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Re: [Haskell] Newbie : How come that cyclic recursive lists areefficient ?
by David Barton 26 Jan '05

26 Jan '05

Benjamin Fransen writes: > There *is no* difference between the two if one views them as pure > mathematical values. Questions of run time speed or memory usage, i.e. > efficiency (which your original question was about) are clearly outside the > realm of pure values, and thus we may perceive them as distinct in this wider > setting. > > My favourite analogy for this is the old joke about a topologist being a > person who cannot see any difference between a cup and a doghnut. The engineer's response, of course, at the thought of ignoring questions about run time speed and memory usage, is that a topologist is a person who doesn't know his ass from a hole in the ground. (I was told this quote was actually from abstract algebraists, when confronted by the famous description of a topologist, but what the heck.) Dave Barton EDAptive Computing

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wxFruit examples
by Dmitri Pissarenko 26 Jan '05

26 Jan '05

Hello! I want to learn to program with wxFruit UI framework. http://zoo.cs.yale.edu/classes/cs490/03-04b/bartholomew.robinson/ Does someone know about any examples apart from paddle ball game? I would like to start with a simple application, which takes a the name of an image file as a command-line argument and displays a window, in which the image is displayed. The window has a "Close" button. If the file in question is not an image, a message box with an error message is displayed. I've looked at the source code of the paddle ball game, but still have no idea how to create the program with aforementioned functionality. TIA Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Visual Functional Languages
by José Miguel Vilaça 26 Jan '05

26 Jan '05

Hi Im searching for Visual Functional Languages, i.e. languages in which the programmer does diagrams instead of writing textual code. The only language I have found until now is VFPE of Joel Kelso's PhD thesis http://www.csse.uwa.edu.au/~joel/vfpe/ Does anyone know other languages or at least projects on this subject? Thankfully for answers José Miguel Vilaça

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File path programme
by Krasimir Angelov 26 Jan '05

26 Jan '05

Hello, Guys The System.FilePath might not be perfect and might not handle some unusual cases very well. Most of functions in this module are collected from Cabal/Alex/Happy/Haddock. Some of these tools already use their own functions for file path handling. Special care was taken to make them more portable but probably there is still enough room for optimizations. >> What about splitFileExt "foo.bar."? ("foo", "bar.") or ("foo.bar.", "")? > > The latter makes more sense to me, as an extension of the first case > you give and splitting "foo.tar.gz" to ("foo.tar", "gz"). I will take a look at this. I also don't know which case is more natural. > (Win32) > > splitFileName "\\\\server\\share" ==> ("\\\\server","share") > (should probably be ("\\\\server\\share","")) Why? "\\\\server" is the server name and "share" is the directory name on this server. > splitFileName "foo:xyz" ==> ("foo:.","xyz") > (should be (".","foo:xyz") -- this refers to the named stream > xyz of foo) System.FilePath currently doesn't support NTFS streams. I don't care much about them because they are rarely used but the things can be improved. The trouble here is that in some cases the drive letter cann't be easily distinguished. What does "c:foo" mean, file "foo" on drive "c" or stream "foo" of file "c"? > joinPaths "c:\\" "\\foo" ==> "\\foo" > (should be "c:\\foo". I realize that "cd c:\\" on Windows doesn't > actually make "c:\\" the current directory, but ";" doesn't > separate shell commands either.) On Windows there is current drive and current directory on each drive. "\\foo" means top level directory "foo" on the current drive. If the current drive is "c" then I can agree that the joined path is "c:\\foo" but if the current drive is "a" then this is wrong. In both cases "\\foo" is the right answer. If you want something like "c:\\foo" you can use System.Directory.canonicalizePath. canonicalizePath "\\foo" returns either "c:\\foo" or "a:\\foo". > (Posix) > > splitFileName "/foo" ==> ("/","foo"), > splitFileName "/foo/" ==> ("/foo","") > (arguably makes sense, but why isn't it documented?) OK > splitFileName "/foo/bar" ==> ("/foo","bar") > splitFileName "/foo//bar" ==> ("/foo/","bar") > (definitely a bug) Is "/foo//bar" valid file path and what does "//" mean? > pathParents "/foo///bar" ==> ["/","/foo","/foo","/foo","/foo/bar"] Again what does "///" mean? > pathParents "foo/../bar" ==> [".","foo/../bar"] > (what if foo doesn't exist and we wanted to create it?) If "foo" doesn't exist then you can create it or raise an error Cheers, Krasimir

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Newbie Monad question
by Simon Ulfsbäcker 26 Jan '05

26 Jan '05

Hi, I'm a functional programming newbie trying to get the hang of the concept of Monads, but one thing about bind puzzles me. Let's take an IO Monad-example: readLn >>= \i -> readLn >>= \j -> return (i + j) I fail to see how (\j -> return (i + j)) can know about the value of i. Isn't just the value of the previous readLn passed to \j -> return (i + j)? /Smalmatskungen

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How to convert from "IO String" to String
by Dmitri Pissarenko 26 Jan '05

26 Jan '05

Hello! I have a function, which reads the contents of a file into a string: <function1> readImage :: String -> IO String readImage filename = do fileContent <- readFile filename return fileContent </function1> The result of this function is fed into another function, which converts the string content of the file (it is a PGM image file) into a list of integer values (one integer for each pixel, one-dimensional array): <function2> convertToList fileContent = drop 3 fileContentNumbers where fileContentNoComments = removeComments fileContent fileContentTokens = words fileContentNoComments fileContentNumbers = map read (tail fileContentTokens) fileContentNumArr = listArray (1, length fileContentNumbers) fileContentNumbers </function2> Then, I wanted to verify that this function works properly and wrote a test case, which reads an image and tests whether the resulting list is correct: <test-case> testConvertToList01 = TestCase (assertEqual "" [0, 255, 255, 255, 255, 0, 255, 255, 255, 255, 0, 255, 255, 255, 255, 0] (convertToList (readImage "../data- test/diagonalImage.pgm"))) </test-case> When I load the files (you will find them all in attachment) into GHCi, I get following error <error-message> Couldn't match `String' against `IO String' Expected type: String Inferred type: IO String In the application `readImage "../data-test/diagonalImage.pgm"' In the first argument of `convertToList', namely `(readImage "../data-test/diagonalImage.pgm")' </error-message> I suppose that the error is due to this call in the test case: (convertToList (readImage "../data-test/diagonalImage.pgm")) readImage returns IO String and convertToList accepts String. How can I re-write this test case so that it works? Thanks in advance Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Haskell programs in C
by Dmitri Pissarenko 26 Jan '05

26 Jan '05

Hello! Imagine I wrote a program in Haskell and want to use it on a microcontroller device. AFAIK, Haskell programs are first converted to C code and then compiled by gcc. Is it possible (at least theoretically) to write a program in Haskell, then convert it into C and then compile the C program into an executable, which is optimized for the microcontroller? Thanks in advance Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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~1k memory per elem for FiniteMap!!!?
by S. Alexander Jacobson 26 Jan '05

26 Jan '05

Is it possible/reasonable that Data.FiniteMap takes around 1k per element? I just profiled this code: import Data.FiniteMap zipped =zip [1..] [1..50000] addFMItem (index,item) fm = (addToFM $! fm) item index produce = foldr addFMItem emptyFM zipped fm = take 1 $ keysFM produce main = print $ fm And it apparently consumes more than 45MB of heap and won't run unless I give it at least 3M of stack space. I assume I doing something wrong here, but have no idea what it is. -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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RE: [Haskell-cafe] Re: Can't do basic time operations with System.Time
by Simon Marlow 26 Jan '05

26 Jan '05

On 21 January 2005 21:50, John Goerzen wrote: > On 2005-01-21, Peter Simons <simons(a)cryp.to> wrote: >>> 24 * ((fromIntegral $ tdDay td) + >>> 30 * ((fromIntegral $ tdMonth td) + >>> 365 * (fromIntegral $ tdYear td))))) >> >> I was wondering: Does this calculation account for leap >> years? Does it have to? > > I also wondered about all months being 30 days. But this exact > algorithm is used in fptools' normalizeTimeDiff, and it seems to work > out correctly, so I guess the timeDiff functions assume that every > month has 30 days and every year has 365. normalizeTimeDiff (and TimeDiff in general) is wrong. I wouldn't recommend using it. There's the TimeExts library in the lang package, which might be useful to you. Cheers, Simon

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what is inverse of mzero and return?
by S. Alexander Jacobson 26 Jan '05

26 Jan '05

I assume there is a standard name for this class/function: instance Foo [] where foo [] = mzero foo (x:_) = return x instance Foo (Maybe x) where foo Nothing = mzero foo Just x = return x -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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