September 2007 - Haskell-Cafe

Serial Communications in Haskell
by Ronald Guida 08 Sep '07

08 Sep '07

I'm on a Windows box and I'm looking for a way to talk to a serial port (for example, RS-232) from Haskell. I couldn't find a library to do this, so I am wondering how to create one. I have a fairly thorough understanding of how to open and use a serial port with the Windows API. In particular, to open a serial port, I have to use CreateFile, which is the same API call that opens files. In fact, if I call openFile from GHC, and pass "COM1:" as the filename, then I can get a writable serial port. > module Serial > where > import System.IO > > main = do > h <- openFile "COM1:" ReadWriteMode > hPutStrLn h "Hello World" I can't read from the port (I always get an immediate EOF), and I have no way of configuring things like the baud rate or the parity settings. Nevertheless, this demonstrates that openFile can at least open the serial port. What I would like to do is create some functions that would allow me to open and configure a serial port, and get a Handle back so that I can use the existing IO functions like hGetChar and hPutChar. I am assuming that hGetChar eventually calls win32::ReadFile and hPutChar eventually calls win32::WriteFile. These same two API calls would work for serial ports. In Windows, there are 23 API functions that apply specifically to serial ports. Out of these 23 functions, only a few of them are actually necessary if I just want to send and receive data. Of course, I don't know how to call Windows API functions from Haskell, and I have no idea how to hook things to the IO library so that I can use a Handle for a serial port. I'm looking for some advice on how to proceed. -- Ron

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generate a news summary page from web services
by brad clawsie 08 Sep '07

08 Sep '07

i recently worked up a little example program i use to generate a general news summary page using yahoo web services if you are interested in an example of XHT and Text.XHtml.Strict in action, you can see it here http://www.b7j0c.org/content/haskell-newspage.html

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Block-wise lazy sequences in Haskell
by Henning Thielemann 07 Sep '07

07 Sep '07

I want to have a data structure like Data.ByteString.Lazy, that is block-wise lazy, but polymorphic. I could use a lazy list of unboxed arrays (UArray) but the documentation says, that the element types are restricted. But I will need (strict) pairs of Double and the like as elements. It seems that I need generalized instances in order to use these arrays. I thought it must be possible to define an unboxed array type with Storable elements. I cannot find such a type in the standard libraries. Further on, I wonder why pairs are not instances of Storable.

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Extending the idea of a general Num to other types?
by Peter Verswyvelen 07 Sep '07

07 Sep '07

I like the fact that Haskel treats numbers in a generic way, so you can lift them to any other datatype by instantiating Num. Can the same be done on other builtin constructs? For example, if I have [a], can this list be lifted to other types? I guess not, because no type class exists for the list type? Does this make sense? I'm still searching for correct terminology... Thanks, Peter

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Learning advice
by Chris Saunders 07 Sep '07

07 Sep '07

I wish to learn Haskell. I purchased a book - An Introduction to Functional Programming Systems Using Haskell but I think this might be too outdated. I'm using GHC and it seems that some of the functions used in the book are no longer a part of Haskell. I'm looking for advice on books or tutorials that might be more up to date. So far I'm finding Haskell difficult - I may be too thick. Regards Chris Saunders

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Fwd: [Haskell-cafe] turning an imperative loop to Haskell
by Stephen Dolan 07 Sep '07

07 Sep '07

Sorry, forgot to cc list > I think this is called taking a good thing too far, but cool too: > > > f1 u = u + 1 > f2 u v = u + v > f3 u v w = u + v + w > > -- functions renamed for consistency) > zipWith1 = map > zipWith2 = zipWith > > -- and hey presto! > us1 = 3 : zipWith1 f1 us1 > us2 = 2 : 3 : zipWith2 f2 (drop 1 us2) us2 > us3 = 2 : 3 : 4 : zipWith3 f3 (drop 2 us3) (drop 1 us3) us3 How about this: import Data.List -- for transpose f = sum zipWithN fn = map fn . transpose us k = s where s = [2..k+1] ++ (zipWithN f $ x $ map (flip drop s) [0..k-1]) take 10 (us 1) [2,2,2,2,2,2,2,2,2,2] take 10 (us 2) [2,3,5,8,13,21,34,55,89,144] take 10 (us 3) [2,3,4,9,16,29,54,99,182,335] take 10 (us 4) [2,3,4,5,14,26,49,94,183,352] We can always take it further :D Stephen

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Typeclasses and implicit parameters
by ajb＠spamcop.net 07 Sep '07

07 Sep '07

{-# OPTIONS -fglasgow-exts #-} -- G'day everyone. -- This is okay. f1 :: (?foo :: String) => String f1 = ?foo -- So is this. f2 :: (Show a, ?foo :: a) => a -> String f2 _ = show ?foo -- Hugs allows this. GHC rejects it on the grounds that "a" is unused -- on the right-hand side of the (=>). I think this is arguably a bug -- in GHC. f3 :: (Show a, ?foo :: a) => String f3 = show ?foo -- GHC rejects this. Hugs compiles it, but I can't call it as -- let ?foo = "Hello" in show Foo -- -- Is there a good reason to disallow this? data Foo = Foo instance (?foo :: String) => Show Foo where showsPrec _ Foo = showString ?foo . showString "Foo" -- Cheers, -- Andrew Bromage

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ANNOUNCE: xmonad 0.3
by dons＠cse.unsw.edu.au 07 Sep '07

07 Sep '07

The xmonad dev team is pleased to announce the 0.3 release of xmonad. xmonad: a tiling window manager http://xmonad.org About: xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. All features of the window manager are accessible from the keyboard: a mouse is strictly optional. xmonad is written and extensible in Haskell. Custom layout algorithms, and other extensions, may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating windows * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Easy, powerful customisation and reconfiguration * Large extension library * Extensive documentation and support for hacking Since xmonad 0.2, the following notable features and bug fixes have appeared: New features: * floating layer support: transients windows are not tiled by default, and windows may be dragged to and from a traditional floating layer (which allows mouse-resizing, and overlapping windows). * improved Xinerama support: workspace switching reuses multiple displays more effectively. * huge new extension library. Over 50 extensions to xmonad have been contributed by users, and are available all in a standard library, with documentation. More information, screenshots, documentation and community resources are available from: http://xmonad.org Xmonad is available from hackage, and via darcs. Happy hacking! The Xmonad Team: Spencer Janssen Don Stewart Jason Creighton Xmonad has also received contributions from at least: Alec Berryman Andrea Rossato Chris Mears Daniel Wagner David Glasser David Lazar David Roundy Hans Philipp Annen Joachim Fasting Joe Thornber Kai Grossjohann Karsten Schoelzel Michael Sloan Miikka Koskinen Neil Mitchell Nelson Elhage Nick Burlett Peter De Wachter Robert Marlow Sam Hughes Shachaf Ben-Kiki Shae Erisson Simon Peyton Jones Stefan O'Rear as well as many others on the IRC channel and mailing list. Thanks to everyone!

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Speed of character reading in Haskell
by ok 07 Sep '07

07 Sep '07

I'm writing a tokeniser for a legacy programming language in Haskell. I started with the obvious main = getContents >>= print . tokenise where tokenise maps its way down a list of characters. This is very simple, very pleasant, and worked like a charm. However, the language has an INCLUDE directive, so I'm going to have to call readFile or something in the middle of tokenising, so the main tokeniser loop can't be a pure String -> [Token] function any more. It occurred to me to wonder whether it would be better to try to stick with the list of characters style as much as possible (and use readFile) or to switch over to character at a time processing (with openFile, hIsEOF, hGetChar, hClose). So I decided on an experiment: count the lines both ways. Method 1A (pure list processing) main = getContents >>= print . doit 0 doit n ('\n':cs) = doit (n+1) cs doit n ( _ :cs) = doit n cs doit n [] = n Method 1B (doit is in the IO monad) main = getContents >>= doit 0 doit n ('\n':cs) = doit (n+1) cs doit n ( _ :cs) = doit n cs doit n [] = print n Method 2 (character at a time) main = doit 0 doit n = isEOF >>= \eof -> if eof then print n else getChar >>= \c -> doit (if c == '\n' then n+1 else n) I tried this using GHC 6.4 on a 2.8 GHz PC running linux 2.6.20-1.2933.fc6 I don't know whether gcc was involved at all, but in case it was, the gcc version was 4.1.1. All test programs were compiled using ghc -o foobar -O foobar.hs The input was a 3.8 MB file. wc: 0.08 seconds total Method 1A: 0.30 seconds total Method 1B: 0.47 seconds total Method 2: ran out of stack Method 2': 2.21 seconds total. When I got the stack overflow, I suspected that this might be due to lazy evaluation of the integer argument to doit, so I changed the last line to doit $! (if c == '\n' then n+1 else n) and that worked. In *retrospect*, it is really obvious why this was necessary, but I must say that in *prospect* I wasn't expecting it. Other things being equal, I would have expected Method 2' to be faster. I'm now wondering if there is some other consequence of laziness that I am just not seeing. I'm also a little puzzled that there is a measurable difference between method 1A and method 1B. Things that I am not asking about: - ghc version; that's what's installed on the machine and if I want something different I may have to wait many days. - style comments on the list processing versions; yes I know how to write getContents >>= print . length . filter (=='\n') and that really has no relevance to the full tokeniser and in any case is slower than Method 1A.

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wanted: HAppS example combining state and io
by Thomas Hartman 06 Sep '07

06 Sep '07

In the latest happs (darcs pulled, updated head is 0.9.1 iirc), I am experimenting with the example file in src/HAppS/Examples/HTTP1.hs. I would like to combine state with io. Eventually io will mean stuff like reading from a database, but for now I'm just reading a file. The example file HTTP1.hs has an example that demonstrates state with macid. I added an example that allows you to execute arbitrary io. I tried, but was unable to, add a handler that combines state and io. , h ["iohandler"] GET $ ioReadFileHandler , h ["statehandler"] GET $ stateHandler --, h ["ioandstatehandler"] GET $ ioAndStateHandler ..... -- displays contents of HAPPS.hs in current directory ioReadFileHandler = iohandler $ readFile "./HAppS.hs" -- displays incremented state counter stateHandler = ok $ \() () -> modify (+(1::Int)) >> get >>= respond . show -- should combine effect of iohandler with statehandler -- specifically, should display contents of HAppS.hs, and under that an incremented state handler -- is this possible ioAndStateHandler = undefined undefined Is mixing state and io possible with HAppS? If so, an example showing how to do it would be extremely helpful. best, thomas. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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