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Re: layout rule infelicity
by Frank Atanassow 30 May '02

30 May '02

[redirected to haskell-cafe] Ashley Yakeley wrote (on 30-05-02 03:18 -0700): > At 2002-05-30 02:54, Lennart Augustsson wrote: > > >If you look at C (& offspring), it's not the {;} that makes the code > >readable, it's the indentation that does. So why not acknowledge that? > > In C, the indentation is an important visual clue, but there are many > different indentation styles. I think there are not so many different _indentation_ styles. The differences seem mostly to revolve around where to put _the braces_. And one might argue that the reason for that is precisely that it's so arbitrary, since the indentation is the main visual clue to structure anyway. Personally, I always try to factor out arbitrariness from my programs. I think layout does the same thing for syntax. > If you're used to braces, complicated Haskell expressions with layout > look confusing, since it's not immediately clear which indentation style > the layout rules are trying to enforce. If you're used to C, then layout and indentation will be the least of your difficulties when you start using Haskell... Anyway, I have the feeling that, for every person on this list who complains about layout being unintuitive, there are 10 people who would say the opposite. Shall we take a poll? -- Frank Atanassow, Information & Computing Sciences, Utrecht University Padualaan 14, PO Box 80.089, 3508 TB Utrecht, Netherlands Tel +31 (030) 253-3261 Fax +31 (030) 251-379

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Space problem: reading a list of records from a binary database
by Eray Ozkural 28 May '02

28 May '02

Hi there, While trying to read a list of records from a binary database I think I've encountered a space leak but I'm not proficient enough to detect the cause of the error. The culprit is the readSessionDB function in SessionDB module which I attach with other relevant code. What I do in Session.readSession is using FFI to read a fixed size record from an input stream. That far, okay. (I have no idea if I'm doing it right, though) The problem is, combined with my approach in Session.readSessionDB, it results in huge amounts of space allocation, about 1MB, whereas the raw data would consume about 50K. It would be a [Session] with about 3000 elements, each of which contains 3 Ints and 6 Floats. I wonder if the following code in SessionDB is the proper way to read a list of records from a file. I get the feeling that the (s:rest) in the last return statement causes my problems. readSessionList :: Handle -> IO [Session] readSessionList h = do session <- readSession h case session of Nothing -> return [] Just s -> do rest <- readSessionList h return (s:rest) Comments appreciated, -- Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo Malfunction: http://mp3.com/ariza GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C

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RE: toEnum with international chars
by Simon Marlow 28 May '02

28 May '02

> Oh, my bad, I think I gave a wrong list there. 66 is of > course 'B', I was a > little sleepless at the time of writing. Now having enjoyed a > few hours of > sleep I can express more eloquently :) > > It seems to be the fourth character that is the problem, but > then again that's > my mistake. Thank you. > > Still it attracts my attention that I cannot type þ in ghci > while I can type > it in bash. I think I can't type any of those international > chars in ghc. > Maybe some bug fix is in order. Perhaps you could give an example? Bash and GHCi both use the Readline library, and as far as I can tell you can type exactly the same characters to both. Cheers, Simon

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toEnum with international chars
by Eray Ozkural 28 May '02

28 May '02

I have a code that reads a string from a list of CChar's > shortName = map (toEnum.fromIntegral) shortname When I have a turkish character like Ý the interpreter will: StockList> a <- readStockInfo h [66,105,108,105,-2,105,109,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32] StockList> a *** Exception: Prelude.chr: bad argument The problem is the 66 in the beginning of the list. I'm not giving the entire code, it's not needed... What should I be using to work with such strings? Thanks, -- Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo Malfunction: http://mp3.com/ariza GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C

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Critique required
by Andy Fugard 27 May '02

27 May '02

Hi all, I'm a Haskell newbie. Recently I threw together an answer to Exercise 3.2 on p47 of Hudak, which is attached below. Essentially all it does is draw a star, then on each point of the star draw another star, and so on, recursively. Would it be possible for someone to comment briefly on my answer. For example is there anything in there that is immediately annoying to an experienced Haskell programmer? How are programs usually commented? (I come from an imperative background, where functions are often longer -- there it makes sense to write pre- and post-conditions for each function, but I'm not sure if that's relevant for Haskell.) Any ideas for making it more efficient? So, essentially I'm asking someone to rip me to shreds :) I don't yet have any technical queries, but have just reached the section on "fold", so I expect it won't be long.... Cheers, Andy ---------------------------------------------------------- import SOEGraphics intToFloat :: Int -> Float intToFloat n = fromInteger (toInteger n) starOfDavidList :: Point -> Int -> [Point] starOfDavidList (x,y) side = [ ( x, y - halfHeight ), ( x - halfWidth, y + halfHeight ), ( x + halfWidth, y + halfHeight ), ( x, y + halfHeight + offset ), ( x - halfWidth, y - halfHeight + offset ), ( x + halfWidth, y - halfHeight + offset ) ] where s = intToFloat side height = sqrt (s^2 - (s^2 / 4)) halfHeight = round (height/2) halfWidth = round (s/2) offset = round (1/3 * height) drawStar :: Window -> Color -> Point -> Int -> IO () drawStar w c pos side = do drawInWindow w equiTriUp drawInWindow w equiTriDown where equiTriUp = withColor c (polygon (take 3 list)) equiTriDown = withColor c (polygon (drop 3 list)) list = starOfDavidList pos side snowFlake :: Window -> [Color] -> Int -> Int -> Point -> IO () snowFlake w colors side itr pos | length colors < itr = error "Not enough colours in list" | itr > 0 = do drawStar w (head colors) pos side sequence_ (map funct ptList) | otherwise = return () where funct = snowFlake w (tail colors) newWidth (itr - 1) ptList = starOfDavidList pos side newWidth = round (1/3 * intToFloat(side)) main = runGraphics( do w <- openWindow "A Snowflake Fractal" (700,650) snowFlake w [Cyan, Blue, Magenta, White, Blue] 400 5 (300,250) k <- getKey w closeWindow w ) -- [ Andy Fugard ] [ +44 (0)7901 603075 ]

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Constructing Cases
by Carl McTague 27 May '02

27 May '02

Hi there, is there a simple way to carry out the following type of conversion? Suppose I have a list (finite) of (value,image) pairs such as: list = [(0,1),(1,0)] >From this I want to generate a function f 0 = 1 f 1 = 0 Is there a way to do this? Note, I don't want to define a function that searches through the list each time it is invoked, I want to generate the function once and have it be as fast as the pattern-matcher can make it. So, I'm looking for a function g :: [(a,b)] -> a -> b Is this simple to do? Thanks, Carl

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Segmentation fault
by ck15 22 May '02

22 May '02

Hi, I am getting a segmentation fault , when I execute a code in hugs. (only some times) What are the conditions that give rise to a segmentation fault , when executing a Haskell code in hugs? I would appreciate very much if some could spare some time to help me. Thanks Chitra

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by web 22 May '02

22 May '02

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RE: Re: Fixed point of n-mutually recursive functors
by gonflores＠guay.com 22 May '02

22 May '02

> I don't think this is possible in plain Haskell 98. > It is possible in Generic Haskell, see: > http://www.generic-haskell.org/ > [...] > Generic Haskell's type system is sufficient. > > Is it possible using dependent types? > > I would expect so. I have been looking to Generic Haskell and it may be possible to define a generic map function like: mapN :: (a1 -> b1) -> ... -> (aN -> bN) -> f a1 ... aN -> f b1 ... bN However, I still don't know how could I define something like: > data FixN1 f1 ... fN = InN1 (f1 (FixN1 f1 ... fN) ... (FixNN f1 ... fN)) > ... > data FixNN f1 ... fN = InNN (fN (FixN1 f1 ... fN) ... (FixNN f1 ... fN)) I think that I would need dependent types, because I want to define a number of data types that depend on a given natural...any suggestions? Best regards, Gonzalo Flores

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convering Fds to Handles
by Abraham Egnor 22 May '02

22 May '02

Is there a way to convert an Fd variable to a Handle variable? Specifically, I'd like to use "pipe" from the Posix library to create a pipe and then pass one end of it to runProcess as its stdin. Abe

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