January 2005 - Haskell-Cafe

Newbie Monad question
by Simon Ulfsbäcker 25 Jan '05

25 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 25 Jan '05

25 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 25 Jan '05

25 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 25 Jan '05

25 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 25 Jan '05

25 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 25 Jan '05

25 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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Re: [Haskell-cafe] what is a stack overflow?
by S. Alexander Jacobson 25 Jan '05

25 Jan '05

Thank you iavor. But the -K option doesn't appear to work with ghci. And I guess the bigger question is what sort of code causes a stack overflow. If 5M is enough stack for most programs then I obviously have some basic coding error which is causing a stack overflow... What sort of code causes that? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com On Mon, 24 Jan 2005, Iavor Diatchki wrote: > hi, > programs compile with GHC have a bunch of command line switches. > you can see them by typing: > myProg +RTS -help > one of them enables you to specify stack space, e.g. > myPorg +RTS -K5M > > (very briefly) the stack is a part of memory used by the compiler to > pass around arguments > to functions, and for temporary computations. > -iavor > > > > > On Mon, 24 Jan 2005 17:16:08 -0500 (Eastern Standard Time), S. > Alexander Jacobson <alex(a)alexjacobson.com> wrote: >> GHC assumes the user knows the difference between >> the heap and the stack. I don't. No matter how >> much heap I specify on the GHCi command line, I >> get a stack overflow exception. I have no idea >> what that means or how to remedy it. Hints? >> >> Note: My program is basically creating a few 100k >> item FiniteMaps. I don't think that should exceed >> the memory on my laptop.... >> >> -Alex- >> >> ______________________________________________________________ >> S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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Hugs for Zaurus
by John Goerzen 25 Jan '05

25 Jan '05

Hello, I have built a fixed Hugs for the Zaurus PDA running the OpenZaurus distribution. Download here: http://quux.org/devel/zaurus/hugs_hugs98-Nov2003-r1_arm.ipk Every module except Posix signals appears to be working. -- John

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Recantation and updated views
by Daniel Fischer 25 Jan '05

25 Jan '05

Okay, I was stupid, having ignored my own advice to always look at the sources. I stupidly assumed IO a was not a function-type. On retrospect, I can't imagine, how that came to me. I've looked it up now, and for ghc, I found newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)) which is more or less what I've arrived at, eventually. For hugs, it's newtype IO a = IO ((IOError -> IOResult) -> (a -> IOResult) -> IOResult), which I find somewhat more contrived. So I apologize for all the nonsense that came from that and happily agree that getChar &c are well defined values of IO a. Whether we have putStrLn "hello" >> mzero === mzero depends on what raiseIO# exactly does, I've no idea. Again, sorry for all the fuss. Thanks to (in alphabetical order) Jorge, Jules and Ketil for helping me out of my error. Daniel

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RE: [Haskell-cafe] can't 'make install-docs'
by Simon Marlow 25 Jan '05

25 Jan '05

On 21 January 2005 23:24, Daniel Fischer wrote: > I've just installed ghc-6.2.2, but couldn't get the docs made. > I got the following: > Text/Regex.raw-hs Text/Regex/Posix.raw-hs Text/Show.raw-hs > Text/Show/Functions.raw-hs \ > --package=base \ > --dump-interface=base.haddock \ > --use-index=../doc-index.html --use-contents=../index.html \ > > /bin/sh: line 1: t: command not found > gmake[2]: [html/haddock.css] Error 127 (ignored) > /usr/bin/install -c -m 644 html/* > /usr/local/share/ghc-6.2.2/html/libraries/base > /usr/bin/install: cannot stat `html/*': No such file or directory > gmake[2]: *** [install-docs] Error 1 > gmake[1]: *** [install-docs] Error 1 > gmake[1]: Leaving directory `/usr/lib/ghc-6.2.2/libraries' > gmake: *** [install-docs] Error 1 > > What can I do about it? Looks like you need to install Haddock. Cheers, Simon

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