invoice") > > invoices = > html > [ hhead > [ htitle [ ("Invoices"!)] ], > hbody > [ customers `o` children `with` tag "customer" ] > ] > > customers = > cat > [ h2 [ ("Customer"!) ], > contracts `o` children `with` tag "contract" > ] > > contracts = > cat > [ h3 [ ("Id:"!), ("id"?)], > hpara [ ("Access:"!), keep /> txt ] `o` children `with` tag "access", > hpara [ ("Intl:"!), keep /> txt] `o` children `with` tag "inter" > ] This program can process following file: <?xml version='1.0'?> <invoice> <customer> <contract id='1'> <access>1</access> <inter>1</inter> </contract> <contract id='2'> <access>2</access> <inter>2</inter> </contract> </customer> </invoice> (I use "runhugs translate.hs invoice.xml invoice.html") Now increase amount of <customer>s to 10, and amount of <contract>s within each customer to 999. After that, "runhugs -h6000000 translate.hs invoice.xml invoice.html" dumps core :( What's the reason: bug in hugs, bug in HaXml, or my own bad programming techniques? -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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how to store new values in variables?
by Anders Elfgren 31 Oct '01

31 Oct '01

Hi, its me with the dungeon master-game again.. I'm in the process of making me able to move around, which I have succeeded with. Kind of. Remember the map type Pic = [[Char]] maplevel1 = ["##############", "# #", "### #", "# #", "# # #", "#P# #", "# #m #", "##############"] Now, I've made a function called s (for south) which just moves the 'P' one step down. It looks like this s :: Pic s = remap (findPos 'P' 1) (0,1) 1 findPos just finds the positin of the P, and 0,1 tells remap to move it one step on the y-axis. The remap function looks overly complicated right now so I'm not gonna put it in here, nad I dont think it really matters. What I want to do tho, is something like this s :: Pic s = maplevel1 <- (remap (findPos 'P' 1) (0,1) 1) so that it not only returns the new Pic, but also sets the maplevel1 variable to look like it should after the move. If I do putStr (unlines s) twice, it should move down two steps.. but it doesnt. We didn't learn this, well I didn't :), in the Haskell course so if someone can just give me an example of how I should do it it'd be nice.. Thanks again. The dm2.hs can be downloaded from www.dtek.chalmers.se/~srekel/ptf and yes, the draw2 function works :) ------------------------------------------------------------------------- Anders Elfgren || AKA: Srekel and Farfar Homepage: http://hem.passagen.se/srekel || Alt email: srekel(a)icqmail.com ICQ: 4596105 || AIM: Srekel - Kaka e gott || - Cs e halva mitt liv -------------------------------------------------------------------------

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Some more news about DM-haskell..
by Anders Elfgren 31 Oct '01

31 Oct '01

Hehe sure :) If I DO finish it that is :) It should actually be close to being at the state where I just have to add different images and when to display them. Then I need to work on some kind of function to allow it to take 'w','s','n'and 'e' as input, or if I can, the arrow keys. Btw, I see that when I hit reply in Pine I only send to the person that sent me the mail, not the whole list, so I'm going to make an alias for it. For those of you that don't know, I've managed to find the problem with the help of some fellow members of this list :) On Tue, 30 Oct 2001, Andre W B Furtado wrote: > Will you release your game when it is done? It seems very interesting and I > woud like to check how it will look like... > > -- Andre PS. Andre, there's a.. um.. game I guess, for Linux called Text-mode Quake. Search for it on www.google.com and you should find something interesting :D ------------------------------------------------------------------------- Anders Elfgren || AKA: Srekel and Farfar Homepage: http://hem.passagen.se/srekel || Alt email: srekel(a)icqmail.com ICQ: 4596105 || AIM: Srekel - Kaka e gott || - Cs e halva mitt liv -------------------------------------------------------------------------

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What does this error message mean
by Anders Elfgren 31 Oct '01

31 Oct '01

Short intro (skip down for the real reason for this email) : Hi, I'm a student at Chalmers and have finished the Haskell course but actually (is IS weird, but I'm pretty sure I like it... :) ) found the language to be pretty cool and decided to try and make an ascii-based Dungeon Master type game. For anyone who doesn't know what that is : Its a game in which you walk in a dungeon with corridors in a firstperson view. These are pictures that show what it looks like when you are looking down a corridor that turns right two and respectively one "moves" ahead of the current position. (Yes, I plan to make these better looking :) ) corrl_2 = ["##############", "# \\ / #", "# \\ ____ / #", "# | } #", "# |____} #", "# / \\ #", "# / \\ #", "##############"] corrl_1 = ["##############", "# \\ / #", "# \\ ______| #", "# | | #", "# |______| #", "# / | #", "# / \\ #", "##############"] This is the whole program. draw simply prints out all three pictures (including the overhead map found below) My (current) problem lies in whatPic, which returns the error Program error: {whatPic Facing2_North (1,6)} if I type in whatPic North (1,6) or just call it through the draw2 function (which is supposed to be the main drawing function, and it actually works.. Well it did yesterday until I did something :D (sorry about pine wrapping the lines.. if you're in luck you wont notice it.) import Observe draw = putStr ((unlines corrl_2) ++ (unlines corrl_1) ++ (unlines maplevel1)) data Facing2 = North | South draw2 = putStr (unlines (whatPic North (findPos 'P' 1))) findPos :: Char -> Int -> (Int,Int) findPos obj location | head (drop (location) (unlines maplevel1)) == obj = (location `div` 14,location `mod` 15) | otherwise = findPos obj (location + 1) whatPic :: Facing2 -> (Int,Int) -> Pic whatPic facin (x,y) | corridorType facin (x,y) == (2, "rTurn1") = corrl_1 | corridorType facin (x,y) == (2, "rTurn2") = corrl_2 corridorType :: Facing2 -> (Int,Int) -> (Int, String) corridorType facing (x,y) | observe "distWall" distWall facing (x,y) (x,y) == 4 && (whatIs (findPos 'P' 1) (1,-2) == ' ') = (2, "rTurn") | otherwise = (2, "rTurn") distWall :: Facing2 -> (Int,Int) -> (Int,Int) -> Int distWall North (x,y) (newx,newy) | whatIs (x,y) (x,y-1) == '#' = abs (y - newy) | whatIs (x,y) (x,y-1) == ' ' = distWall North (x,y) (x,y-1) --distWall (North) (x,y) (newx,newy) = 2 --distWall (South) (x,y) (newx,newy) = 2 whatIs :: (Int,Int) -> (Int,Int) -> Char whatIs (x,y) (xdiff,ydiff) = head (drop (x + xdiff + y + ydiff ) (unlines maplevel1)) --[1,2,3,4,5] type Pic = [[Char]] maplevel1 = ["##############", "# #", "### #", "# #", "# # #", "# # #", "#P#m #", "##############"] corrl_2 :: Pic corrl_2 = ["##############", "# \\ / #", "# \\ ____ / #", "# | } #", "# |____} #", "# / \\ #", "# / \\ #", "##############"] corrl_1 = ["##############", "# \\ / #", "# \\ ______| #", "# | | #", "# |______| #", "# / | #", "# / \\ #", "##############"] I'm sure its something simple.. I just can't make out what could be wrong right now. Thanks for any replies :) ------------------------------------------------------------------------- Anders Elfgren || AKA: Srekel and Farfar Homepage: http://hem.passagen.se/srekel || Alt email: srekel(a)icqmail.com ICQ: 4596105 || AIM: Srekel - Kaka e gott || - Cs e halva mitt liv -------------------------------------------------------------------------

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Vim syntax files for .lhs files using \begin{code} and \end{code}
by Nicholas Nethercote 30 Oct '01

30 Oct '01

Hi, The Vim syntax file for literate Haskell programs handles code lines that begin with a '>' character fine. But in GHC much of the code is defined between "\begin{code}" and "\end{code}" pairs, and that shows up as one giant comment. Does anybody know where I might be able to obtain a Vim syntax file that handles these constructs? I made a feeble attempt myself, but Vim's syntax highlighting language defeated me. Thanks. -- Nick Nethercote njn25(a)cam.ac.uk

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Re: HaXml, memory usage and segmentation fault
by Marcin 'Qrczak' Kowalczyk 27 Oct '01

27 Oct '01

27 Oct 2001 01:44:50 +0300, Dmitry Astapov <adept(a)umc.com.ua> pisze: > well, ghc-5.02 seems to dislike something inside XmlLib.hs - it > could not find interface defs file for modules IOExts .. Perhaps the option -package lang would help. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Project: creating a table using haskell
by MR NICHOLAS EDWARD SUTCH 22 Oct '01

22 Oct '01

To whome it may concern, I am currently involved in project using haskell to create a sample time table for a company. I am a newcomer to haskell and would like some help with creating an table for such a project which will print to screen. The data contained needs to be accessible in that I can take a location or a particular time from the table and individually print this to screen. Can anyone help? N. Sutch

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RE: ghci
by Simon Marlow 22 Oct '01

22 Oct '01

> The following code works fine under ghc, but produces a strange error > under ghci: > > Main> main > *** Exception: failed > Action: connect > Reason: Unknown error 141312000 > > It is deterministic - i.e I have verified that it isn't a network > fault. I'm afraid there's a bug in 5.02's SocketPrim library that causes this. The fix will be in 5.02.1, due out shortly (or pull it from CVS if you're brave :-). > Additionally, I'm a little confused about how to > import the PortNumber constructor from Socket, without > importing everything else. It's behavior seems odd. There isn't a PortNumber constructor, just a type constructor - is that what you meant? If so, doesn't just "import Socket(PortNumber)" do the right thing? Cheers, Simon

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Re: A small doubt
by Marcin 'Qrczak' Kowalczyk 22 Oct '01

22 Oct '01

Sat, 20 Oct 2001 10:58:02 +0800 (GMT-8), Saswat Anand <saswatan(a)comp.nus.edu.sg> pisze: > I am wondering why this function should not work with input [x,y] > (list with two elements) too, since third element is not referenced. What a pattern matches is independent from which of the variables it binds are used. The pattern [a,b,c], or equivalently a::b::c::[], matches lists of length 3. You can write a::b::_ to match all lists of length 2 or more. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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ghci
by Julian Assange 22 Oct '01

22 Oct '01

The following code works fine under ghc, but produces a strange error under ghci: Main> main *** Exception: failed Action: connect Reason: Unknown error 141312000 It is deterministic - i.e I have verified that it isn't a network fault. module Main(main) where import Socket import IO(hPutStr,hFlush,hGetLine) alexa_host = "iq.org." main = connectUp alexa_host 80 connectUp :: Hostname -> Integer -> IO () connectUp host port = do h <- connectTo host (PortNumber (fromIntegral port)) hPutStr h "GET / HTTP/1.0 \r\n\r\n" hFlush h line <- hGetLine h putStrLn line Additionally, I'm a little confused about how to import the PortNumber constructor from Socket, without importing everything else. It's behavior seems odd.

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A small doubt
by Saswat Anand 21 Oct '01

21 Oct '01

Hi, I am wondering why this function should not work with input [x,y] (list with two elements) too, since third element is not referenced. Why is it so eager to pattern match. fun = \list -> let [a,b,c] = list in [a,b] Thanks, Saswat

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How to compare the time-efficiency of Haskell functions?
by Dimitre Novatchev 18 Oct '01

18 Oct '01

Dear Haskell programmers, I am a complete newbie and just wrote and tested my first few Haskell functions using Hugs98. I have (just copied) the quicksort function and I wrote another one -- dvcSort, which stands for "Divide and Conquer Sort". It seems that dvcSort runs much faster in the quicksort worst case and I wanted to provide more exact timing comparison b/n the two implementations based on various input lists. The first and relatively minor problem is that it seems there's no time library in Hugs. The real reason I'm asking for help is that I don't know what exactly means to "time a function" in Haskell. If you thought about the laziness of the language, then you're right. In my concrete case quicksort outputs the first list element (I use [-10000..10000]) almost immediately, then outputs the next number with a speed approximately one per second. From the code of quicksort it is straightforward to see, that in this case every element from left to right is compared with all the rest, and being the minimal, it can be immediately output. In this case quicksort performs like finding all the minimums and outputting them, and this has an O(N^2) complexity. Therefore, it would be incorrect to state that the time for executing quicksort is the time for its producing the first element of the result list. On the other side, based on analysis of dvcSort, it can be concluded that when it produces the first element of the result, then it has already calculated all elements of the sorted list. The time it takes to output 20001 numbers also affects the correctness and credibility of any timing. I had to use a trick like this: resSort :: (Ord a) => ([a] -> [a]) -> [a] -> (a, a) resSort f xs = let z = f(xs) in (z!!0, (last z)) and I'm timing (resSort quicksort) and (resSort dvcSort) now. However, this is a trick, based on intuition and on my knowledge of these two algorithms -- the same trick is not likely to be universally applicable and useful. My question is: Is there a better and general approach for timing Haskell functions? Any thoughts and recommendations will be invaluable for me. Thanking you in advance, Cheers, Dimitre Novatchev. __________________________________________________ Do You Yahoo!? Make a great connection at Yahoo! Personals. http://personals.yahoo.com

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Support for SOAP, Xml Schema?
by Siegfried Klein 18 Oct '01

18 Oct '01

Are there any haskell modules available for SOAP or Xml Schema (instead or in addition to DTD)? -- Thanks in advance Siegfried

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please add me
by lo janus 18 Oct '01

18 Oct '01

me(a)lojanus.com

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(no subject)
by Nuno Carvalho 17 Oct '01

17 Oct '01

subscribe

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Newbie question about classes and class parameters
by Dmitry Astapov 17 Oct '01

17 Oct '01

Let's say we have infamous Set class defined this way: > class Set s a where > empty :: s a > isEmpty :: s a -> Bool Lets also say that we could define sets as lists: > data SetAsList a = SL [a] deriving (Eq,Ord,Show) > instance Set SetAsList a where > empty = SL [] > isEmpty (SL []) = True > isEmpty _ = False So far, so good. Lets define type synonim for set of integers: > type IntegerSet = SetAsList Integer And set of sets of integers: > data IntegerSuperSet = ISS (SetAsList IntegerSet) Testing in Hugs show that everything seems to be working: Main> :t ISS (SL [ (SL [1,2,3])::IntegerSet, (SL [4,5,6])::IntegerSet ]) ISS (SL [SL [1,2,3],SL [4,5,6]]) :: IntegerSuperSet However: Main> isEmpty (ISS (SL [(SL [1,2,3])::IntegerSet, (SL [4,5,6])::IntegerSet])) ERROR - Type error in application *** Expression : isEmpty (ISS (SL [SL [1,2,3],SL [4,5,6]])) *** Term : ISS (SL [SL [1,2,3],SL [4,5,6]]) *** Type : IntegerSuperSet *** Does not match : a b This is completely understood - I need to state that IntegerSuperSet is an instance of Set for isEmpty to be working. But: > instance Set IntegerSuperSet where > empty = ISS (SL []) > isEmpty (ISS (SL [])) = True > isEmpty _ = False gives me: Reading file "/tmp/bug.lhs": ERROR /tmp/bug.lhs:38 - Wrong number of arguments for class "Set" Obviously I am missing something very important here. Could someone enlighten me? -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua Information Systems Expert Office: +380-50-110-3876 Ukrainian Mobile Communications Mobile: +380-50-330-2019 GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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Re: Haskell 98 - Standard Prelude - Floating Class
by Dylan Thurston 17 Oct '01

17 Oct '01

On Mon, Oct 15, 2001 at 03:52:06PM +0200, Kent Karlsson wrote: > > Simon Peyton-Jones: > > > Russell O'Connor suggests: > > > | but sinh and cosh can easily be defined in terms of exp > > > | sinh x = (exp(x) - exp(-x))/2 > > > | cosh x = (exp(x) + exp(-x))/2 ... > > > This looks pretty reasonable to me. We should have default methods > > > for anything we can. > Mathematically, yes. Numerically, no. Even if 'exp' is implemented > with high accuracy, the suggested defaults may return a very > inaccurate (in ulps) result. Take sinh near zero. sinh(x) with x > very close to 0 should return x. With the above 'default' sinh(x) > will return exactly 0 for a relatively wide interval around 0, which > is the wrong result except for 0 itself. Hmm, on these grounds the current default definition for tanh x is even worse behaved: tanh x = sinh x / cosh x For moderately large floating point x, this will overflow. Frankly, I don't think the whole discussion matters very much; nobody who cares will use the default definitions. But remember to think about branch cuts. And why not go further? cos x = (exp (i*x) + exp (-i*x))/2 where i = sqrt (-1) etc. > In general, this is why LIA-2 (Language Independent Arithmetic, part > 2, "Elementary numerical functions", ISO/IEC 10967-2:2001) rarely > attempts to define one numerical operation in terms of other > numerical operations. That is done only when the relationship is > exact (even if the operations themselves are inexact). That is not > the case for the abovementioned operations. But it is the case for > the relationship between the complex sin operation and the complex > sinh operation, for instance. (Complex will be covered by LIA-3.) This sounds like a very interesting standard. I am constantly annoyed by ISO's attempts to hide their standards; one might wonder what the purpose is of having unavailable "standards". Is the content available somewhere? Best, Dylan Thurston

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ExitFailure 127
by Sebastian Schulz 16 Oct '01

16 Oct '01

hi. I'm trying to run an system command (wget) within a CGI script and get the following error: >Exitfailure 127 How can I get more information out of an ExitFailure x error? Can I use ioeGetErrorString? But this function doesn't work on ExitCode, which system returns. TIA sebastian

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fixed point
by Song Li 15 Oct '01

15 Oct '01

Hi, anybody can explain how Y combinator work in find fixed point ? say : Y = 'lam' f . ('lam' x . f(xx))('lam' x . f(xx)) X = Y F -> ( 'lam' x .F(xx))('lam' x . F(xx)) -> F(('lam' x . F(xx))('lam' x . F(xx))) <- How can this expression be derived ???????????? ->F X <- And why ? Thanks a lot! Song

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a newbie's question
by Song Li 15 Oct '01

15 Oct '01

Hi, there, Could anybody explain what does this type defination mean : data xxx a = xxx (a ->b) looks xxx can use itself as constructor(like tree) but change the type...... Thanks! Song

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Closed Classes
by Eric Allen Wohlstadter 12 Oct '01

12 Oct '01

Are there any plans to add closed classes as a hugs extension? This was described in the paper: Object-Oriented Style Overloading for Haskell, Mark Shields and Simon Peyton Jones; BABEL workshop '01. Eric Wohlstadter

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Re: = vs ->
by Ashley Yakeley 12 Oct '01

12 Oct '01

At 2001-10-09 11:55, Mark Carroll wrote: >What is the rationale for when Haskell demands a "=" and when it demands >a "->"? What? Example please... -- Ashley Yakeley, Seattle WA

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Re: performUnsafeIO
by Ashley Yakeley 12 Oct '01

12 Oct '01

At 2001-10-11 11:55, Sebastian Schulz wrote: >I need to extract an IO String to String. No!!! Don't do it. Make the final result of your Haskell calculation be an "IO String". Then when you have a look at it, it will be automatically executed and you'll see the String. -- Ashley Yakeley, Seattle WA

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help for exercise 4.10
by rock dwan 11 Oct '01

11 Oct '01

Iam having some difficulties doing exercise 4.10 from craft of functional programming book second edition ..is their a possible solution for this ? _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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performUnsafeIO
by Sebastian Schulz 11 Oct '01

11 Oct '01

hi. I need to extract an IO String to String. Remembering some function like 'performUnsafeIO :: IO a -> a', I searched the hugs libs, but without a match. Does a function like this still exists? Or how can I transform IO a to a? regards seb

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Re: Reasons behind the "one instance per type" limitation
by Ashley Yakeley 10 Oct '01

10 Oct '01

At 2001-10-08 09:27, Diego Dainese wrote: >what are the reasons behind the rule stating that a type must not be >declared as an instance of a particular class, more than once in the >program? It's so that the members of the class are unambiguous. -- class C t where foo :: t -> Integer instance C Bool where foo _ = 3; instance C Bool where foo _ = 5; ambiguous = foo True; -- GHC has a flag that will turn the rule off. -- Ashley Yakeley, Seattle WA

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Re: = vs ->
by Ashley Yakeley 10 Oct '01

10 Oct '01

At 2001-10-09 17:36, Mark Carroll wrote: >So, for instance, how come function definitions and guards use "=" but >lambdas and cases use "->"? It's like this: f x = fx f = \x -> fx f :: X -> FX f x = case x of x' -> fx' x'' -> fx'' Make sense? -- Ashley Yakeley, Seattle WA

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Opsss... Sorry (SuSE 7.3)
by Jorge Adriano 09 Oct '01

09 Oct '01

I just accidently sent a mail written in portuguese about SuSE 7.3. I'm really sorry, it will not happen again. My apologies J.A.

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SuSE 7.3
by Jorge Adriano 09 Oct '01

09 Oct '01

Está para sair a 22 de Outubro... até eu estou a ficar surpreendido com a evolução destes tipos. Qd penso do 1o Linux que instalei (à cerca de dois anos? - andava eu no 4o ano) sinto-me como aquelas velhas que dizem "eu ainda sou do tempo...", err... em que tinha de andar a alterar XF86Config à unha para configurar a resolução default do monitor (Lembras-te Canelas? :) De instalação para instalação a distribuição da SuSE tem ficado mais fácil de instalar e configurar, e até mais bonita. O Kernel tem-se desenvolvido bastante bem e as Desktops nem se fala. Tudo junto dá um sistema excelente. Eles tentam passar a ideia de que a distribuição é excelente para empresas, assim como familias comuns... e se há algum tempo tinha as minhas reserva, agora começo a achar que isso já não é bem assim. Vamos lá ver no que é que dá o Star Office 6, ou como (a mais longo prazo) evoluirá o KOffice - já para não falar do Hancom Office parece espetacular (mas esse é comercial) http://www.suse.com/us/products/suse_linux/i386/new_features.html http://www.suse.com/us/products/suse_linux/i386/games.html :) J.A.

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RE: Unicode support
by Karlsson Kent - keka 09 Oct '01

09 Oct '01

> -----Original Message----- > From: Ketil Malde [mailto:ketil@ii.uib.no] ... > > But as I said: they will not go away now, they are too > firmly established. > > Yep. But it appears that the "right" choice for external encoding > scheme would be UTF-8. You're free to use any one of UTF-8, UTF-16(BE/LE), or UTF-32(BE/LE). And you should be prepared for those encodings from anyone else. In some cases, like e-mail, only UTF-8 (unless encoded by base64) can be used for Unicode. > >> When not limited to ASCII, at least it avoids zero bytes and other > >> potential problems. UTF-16 will among other things, be full of > >> NULLs. > > > Yes, and so what? > > So, I can use it for file names, Millions of people do already, including me. Most of them don't even know about it. (The file system must be made for that, of course, but at least two (commonly used) file system use UTF-16 for *all* [long] file names: NTFS and HFS+. There is another file system, UFS, that uses UTF-8, with the names in normal form D(!), for all file names. (If the *standard* C file API is used, some kind of conversion is triggered.) Those file systems got it right, many other file systems are at a loss when it comes to even that simple level of I18N, rendering non-pure-ASCII file names essentially useless, or at least unreliable.) > in regular expressions, If the system interpreting the RE is UTF-16 enabled, yes, of course. > and in > whatever legacy No: in modern systems. One of the side effects of the popularity of XML is that support for both UTF-8 and UTF-16 (also as external encodings) is growing... B.t.w. Java source code can be in UTF-8 or in UTF-16, as well as in legacy encodings. Unfortunately the compiler has to be steered via a command line parameter, while having the source files self declare their encoding would be much better (compare XML). > applications that expect textual data. > > So will a file filled with image data, video clips, or plainly a > > list of raw integers dumped to file (not formatted as strings). > > But none of these pretend to be text! How is that relevant? If you're going to do anything "higher-level" with text, you have to know the encoding, otherwise you'll get lots of more or less hidden bugs. Have you ever had any experience with any of the legacy "multibyte" encodings used for Chinese/Japanese/etc.? In many of them, if you hit a byte that might be an ASCII letter, it need not be that at all, just a second byte component in the representation of a non-ASCII character. If you think every "A" byte is an "A" (an interpret them in some special way, say a (part of a) command name), you're in trouble! Often hard-to-find trouble. No-one that argues that one can take text in any "ASCII extension" and look at the (apparent) ASCII only (and everything else to be in some arbitrary extension, never affecting the processing) seems to be aware of the details of those encodings. B.t.w. video clips (and images) can and do have Unicode (UTF-16?) texts as components (e.g. subtitles). > > True. But implementing normalisation, or case mapping for > that matter, > > is non-trivial too. In practice, the additional complexity with > > UTF-16 seems small. > > All right, but if there are no real advantages, why bother? Efficiency (and backwards compatibility) is claimed from people who work much more "in the trenches" with this than I do. And I have no quarrel with that. Kind regards /kent k

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= vs ->
by Mark Carroll 09 Oct '01

09 Oct '01

What is the rationale for when Haskell demands a "=" and when it demands a "->"? Ideas that occur to me are: (a) The distinction helps the parser a lot (b) There's a semantic difference that the language's grammar is trying to express that isn't obvious to me -- Mark

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Re: Unicode support
by Ketil Malde 09 Oct '01

09 Oct '01

[Posted to haskell-cafe, since it's getting quite off topic] "Kent Karlsson" <kentk(a)md.chalmers.se> writes: >>> for a long time. 16 bit unicode should be gotten rid of, being the worst >>> of both worlds, non backwards compatable with ascii, endianness issues >>> and no constant length encoding.... utf8 externally and utf32 when >>> worknig with individual characters is the way to go. >> I totally agree with you. > Now, what are your technical arguments for this position? > (B.t.w., UTF-16 isn't going to go away, it's very firmly established.) What's wrong with the ones already mentioned? You have endianness issues, and you need to explicitly type text files or insert BOMs. An UTF-8 stream limited to 7-bit ASCII simply is that ASCII stream. When not limited to ASCII, at least it avoids zero bytes and other potential problems. UTF-16 will among other things, be full of NULLs. I can understand UCS-2 looking attractive when it looked like a fixed-length encoding, but that no longer applies. > So it is not surprising that most people involved do not consider > UTF-16 a bad idea. The extra complexity is minimal, and further > surfaces rarely. But it needs to be there. It will introduce larger programs, more bugs, lower efficiency. > BMP characters are still (relatively) easy to process, and it saves > memory space and cache misses when large amounts of text data > is processed (e.g. databases). I couldn't find anything about the relative efficiencies of UTF-8 and UTF-16 on various languages. Do you have any pointers? From a Scandinavian POV, (using ASCII plus a handful of extra characters) UTF-8 should be a big win, but I'm sure there are counter examples. -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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Reasons behind the "one instance per type" limitation
by Diego Dainese 08 Oct '01

08 Oct '01

Hi, what are the reasons behind the rule stating that a type must not be declared as an instance of a particular class, more than once in the program? Thanks in advance, -- Diego To reply remove the 2 `x' from the address.

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Re: UniCode
by Dylan Thurston 08 Oct '01

08 Oct '01

On Fri, Oct 05, 2001 at 11:23:50PM +1000, Andrew J Bromage wrote: > G'day all. > > On Fri, Oct 05, 2001 at 02:29:51AM -0700, Krasimir Angelov wrote: > > > Why Char is 32 bit. UniCode characters is 16 bit. > > It's not quite as simple as that. There is a set of one million > (more correctly, 1M) Unicode characters which are only accessible > using surrogate pairs (i.e. two UTF-16 codes). There are currently > none of these codes assigned, and when they are, they'll be extremely > rare. So rare, in fact, that the cost of strings taking up twice the > space that the currently do simply isn't worth the cost. This is no longer true, as of Unicode 3.1. Almost half of all characters currently assigned are outside of the BMP (i.e., require surrogate pairs in the UTF-16 encoding), including many Chinese characters. In current usage, these characters probably occur mainly in names, and are rare, but obviously important for the people involved. > However, you still need to be able to handle them. I don't know what > the "official" Haskell reasoning is (it may have more to do with word > size than Unicode semantics), but it makes sense to me to store single > characters in UTF-32 but strings in a more compressed format (UTF-8 or > UTF-16). Haskell already stores strings as lists of characters, so I see no advantage to anything other than UTF-32, since they'll take up a full machine word in any case. (Right?) There's even plenty of room for tags if any implementations want to use it. > See also: http://www.unicode.org/unicode/faq/utf_bom.html > > It just goes to show that strings are not merely arrays of characters > like some languages would have you believe. Right. In Unicode, the concept of a "character" is not really so useful; most functions that traditionally operate on characters (e.g., uppercase or display-width) fundamentally need to operate on strings. (This is due to properties of particular languages, not any design flaw of Unicode.) Err, this raises some questions as to just what the "Char" module from the standard library is supposed to do. Most of the functions are just not well-defined: isAscii, isLatin1 - OK isControl - I don't know about this. isPrint - Dubious. Is a non-spacing accent a printable character? isSpace - OK, by the comment in the report: "The isSpace function recognizes only white characters in the Latin-1 range". isUpper, isLower - Maybe OK. toUpper, toLower - Not OK. There are cases where upper casing a character yields two characters. etc. Any program using this library is bound to get confused on Unicode strings. Even before Unicode, there is much functionality missing; for instance, I don't see any way to compare strings using a localized order. Is anyone working on honest support for Unicode, in the form of a real Unicode library with an interface at the correct level? Best, Dylan Thurston

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Re: Hashtable ADT
by John Hughes 06 Oct '01

06 Oct '01

> As I understand from the concepts of Functional Programming, it is not > possible to implement a Hashtable ADT in Haskell language, where one can > insert, and access values in O(1) complexity. It has to be implemented > with an external language. I don't know if it can be done in standard Haskell 98, but it can certainly be done using extensions provided by most or all implementations (IORef, IOArray). There is no need of using an external language, although it will not fit well the functional style. Better is to use the ST monad (which provides creation, reading, and writing of arrays and references in constant time). Side-effects in the ST monad can be encapsulated using runST :: (forall s. ST s a) -> a You give it a computation using updateable state as an argument, a "state thread", and it gives you the result as a pure value. The "forall s" is a typing trick which prevents an encapsulated state thread from referring to references belonging to a different state thread. Using this you can, for example, create a function share :: Hashable a => [a] -> [a] which takes a list of hashable values, and returns an equal list, in which equal values are always represented by the same pointer. Internally, there's a hash table which lets you map each element of the input to a unique representative. You could compose share with a lexical analyser, for example, to share all the copies of the same identifier. Encapsulated stateful operations like this fit very nicely with the functional style! Take a look at "State in Haskell", John Launchbury and Simon Peyton Jones, http://www.cse.ogi.edu/~jl/Papers/stateThreads.ps which explains all of this at length. John Hughes

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Re: Hashtable ADT
by Marcin 'Qrczak' Kowalczyk 06 Oct '01

06 Oct '01

Sat, 6 Oct 2001 14:10:05 +0300, Cagdas Ozgenc <co19(a)cornell.edu> pisze: > As I understand from the concepts of Functional Programming, it is not > possible to implement a Hashtable ADT in Haskell language, where one can > insert, and access values in O(1) complexity. It has to be implemented > with an external language. I don't know if it can be done in standard Haskell 98, but it can certainly be done using extensions provided by most or all implementations (IORef, IOArray). There is no need of using an external language, although it will not fit well the functional style. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Hashtable ADT
by Cagdas Ozgenc 06 Oct '01

06 Oct '01

Hi, As I understand from the concepts of Functional Programming, it is not possible to implement a Hashtable ADT in Haskell language, where one can insert, and access values in O(1) complexity. It has to be implemented with an external language. Is my argument correct? Thanks for taking time.

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Re: solutions to excercise
by S.J.Thompson 06 Oct '01

06 Oct '01

Sorry about this, but I have no online solutions. Simon

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solutions to excercise
by riz er 06 Oct '01

06 Oct '01

hi iam a Computer Science student who is currently studying Haskell programming using the "Craft of Functional Programming second edition" by Simon Thompson. And i was wondering if you have the solutions to the excersise to the book. Becasue the book does not have them..a few students are having difficulty using the book because the that reason. If possible to send a url site where i can download them or a alternative way .. Thank you _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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Re: newtype | data
by Marcin 'Qrczak' Kowalczyk 05 Oct '01

05 Oct '01

05 Oct 2001 11:53:05 -0700, Carl R. Witty <cwitty(a)newtonlabs.com> pisze: > Data> (T2 undefined) `seq` () > *** Exception: Prelude.undefined > Data> (T3 undefined) `seq` () > *** Exception: Prelude.undefined > > I can't think of a semantic difference between newtype and data > with a single unary strict constructor; It could indeed be represented in the same way, but they behave differently in pattern matching: case undefined of T _ -> () is () in the case of newtype and undefined in the case of strict data. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: newtype | data
by cwitty＠newtonlabs.com 05 Oct '01

05 Oct '01

Mark Carroll <mark(a)chaos.x-philes.com> writes: > Why does "newtype" exist, instead of letting people always use "data" and > still get maximum efficiency? After all, surely the implementation is an > implementation detail - a compiler could see the use of "data" with a > unary constructor and implement it as it does "newtype", instead of making > the programmer worry about how things are actually represented? > > I'm obviously missing something obvious here; I'm hoping to learn what. > (-: newtype is strict; data is not. Given > data T1 = T1 Int > newtype T2 = T2 Int > data T3 = T3 !Int you get the following results. Data> (T1 undefined) `seq` () () Data> (T2 undefined) `seq` () *** Exception: Prelude.undefined Data> (T3 undefined) `seq` () *** Exception: Prelude.undefined I can't think of a semantic difference between newtype and data with a single unary strict constructor; I suppose it would be possible to remove newtype from the language and make people declare things like T3 instead. Carl Witty

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newtype | data
by Mark Carroll 05 Oct '01

05 Oct '01

Why does "newtype" exist, instead of letting people always use "data" and still get maximum efficiency? After all, surely the implementation is an implementation detail - a compiler could see the use of "data" with a unary constructor and implement it as it does "newtype", instead of making the programmer worry about how things are actually represented? I'm obviously missing something obvious here; I'm hoping to learn what. (-: -- Mark

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broken link
by Jorge Adriano 05 Oct '01

05 Oct '01

Just thought I'd report that the PolyGP link: http://www.cs.ucl.ac.uk/staff/T.Yu/research.html at the 'Haskell in Practice' section (http://www.haskell.org/practice.html) seems to be broken. J.A.

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Re: classes and instances
by Ashley Yakeley 05 Oct '01

05 Oct '01

At 2001-10-04 08:31, Russo, Tom wrote: >Main> int 5 >ERROR - Unresolved overloading >*** Type : (Num a, ToInt a) => Int >*** Expression : int 5 > >Can someone explain what the problem is, and how to interpret the error >message I'm getting? Oh, 5 isn't necessarily an Int. It's of type "Num a => a", which is more general. You can restrict it to Int with "(5 :: Int)". -- Ashley Yakeley, Seattle WA

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classes and instances
by Russo, Tom 04 Oct '01

04 Oct '01

Hello, I'm learning haskell and I'm having some trouble with type classes. I understand them conceptually, but when I try to create an instance I run into problems. As a simple example, I create a class called ToInt: > class ToCh a where > int :: a -> Int Then I create an instance: > instance ToInt Int where: > int n = n However, when I try to use the function int :: Int -> Int at the hugs prompt, I get the following error message: Main> int 5 ERROR - Unresolved overloading *** Type : (Num a, ToInt a) => Int *** Expression : int 5 Can someone explain what the problem is, and how to interpret the error message I'm getting? thanks _t

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Re: Small confusion with monads
by Ashley Yakeley 04 Oct '01

04 Oct '01

At 2001-10-04 02:02, Dmitry Olyenyov wrote: >I have the list ["file1","file2","file3"] and I need the >function to return me the list of contents of these files. > >contents :: [String] -> [String] That cannot be safely done with that type. A "pure" function like that is not allowed to do "imperative" stuff such as reading files. -- Ashley Yakeley, Seattle WA

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Small confusion with monads
by Dmitry Olyenyov 04 Oct '01

04 Oct '01

Hello! Can anybody help me with writing function that does following: I have the list ["file1","file2","file3"] and I need the function to return me the list of contents of these files. contents :: [String] -> [String] contents (file:files) = do c <- readFile file ........ Thanks in advance for help! -- With best regards, Dmitry

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always new instance?
by Cagdas Ozgenc 04 Oct '01

04 Oct '01

Hi, Do I ALWAYS need to create a new instance if I want to modify the state of an instance? For example, if I design an index for a simple database with an recursive algebric Tree type, do I need to recreate the whole Tree if I insert or remove an element? How can I improve performance, what are common idioms in these situations? Thanks

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Re: Doing exercises from "Haskell tutorial" ...
by Marcin 'Qrczak' Kowalczyk 03 Oct '01

03 Oct '01

02 Oct 2001 15:16:27 +0300, Dmitry Astapov <adept(a)umc.com.ua> pisze: > I need to define SetsAsLists as an instance of Set by supplying > definitions for all Set methods, but definitions I wrote led me to > adding additional constraints on "union" and "memeber" methods. What constraints? The class already says that these operations require Eq on the element type: > class Set s where > empty :: s a > isEmpty :: s a -> Bool > singleton :: a -> s a > union :: Eq a => s a -> s a -> s a ^^^^ > member :: Eq a => a -> s a -> Bool ^^^^ > choice :: s a -> (a, s a) -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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