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How to getCh on MS Windows command line?
by Ahn, Ki Yung 10 Nov '08

10 Nov '08

What I mean by getCh is the non-buffered non-echoed version of getChar, which Hugs used to provided as an extension but not any more. Is there any way to have a non-buffered non-echoed single character input function on MS Windows command line using only the libraries in the MS Windows distribution packages of either GHC or Hugs? The reason to why this is important for me is because I am translating Graham Hutton's "Programming in Haskell" into Korean (soon to be published), which illustrates interactive programming with the example of a calculator that responds instantly for every keystroke of numbers and arithmetic operations running on text mode. It is very important to consider the readers who only have access to MS Windows systems, because Korean OS market share is completely skewed towards MS Windows for very embarrassing reasons that I do not even want to mention. And, isn't GHC developed in MSR anyway? :-) I remember that this is an old problem for both of the two most widely used Haskell implementation, Hugs and GHC. In ghc 6.8 getChar had a bit strange behavior. As far as I remember it always worked as if it were NoBuffering. Fortunately, in the recently released ghc 6.10, this has been fixed. We now actually have to set the buffering mode to NoBuffering with hSetBufferring to get the non-buffered behavior of getChar. But, it still isn't working on MS Windows. In Hugs, hSetBuffering neither works on Unix terminal nor on MS Windows command line. Surprisingly, it works in WinHugs. However, I cannot use WinHugs for my purpose because the interactive calculator example in the book also uses beep characters and ANSI codes which do not work in WinHugs. Thanks for any hacks or suggestions, -- Ahn, Ki Yung

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foldl vs foldl'
by Daryoush Mehrtash 10 Nov '08

10 Nov '08

Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? thanks, daryoush

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Announce: Reactive library (FRP) and mailing list
by Conal Elliott 09 Nov '08

09 Nov '08

Reactive [1] is a library for functional reactive programming (FRP), similar to the original Fran [2] but with a more modern interface (using standard type classes) and a hybrid push/pull implementation. It is designed to be used in a variety of contexts, such as interactive 2D and 3D graphics, graphical user interfaces, web services, and automatic recompilation/re-execution. It has a simple and precise semantics based on continuous time and is built on a notion of functional future values. The semantics and implementation are described in the paper "Simply efficient functional reactivity" [3]. Reactive now has a mailing list [4] and a feature/bug tracker [5]. [1] http://haskell.org/haskellwiki/Reactive [2] http://conal.net/Fran [3] http://conal.net/papers/simply-reactive [4] http://www.haskell.org/mailman/listinfo/reactive [5] http://trac.haskell.org/reactive

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Physics engines purely in Haskell?
by Mark Wassell 09 Nov '08

09 Nov '08

Has anyone thought about or embarked on the task of developing a 2D or 3D physics engine purely in Haskell? There is Hipmunk but I'm wondering about a purely Haskell implementation; possibly a port of Chipmunk to Haskell. Mark

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Is there a plan for HappS and ghc-6.10.1?
by Hugo Pacheco 09 Nov '08

09 Nov '08

Hi all, The HappS version on Hackage does not yet support ghc 6.10, since GADT pattern matching and the Exception API have changed. One sample error is: src/HAppS/Data/Xml/Base.hs:281:23: GADT pattern match with non-rigid result type `t1' Solution: add a type signature ... Adding a type signature via an auxiliary function instead of a case expression, the problem goes away. readElementRigidity :: (Monad m,Xml t) => Rigidity m -> m ([Element],t) -> Maybe t readElementRigidity Rigid m = ... readElementRigidity Flexible m = ... Is there a plan to update the version on Hackage to support ghc 6.10.1? I don't know either if it may break backwards compatibility. Cheers, hugo -- www.di.uminho.pt/~hpacheco

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Convolution
by Andrew Coppin 09 Nov '08

09 Nov '08

A simple and straight-forward way to define the DSP operations of correlation and convolution is as follows: correlate1 :: [Double] -> [Double] -> Double correlate1 ks = sum . zipWith (*) ks correlate :: [Double] -> [Double] -> [Double] correlate ks [] = [] correlate ks xs = correlate1 ks xs : correlate ks (tail xs) convolute :: [Double] -> [Double] -> [Double] convolute ks = correlate (reverse ks) This very simple code work - and actually works quite well. It has the nice property of generating output from input lazily, as it is demanded. If the correlate function could be altered to use Prelude list functions only, I would think the above code works quite well with stream fusion too. (Presumably you could do this using "inits" or something?) Assuming DPH ever becomes a reality, how would you implement the above with it? (My intuition tells me that unless you have a really *big* filter kernel, breaking correlate1 into parallel chunks is too granular. However, each call to correlate1 could be run independently, presumably confering a more or less linear speedup. Unless having two cores writing to the same memory page is going to upset the cache system...) Presumably if you want to use parallel arrays, then the data must all exist in memory at once, and any chunking must be implemented manually and tediously?

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Dealing with CStringLen
by Maurício 09 Nov '08

09 Nov '08

Hi, How is CStringLen supposed to be used? The only usual C string I'm familiar with is the null terminated sequence of chars. I remember some Pascal version where the first 2 bytes were used as an integer counting the following characters. What is the convention used in Foreign.C.String? I have a C function signature like this: void function (int,const void*,int,const void*) where each (int,const void*) pair is a string. Can (should) I use CStringLen here? How? Thanks, Maurício

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view patterns
by Cetin Sert 08 Nov '08

08 Nov '08

let has [] = False; has _ = True -- this one is ok let empty list = case has list of True -> False; False -> True -- the following is problematic let emp (has -> True) = False; emp (has -> False) = True <interactive>:1:4: Warning: Pattern match(es) are overlapped In the definition of `emp': emp ((has -> True)) = ... emp ((has -> False)) = ... Why do I get this error in ghc or when I try to compile a file with view patterns? (using -fglasgow-exts and -XViewPatterns, ghc 6.10.1)

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Haskell Quick Reference (1-page PDF)
by Malcolm Wallace 08 Nov '08

08 Nov '08

Some time ago, there was a thread about a "CheatSheet" for Haskell beginners. As I recall, the CheatSheet was more than 12 pages long. For a Haskell tutorial I was running at a conference recently, I needed a "Quick Reference Guide" that would fit onto a single side of A4. So I knocked one together quickly, and it is attached as a PDF. I send it to this list, with permission for anyone to distribute it more widely as they wish, in the hope that it might be useful. Doubtless it is incomplete, and I have no particular desire to fix errors or maintain this document, so if anyone is interested and would like to adopt it, I can pass on the editable sources. It was originally created as an Apple Numbers spreadsheet (simply for speed of creation) but could be converted to Excel or CSV, for import into other tools. Regards, Malcolm

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Bit Field Marshalling
by Michael D. Adams 08 Nov '08

08 Nov '08

I'm writing a Storable instance for a data type that marshals to a C structure that contains bit fields such as the following: struct Foo { short x; short y; unsigned int a : 1; unsigned int b : 1; unsigned int c : 1; unsigned int d : 1; unsigned int reserved : 12; } For the "x" and "y" fields I've been using hsc2hs and something like #{peek Foo, x} to extract the fields. However, bit fields such as "a" have me stumped. But as far as I can tell, hsc2hs doesn't support bit fields. On top of that I'm not sure I can make any safe assumptions about what order the bit fields are packed (LSB or MSB first). Have others run into this problem before? What options are there for working around it? Michael D. Adams mdmkolbe(a)gmail.com

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