- Haskell-Cafe - Haskell.org

very elementary "import" problem .. help
by Galchin Vasili 24 Nov '07

24 Nov '07

Hello, import System.Posix --import Network import System.Process --import Directory import STM I am loading the above script in ghci. However, System.Posix, System.Processand STM cannot be found. Network and Directory are found. I assume when ghc is installed on Windows that the Windows registry get populated with paths to GHC 'library" directory paths. Please someone tell me why I am having trouble importing these modules/libraries. Kind regards, vasya

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Interrupting threads and ualarm
by ROBERT DOUGLAS HOELZ 24 Nov '07

24 Nov '07

Hello Haskellers, I have two questions for you: * Is there a Haskell binding to the ualarm() function in unistd.h? I looked in System.Posix.Signals, but there's just a binding for alarm(). * Is there a way to interrupt a blocking operation on another thread? I thought about throwTo, but that seems like overkill. Here's some example code: import Control.Concurrent main :: IO () main = do tid <- myThreadId forkIO $ interruptMain tid getChar interruptMain :: ThreadId -> IO () interruptMain mainThreadId = do threadDelay 5000000 interrupt mainThreadId -- This is probably going to end up being throwTo, isn't it?

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c2hs on Windows?
by Alex Young 24 Nov '07

24 Nov '07

Hi all, Does anyone know if c2hs should be working on Windows? I'm trying to build it under ghc 6.8.0, but this happens: C:\Users\Alex\Documents\Plugins\c2hs\c2hs-0.15.0>runghc Setup.hs configure Configuring c2hs-0.15.0... Setup.hs: Error: Non-empty library, but empty exposed modules list. Cabal may not build this library correctly C:\Users\Alex\Documents\Plugins\c2hs\c2hs-0.15.0>runghc Setup.hs build Setup.hs: error reading dist\setup-config; run "setup configure" command? Apologies if I'm being dense... Thanks, -- Alex

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http/ftp library
by Bulat Ziganshin 23 Nov '07

23 Nov '07

Hello Haskell-Cafe, i write a file-processing utility and want to allow users open URLs like the usual files. for this, i need a library with minimum the following http functionality: getFileSize url readBuf url bufPtr offset size ideally, it should also support ftp and allow to create ftp files: h <- create url writeBuf h bufPtr size close h what libraries can i use? ghc 6.6.1, if it's important -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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ANN: Dimensional 0.7.1 (GHC 6.8.1 compatibility)
by Bjorn Buckwalter 23 Nov '07

23 Nov '07

Dear all, The Dimensional library has been ported to GHC 6.8.1 (it remains backwards-compatible with GHC 6.6.1, and also with Cabal 1.1.6 I believe). The new version number is 0.7.1. Due to a GHC 6.8.1 bug (#1919) the CGS module will not compile and has been disabled. Thanks, Bjorn Buckwalter http://dimensional.googlecode.com http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dimensional http://hackage.haskell.org/trac/ghc/ticket/1919

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Digest size
by Simon Marlow 23 Nov '07

23 Nov '07

[ with mailing list maintainer's hat on ] Someone asked me if they could get fewer digests per day from haskell-cafe. The threshold for sending out a digest is currently 30k, which is probably the default, but seems a bit small to me. Any objections to bumping it, to say 100k? Cheers, Simon

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Source code for Visual Haskell
by Felix Martini 23 Nov '07

23 Nov '07

Hi all, The documentation of Visual Haskell mentions that the source code is available under a BSD license. The code is not available from the download page (http://www.haskell.org/visualhaskell/downloads.html) Does anone know where to get it?

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Tetris
by Andrew Coppin 23 Nov '07

23 Nov '07

If you were going to implement Tetris in Haskell, how would you do it? (For that matter, has anybody already *done* it? It would probably make a nice example program...) I'm particularly interested to know 1. How exactly would you do the graphical components? (Presumably there's some deep trickery with Gtk2hs that can draw free-form stuff like this.) 2. How do you implement a program that is fundamentally about state mutation in a programming language which abhors state mutation?

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expanded standard lib
by brad clawsie 23 Nov '07

23 Nov '07

i would categorize myself as a purely practical programmer. i enjoy using haskell for various practical tasks and it has served me reliably. one issue i have with the library support for practical problem domains is the half-finished state of many fundamental codebases such as networking and database support. in the perl world, support for these domains is provided through cpan, and this model is viable due to the (once) massive number of perl coders out there. in the java, c# etc world, a "batteries included" approach implies a narrowing of options, but also an immediate delivery of functionality. so far the haskell community has taken the cpan route for most practical libs but i wonder if a "batteries included" approach might help get some key libraries to a more complete state. in particular, i would like to see support for basic internet protocols, database connectivity, and potentially xml parser support rolled into the ghc standard libs. there is always a strong debate on where the line is drawn, but this functionality at least shows up in a plurality of practical projects. the "batteries included" approach does imply choosing preferred solutions when more than one library is available, this can also be difficult. that said, i think haskell would pick up a lot of new coders if it was obvious that the functionality they were looking for came out of the base libs. i know that people will say they don't use a database or xml, but there will always be parts of a standard library that any particular coder will never touch...but still see value in the inclusion for others. comments?

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Dynamically find out instances of classes (plugin system for haskell)
by Radosław Grzanka 23 Nov '07

23 Nov '07

Hi, Is there any plugin system for haskell? For example, in Java, I can load all compiled classes from given directory, check their interfaces and run some methods through reflection etc. Is it possible in haskell, to load modules from given directory, and if in module there is instance of class then return instantiation of that class? I hope this is understandable what I'm trying to achieve here. Thank you, Radek. -- Codeside: http://codeside.org/ Przedszkole Miejskie nr 86 w Lodzi: http://www.pm86.pl/

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question about ghci on windows ...
by Galchin Vasili 23 Nov '07

23 Nov '07

Hello, I am reading through http://www.haskell.org/ghc/docs/latest/html/users_guide/index.html in particular the description of the boot libraries. I don't see how I can display function signatures from compiled code (i.e. .hi). ??? Kind regards, Vasya

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How to abort a computation within Continuation Monad?
by Dimitry Golubovsky 23 Nov '07

23 Nov '07

Hi, I have been using plain non-monadic CPS for a while in my web-browser related stuff. Now I am tempted to switch from plain CPS to syntactically sweetened monadic style based on Continuation Monad, but I feel stuck with one important issue that I need an advice on. In plain CPS, I may write: type CPS x y = (y -> x) -> x a :: x a = f1 x $ \r -> case r of foo1 -> bar -- of type x foo2 -> f2 r $ \p -> ... -- something finally evaluating to a value of type x So, if at any time I return a value of a final type (x) instead of doing something with given continuation, I abort/suspend the whole computation. This can happen at any function call depth. I can also save the continuation reference in some persistent place, to resume the remainder of computation later. Now, I am trying to do the same with a Continuation Monad. But does anything similar exist in this monad? callCC is not exactly what I need because it only helps abort the inner computation, and what is returned, is a monadic value, not a final value. Besides, callCC defines a name of the function corresponding to the current continuation, and in order to use it, this name should be visible. If I have callCC $ \exit -> do foo ... I cannot jump to `exit' from within foo unless `exit' is given to foo as an argument. Any suggestions? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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The Yampa Arcade: source code available?
by Peter Verswyvelen 22 Nov '07

22 Nov '07

I can find the paper, but is the source code for that Space Invaders alike game also available somewhere? Thanks, Peter

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Problems with do notation
by Peter Verswyvelen 22 Nov '07

22 Nov '07

worksFine = if True then putStrLn "True" else putStrLn "False" worksNOT = do if True then putStrLn "True" else putStrLn "False" worksAgain = do if True then putStrLn "True" else putStrLn "False" Of course the worksFine function returns an IO action, so has different behavior, but I mean the indentation is different. Is this by design? Also the following is rather strange: doubleDefError = let x = 1 x = 2 in x * x alsoDoubleDefError = do let x = 1 x = 2 return (x*x) doubleDefNOERROR = do let x = 1 let x = 2 return (x*x) Now I understand why this is (the second let starts a new invisible scope no?), but for newbies, this is all very strange :-) Cheers, Peter

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Scheme in Haskell, Parsec Example, how to add scheme comments
by Berlin Brown 22 Nov '07

22 Nov '07

I am sure many of you have looked at the scheme in haskell example that is on the web by Jonathan Tang. If you are familiar with the code, I need a little help trying to add scheme style comments: "; This is my comment" I added this code here and I think it works (I replaced the name parseSpaces with his "spaces" function). But, if I start a line with a comment, it errors out with unbound variable. Anybody have any ideas? -- Added the ';' symbol :: Parser Char symbol = oneOf ";!$%&|*+-/:<=>?@^_~#\n" -- -- Handle whitespace and comments parseSpaces :: Parser () parseSpaces = (try oneLineComment) <|> whiteSpace <|> return () where whiteSpace = do skipMany1 space parseSpaces oneLineComment = do { try (string ";") ; skipMany (satisfy (/= '\n')) ; parseSpaces } http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/overview.html

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Re: [Haskell-cafe] expanded standard lib
by Magnus Therning 22 Nov '07

22 Nov '07

On Wed, Nov 21, 2007 at 20:40:01 +0000, Alex Young wrote: > Magnus Therning wrote: >> On Tue, Nov 20, 2007 at 12:33:21 +0000, Vladimir Zlatanov wrote: >>> Yes, those are good points. Maybe adding functionality similar to plt's >>> planet http://planet.plt-scheme.org and >>> http://download.plt-scheme.org/doc/371/html/mzscheme/mzscheme-Z-H-5.html#no… >>> >>> In plt scheme including a module, not present in the local repository , >>> but included via planet, resolves the module, including version, >>> etc..., downloads it from planet, and uses it appropriately. It makes >>> following various dependencies extremely easy. Updating with a new >>> version is updating the appropriate local module definitions. >>> >>> I have no clue how it would be best to implement this for haskell, but >>> it is a very user friendly no hassle way to work, so I reckon worth >>> investigating. >> >> Many other programming languages have packaging strategies that sound >> very similar. Several of them have managed to have a negative impact on >> platforms that already have good packaging technologies (i.e. almost >> every platform apart from Windows ;-). I'd hate to see Haskell go in a >> direction where packaging for e.g. Debian is made more difficult than it >> is at the moment. >> >> See [1] for the Debian Ruby packagers' opinion of RubyGems. IIRC >> similar concerns have been raised for Python's eggs. >> >> /M >> >> [1]: http://pkg-ruby-extras.alioth.debian.org/rubygems.html > Much of that's either outdated or just plain wrong, as I understand > it. In the interest of balance, note the following thread on > ruby-talk, which devolved fairly rapidly into a bunfight over Debian's > policies (and a comparison with Apple's approach to the same problem): > > http://www.nabble.com/-ANN--RubyGems-0.9.5-tf4840470.html > > There are arguments on both sides, but the utility of having RubyGems > available far outweighs the minor inconvenience of having to install > RubyGems outside apt. All I was trying to say is that if discussions like the one you link to can be avoided altogether then that would be good. I have never been heavy into Ruby programming, but I thought that the location for gems was the least of the problems they list. The major one, IFAIU is: “Rubygems is source-intrusive. The require instruction is replaced by a require_gem instruction to allow for versioned dependencies. Debian and most other systems think that dealing with versioned dependencies outside of the source is a better idea.” And it's not only a problem for Ruby in Debian: http://people.debian.org/~terpstra/message/20070904.152810.4f84c924.en.html “There are currently no plans to improve RubyGems to ease the work of Debian and RPM packagers.” I think that making it easy to package Haskell libraries, and programs, for any OS/distribution (Debian, Gentoo, PC-BSD, windows, etc) is /extremely/ important. I for one would like to see programs written in Haskell make it into as many places as possible. I'd hate to see a Haskell-specific packaging system prevent that in a similar way that RubyGems does. From the Debian Ruby team again: “[Users can] continue to install their apps the way they are used to (using apt-get), since most of them do not care about the language their apps are written in or about other ways this application/library is made available.” /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus

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"Using GHC as a library"?
by Galchin Vasili 22 Nov '07

22 Nov '07

Hello, I am reading 1) http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/API 2) http://haskell.org/haskellwiki/GHC/As_a_library Conceptually by allowing importation of GHC itself into a problem is this some kind of reification? Kind regards, vasya

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Best Linux for Haskell?
by Peter Verswyvelen 22 Nov '07

22 Nov '07

Each year I give Linux a try. And usually I kick it off my harddrive after a month, and stick to Windows. However, it does get better each year, so. So which kind Linux works best for running GHC (6.8.1) and related tools? (I want to give Yi a go, I can't get it to work on Windows). Debian? Fedora? Ubuntu? This is most likely a very stupid question, but please understand I'm a Windows user, so I'm supposed to be stupid J Thanks, Peter

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gtk2hs problem
by Gregory Propf 22 Nov '07

22 Nov '07

I've written a small program using the gtk2hs library and it crashes at unpredictable times with X windows errors like the one below. I looked for the messages online and found various people talking about buggy gtk libraries but no clear solutions. I don't know a lot about X windows or what these errors mean. Has anyone else had this problem or know what to do? The program is a simple implementation of the Conway Life game. It doesn't have very much going on except that the main window is an animation showing the evolution of the game. I tried running with the --sync option as it said and you get an almost immediate error like "Xlib: unexpected async reply (sequence 0x1652)!". Even these are not predictable and often come after several screen updates. I'm using the Gtk.timeoutAddFull function to do the animation. Note that the error_code, request_code, etc.. changes with each crash. It's as though it picks a random error code each time. I've compiled with both ghc 6.4.2 and 6.6.1 and gtk2hs 0.9.12 and 0.9.12.1. I've tried various combinations of -O flags as well. Same problem every time. A typical crash message ----------------------------------------------------------------------- The program 'hlh' received an X Window System error. This probably reflects a bug in the program. The error was 'BadDrawable (invalid Pixmap or Window parameter)'. (Details: serial 236321 error_code 9 request_code 62 minor_code 0) (Note to programmers: normally, X errors are reported asynchronously; that is, you will receive the error a while after causing it. To debug your program, run it with the --sync command line option to change this behavior. You can then get a meaningful backtrace from your debugger if you break on the gdk_x_error() function.) --------------------------------- Never miss a thing. Make Yahoo your homepage.

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A small question
by Andrew Coppin 22 Nov '07

22 Nov '07

I notice that in GHC 6.8.1, if I compile a runnably program, as well as generating foo.exe, GHC now also generates a file foo.exe.manifest, which appears to contain some kind of XML data. Anybody know anything about this mysterious file?

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yhc install fails
by Thomas Hartman 22 Nov '07

22 Nov '07

I'm having the devil of a time getting yhc to install. http://hpaste.org/4028 or for posterity root@none:~/yhc-install/yhc>scons ... blah blah blah... Compiling PreludeAux ( src/packages/yhc-base-1.0/PreludeAux.hs ) YHC_build(["src/packages/yhc-base-1.0/Foreign/Util.hbc"], ["src/packages/yhc-base-1.0/Foreign/Util.hs"]) inst/bin/yhc --core --cpp -c src/packages/yhc-base-1.0/Foreign/Util.hs Compiling Foreign.Util ( src/packages/yhc-base-1.0/Foreign/Util.hs ) YHC_build(["src/packages/yhc-base-1.0/Foreign/C/String.hbc"], ["src/packages/yhc-base-1.0/Foreign/C/String.hs"]) inst/bin/yhc --core --cpp -c src/packages/yhc-base-1.0/Foreign/C/String.hs Error: File not found, Foreign.Storable Reason: imported from Foreign.C.String Looked in: /root/yhc-install/yhc/src/packages/yhc-base-1.0/ /root/yhc-install/yhc/inst/lib/yhc/packages/yhc-base/1.0 scons: *** [src/packages/yhc-base-1.0/Foreign/C/String.hbc] Error 1 scons: building terminated because of errors. root@none:~/yhc-install/yhc>ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.1 advice? thomas.

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Re: wxHaskell / Proxima (was Tetris)
by Gerbo Engels 22 Nov '07

22 Nov '07

Peter Verswyvelen <bf3(a)telenet.be> wrote: > > Talking about wxHaskell, one of the applications shown on the website is > Proxima. I've been looking for something like that for a long time > (combined structural and freestyle editing). The screenshot > <http://wxhaskell.sourceforge.net/images/proxima-mac.png> seems to imply > that it supports Haskell. Anyone has experience with that? Recently, Proxima changed the GUI from wxHaskell to Gtk2Hs. And the screenshot is an instantiation of a Helium [1] editor (using Helium as the backend to show the type information and values in the editor), so no support for (full) Haskell yet. But I'm about to start my Master thesis project, which will (partially) be about creating a Proxima instance using GHC as the backend (and thus hopefully opening ways to implement a Haskell IDE). So maybe in some months Proxima does support full Haskell :) Gerbo [1] http://www.cs.uu.nl/helium/

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[Offtopic?] Cabal package license and copyright holder question
by Alfonso Acosta 22 Nov '07

22 Nov '07

Hi all, I don't know if this is the right place to ask but, since it's somehow Haskell-related I decided to use haskell-cafe. I'm cabalizing a library which, apart from my own code, has code taken from others. As it could be expected, each piece has its own copyright holder and (slightly) different license. The concrete holder and licenses are clearly annotated in every module. But, even with that, I'm not sure if I'm entitled to set an overall copyright holder and license for the whole cabal package. So, right now, I'm not sure about what license and holder filelds should I set in my .cabal file. Could anyone who had encounterd this problem before suggest a solution? Thanks, Fons

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More accessible papers
by Peter Verswyvelen 22 Nov '07

22 Nov '07

Most research papers have the same layout: two columns per A4 page. They mostly come as PDF or PS. Although this is standard, it is not really accessible for people with people with bad vision, who prefer larger fonts. When you print this, the fonts are rather small. For those people, a reflowable PDF would make much more sense, so they can choose how big the fonts are on screen & paper. Also, some of the new epaper devices, such as the Sony Reader or IRex, don't really support the format of these research papers (they become unreadable). Do any of you have ideas how to convert a two column per page PDF/PS to a single larger column per page, and maybe even reflowable PDF? Thanks, Peter

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State of the GHC-embedded Haddock (AKA Haddock 2.0)?
by Alfonso Acosta 22 Nov '07

22 Nov '07

Hi, Can someone shed some light on what's the state of GHC-Haddock? The thread [1] mentions a haddock.ghc repository which doesn't exist anymore. Are there any plans of releasing it anytime soon? I have a haddock-annotated library which makes massive use of TH, making Haddock 0.8 fail miserably Cheers, Fons .

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Re: GHC 6.8.1 and HDirect
by Olivier Boudry 21 Nov '07

21 Nov '07

On 11/21/07, Fernand <quarantedeux42(a)yahoo.fr> wrote: > > Hi dear fellow ghc users, > > I simply wanted to let people know that I apparently succeeded in > compiling a patched version of HDirect with GHC 6.8.1 (with typelibs > support). > At least, I could compile some of the examples and had very simple test > programs launch/connect to Word and Excel using automation. I lack time > to really clean up the library (I did not made any Cabal patch and > currently directly link with the « .a » and « .o » of the various > modules), but feel free to contact me if you need some help to use > hdirect with GHC 6.8.1. > > Sincerely yours, > > sL > > Hi Fernand, Congratulations! I once tried to compile HDirect with GHC 6.6.1 but finally stopped trying after having made a bunch of unsuccessful modifications. It's just too bad this library is no more maintained. Having a clean and maintained COM interface would be really valuable for Haskell. I would be interested in getting the patches for compiling HDirect with GHC 6.8.1. Cheers, Olivier.

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Re: [Haskell-cafe] More accessible papers
by Peter Verswyvelen 21 Nov '07

21 Nov '07

Christopher L Conway wrote: > style is attached (I'm sure many on the list already have it), in case > Peter is feeling brave. Note that the ACM has several different I'm feeling brave but tired ;-) Besides I'm spending all of my free time learning Haskell! :) I don't know tex at all, I just wanted to mention the unfortunate fact that people with bad eyes (or tired eyes, like mine) might have problems (well at least I have) reading this standard format. I did some Googling, and at first sight <http://www.ntg.nl/pipermail/ntg-pdftex/2006-May/002213.html> (but my sight is not that good ;-) creating a reflowable PDF from tex is not obvious. But it would already help a lot if instead of having two columns per page, we just had a single column, with less space between the margins, and using a >2x larger font. That can't be that hard I guess? Peter Well, I would Christopher L Conway wrote: > On Nov 19, 2007 2:57 PM, Neil Mitchell <ndmitchell(a)gmail.com> wrote: > >> Hi Peter, >> >> >>> Although this is standard, it is not really accessible for people with >>> people with bad vision, who prefer larger fonts. When you print this, the >>> fonts are rather small. For those people, a reflowable PDF would make much >>> more sense, so they can choose how big the fonts are on screen & paper. >>> >> It is the standard for ACM workshops and conferences, which includes >> ICFP and Haskell Workshop. All these PDF's are produced from a >> standard Latex class file. If you wrote to people expressing this, and >> did whatever magic is required to make the class file produce both the >> current format and a reflowable PDF, you might get somewhere. >> > > Neil, > > The person responsible for this at the ACM is Gerry Murray > (murray(a)hq.acm.org) He has been extremely responsive in the past when > I have had problems with the class file. The standard ACM conference > style is attached (I'm sure many on the list already have it), in case > Peter is feeling brave. Note that the ACM has several different > formats (e.g., journal vs. conference) and they would all need to be > updated. > > Chris >

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Re: [Haskell-cafe] More accessible papers
by Miguel Mitrofanov 21 Nov '07

21 Nov '07

> >Building a .tex file can be rather hard with packages and what-not, > >plus quite a few of us use lhst2tex as a preprocessor. It's not > >impossible, but its not trivial either, and I can't imagine that > >anyone would use a .tex over a PDF. > > > I would prefer the .tex version any day! Why not have both versions. Yeah, me too.

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Re: [Haskell-cafe] More accessible papers
by PR Stanley 21 Nov '07

21 Nov '07

>Hi Peter, > > > Yes, but why don't researchers just publish their TEX file? You can > > regard that as the "source code" for generating PDF/PS whatever no? > >Building a .tex file can be rather hard with packages and what-not, >plus quite a few of us use lhst2tex as a preprocessor. It's not >impossible, but its not trivial either, and I can't imagine that >anyone would use a .tex over a PDF. > >I would prefer the .tex version any day! Why not have both versions. Cheers, Paul

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Stream fusion for Hackage
by Don Stewart 21 Nov '07

21 Nov '07

Just a quick announce: the stream fusion library for lists, that Duncan Coutts, Roman Leshchinskiy and I worked on earlier this year is now available on Hackage as a standalone package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stream-fusion-0.… As described in the recent paper: "Stream Fusion: From Lists to Streams to Nothing at All" Duncan Coutts, Roman Leshchinskiy and Don Stewart. ICFP 2007 This is a drop-in replacement for Data.List. Haddocks here, http://code.haskell.org/~dons/doc/stream-fusion/ (but note the interface is exactly the same as the Data.List library) You might expect some small percent performance improvement for list heavy programs, using ghc 6.8 and this list library with -O2 (use -ddump-simpl-stats and look for: STREAM stream/unstream fusion messages, indicating your intermediate lists are getting removed. To get an idea of what is happening, consider this list program: foo :: Int -> Int foo n = sum (replicate n 1) Compiled with ghc-6.8.1 -O2 -ddump-simpl Normally, as sum is a left fold, an intermediate lazy list is allocated between the call to sum and replicate, as GHC currently does: foo :: Int# -> Int# foo n = Data.List.sum (case <=# n 0 of False -> go n True -> []) where go :: Int# -> [Int] -- intermediate list! go n = case <=# n 1 of False -> 1 : (go (n -# 1)) True -> [1] By using Data.List.Stream instead, you get a strict fused loop instead, with no intermediate structure allocated: loop_sum :: Int# -> Int# -> Int# loop_sum k n = case <=# n 0 of False -> loop_sum (k +# 1) (n -# 1) True -> k foo :: Int# -> Int# foo n = loop_sum 0 n This is a the halfway mark before porting other sequence types -- especially Data.ByteString -- over to the full stream fusion model (in particular, strict bytestrings will benefit a lot, due to the O(n) cost of intermediate bytestrings being removed). The stream fusion types and combinators are also available in stripped down form in the mlton sml compiler's extended prelude. Enjoy. -- Don

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Re: ANNOUNCE: fixpoint 0.1
by Bertram Felgenhauer 21 Nov '07

21 Nov '07

[redirecting from haskell@...] apfelmus wrote: [...] > I wonder whether a multi parameter type class without fundeps/associated > types would be better. > > class Fixpoint f t where > inject :: f t -> t > project :: t -> f t > [...] > Interestingly, this even gives slightly shorter type signatures > > cata :: Fixpoint f t => (f s -> s) -> t -> s > size :: (Fixpoint f t, Foldable f) => t -> Int size can't be used now though, because there is no way to infer f. Bertram

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Knot tying vs monads
by John D. Ramsdell 21 Nov '07

21 Nov '07

This is another Haskell style question. I had some trouble with the pretty printer that comes with GHC, so I translated one written in Standard ML. I have already translated the program into C, so rewriting it in Haskell was quick and easy for me. The Standard ML version uses a reference cell to keep track of the space available on a line. I threaded the value of the reference cell through the computation using a where clause to define two mutually recursive equations. The fixed point implicit in the where clause ties the knot in the circular definitions in a way that allows the output string to be efficiently computed front to back. I showed the code to a colleague, who found the circular definitions opaque. He suggested a better style is to use monads, and describe the computation in a mode that is closer to its origin form in Standard ML. What style do to you prefer, a knot-tying or a monad-based style? I have enclosed the pretty printer. The printing function is the subject of the controversy. John A simple pretty printer The alogithm is by Lawrence C. Paulson, who simplified an algorithm by Derek C. Oppen. Derek C. Oppen, Prettyprinting, ACM Transactions on Programming Languages and Systems, Vol 2, No. 4, October 1980, Pages 465-483. A pretty printer based on ML programs with the following copyright (**** ML Programs from Chapter 8 of ML for the Working Programmer, 2nd edition by Lawrence C. Paulson, Computer Laboratory, University of Cambridge. (Cambridge University Press, 1996) Copyright (C) 1996 by Cambridge University Press. Permission to copy without fee is granted provided that this copyright notice and the DISCLAIMER OF WARRANTY are included in any copy. DISCLAIMER OF WARRANTY. These programs are provided `as is' without warranty of any kind. We make no warranties, express or implied, that the programs are free of error, or are consistent with any particular standard of merchantability, or that they will meet your requirements for any particular application. They should not be relied upon for solving a problem whose incorrect solution could result in injury to a person or loss of property. If you do use the programs or functions in such a manner, it is at your own risk. The author and publisher disclaim all liability for direct, incidental or consequential damages resulting from your use of these programs or functions. ****) > module Pretty(Pretty, pr, blo, str, brk) where > data Pretty > = Str !String > | Brk !Int -- Int is the number of breakable spaces > | Blo ![Pretty] !Int !Int -- First int is the indent, second int > -- is the number of chars and spaces for strings and breaks in block Constructors Strings > str :: String -> Pretty > str = Str Break points > brk :: Int -> Pretty > brk = Brk Indentation blocks > blo :: Int -> [Pretty] -> Pretty > blo indent es = > Blo es indent (sum es 0) > where > sum [] k = k > sum (e:es) k = sum es (size e + k) > size (Str s) = length s > size (Brk n) = n > size (Blo _ _ n) = n Pretty prints the constructed object > pr :: Int -> Pretty -> ShowS > pr margin e s = > s1 > where > (_, s1) = printing margin [e] margin 0 (margin, s) The state of the computation is maintained as a pair consisting of an integer and a string. The integer is the number of unused character positions in the current line of output. The printer adds content to the front of the given string. > printing :: Int -> [Pretty] -> Int -> Int -> (Int, String) -> (Int, String) > printing _ [] _ _ p = p > printing margin (e:es) blockspace after (space, s) = > (space1, s1) > where > (space2, s1) = -- Result of first item > case e of > Str str -> -- Place a string > (space - length str, showString str s2) > Brk n -> -- Place breakable space > if n + breakdist es after <= space then > blanks n (space, s2) -- Don't break > else > (space3, showChar '\n' s3) -- Break > where > (space3, s3) = > blanks (margin - blockspace) (margin, s2) > Blo bes indent _ -> -- Place a block > printing margin bes (space - indent) > (breakdist es after) (space, s2) > (space1, s2) = -- Result of the remaining items > printing margin es blockspace after (space2, s) Find the distance to the nearest breakable space. > breakdist :: [Pretty] -> Int -> Int > breakdist (Str s : es) after = length s + breakdist es after > breakdist (Brk _ : _) _ = 0 > breakdist (Blo _ _ n : es) after = n + breakdist es after > breakdist [] after = after Place spaces > blanks :: Int -> (Int, String) -> (Int, String) > blanks n (space, s) > | n <= 0 = (space, s) > | otherwise = blanks (n - 1) (space - 1, showChar ' ' s)

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Re: [Haskell] recursive deriving
by Duncan Coutts 21 Nov '07

21 Nov '07

On Tue, 2007-11-20 at 19:18 -0500, Alex Jacobson wrote: > When you want automated deriving of show/read etc., you need all the > components of your type also to be instances of show/read but you won't > want to *require* them to be automatically generated verions. > > Standalone deriving does the wrong thing here. Standalone deriving > should not cause an overlapping instance error if someone derives an > instance manually. Instead, the manually derived instance should be > treated as more specific and win out. > > The other part of this problem is that you can't do automatic recursive > deriving and this results in a ridiculous amount of boilerplate. I know > some people have a theory that they want to avoid accidentally creating > instances for things that shouldn't have them, but the solution to that > is probably to add some declaration for types that prohibits automatic > deriving for those types. The 99% case is that automatic deriving is ok. > > Proposed syntax: > > derive instance Show T recursively > data T = T no-deriving (Ord,Eq) I would expect that if the data constructor for T is not exported then standalone deriving should not work. However this appears not to be the case which breaks module abstraction. Foo.hs: module Foo ( T, t ) where data T = T t = T Bar.hs: import Foo deriving instance Eq T $ ghci Bar.hs -XStandaloneDeriving [1 of 2] Compiling Bar ( Bar.hs, interpreted ) [2 of 2] Compiling Main ( Baz.hs, interpreted ) Ok, modules loaded: Bar, Main. *Main> t == t True You could write that Eq instance by hand since they do not have access to the T constructor, then StandaloneDeriving should not be able to so either. I think it's a design flaw in standalone deriving. Does anyone else agree? Should we file a bug report? Duncan

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SingHaskell 2007, November 28 2007
by Tom Schrijvers 21 Nov '07

21 Nov '07

We're pleased to invite you to SingHaskell: What is SingHaskell? Sing(apore)Haskell is a Haskell (and related languages) meeting in Singapore. The meeting is organized by Tom Schrijvers and Martin Sulzmann and will be hosted by the National University of Singapore. Date and location Sing(apore)Haskell takes place on Wed 28 Nov 2007 (right before APLAS'07 http://flint.cs.yale.edu/aplas2007/) The meeting will be held somewhere on the National University of Singapore campus (details will follow shortly). Let Tom or Martin know if you are interested in coming. Either to attend the meeting or even give a talk. For more information and tentaive talks: http://taichi.ddns.comp.nus.edu.sg/taichiwiki/SingHaskell2007 Best regards, Martin Sulzmann and Tom Schrijvers -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers(a)cs.kuleuven.be url: http://www.cs.kuleuven.be/~toms/

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Best way to format a number
by Maurício 21 Nov '07

21 Nov '07

Hi, I would like to get a string for a number using always 2 digits (i.e., 75 would be "75", 3 would became "03"). What is the best way to do that? I tried printf "%02d\n" 3 which gives me the correct string, but ghci always ends that call with a *** Exception: Prelude.undefined message. What else should I try? Can Text.PrettyPrint do that? Thanks, Maurício

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Re: Tetris
by nick ralabate 21 Nov '07

21 Nov '07

Speaking of Tetris and Space Invaders, you might be interested in this project: http://www.geocities.jp/takascience/haskell/monadius_en.html It's a clone of Gradius written in Haskell. -Nick

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timeout and waitForProcess
by Tim Bauer 21 Nov '07

21 Nov '07

Does anyone know if the new System.Timeout.timeout combinator can wakeup from System.Process.waitForProcess? I have not been able to make this work, and it appears as though the timeout exception is thrown only after waitForProcess returns normally (if it ever does). I am using Windows and GHC 6.8.1. import Control.Concurrent import System.IO import System.IO.Error(isEOFError) import System.Process(runInteractiveProcess, waitForProcess, ProcessHandle) import System.Timeout(timeout) halfSecond = 500000 example = timeout halfSecond sleepy sleepy = do (inp,out,err,pid) <- runInteractiveProcess "sleep.exe" ["10"] Nothing Nothing forkOS (ioGobbler out) forkOS (ioGobbler err) waitForProcess pid -- CAN timeout INTERRUPT THIS? return "PROCESS FINISHED" ioGobbler :: Handle -> IO () ioGobbler h = catch (ioDiscarder h) eofHandler ioDiscarder :: Handle -> IO () ioDiscarder h = do hGetChar h >> ioDiscarder h eofHandler :: IOError -> IO () eofHandler e | isEOFError e = return () | otherwise = ioError e Executing `example' from ghci will spawn (sleep.exe 10) which sleeps for 10 seconds. The timeout should happen in half a second (giving the thread time to get blocked into waitForProcess). However, instead, things block for ten seconds (until sleep.exe exits and waitForProcess returns normally). While a workaround is to spin and sleep with the getProcessExitCode and threadDelay, it seems like an inappropriate hack. The documentation for timeout lists some functions that are interruptible, but is not mean to be a complete list. Thanks all, - Tim -- View this message in context: http://www.nabble.com/timeout-and-waitForProcess-tf4847280.html#a13868823 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: Exposed module still hidden, why?
by Olivier Boudry 21 Nov '07

21 Nov '07

On 11/20/07, Greg Fitzgerald <garious(a)gmail.com> wrote: > > Using GHC 6.8.1 on Windows XP, after having used ghc-pkg to expose ' > directory-1.0.0.0', I am getting an error when I build haddock that says > the package is hidden. When I type "ghc-pkg list", the package is not in > parenthesis. Typing "ghc -v" says that it is using the file from > "C:\ghc\ghc- 6.8.1\package.conf". That package.conf file has the > 'exposed' set to True for that file. Why does GHC still think the package > is hidden? Hi Greg, You need to add "directory" to the Build-Depends instruction in the cabal file. The base package has been split into separate modules in 6.8.1. Build-Depends: base, directory Look at this: http://groups.google.com/group/fa.haskell/msg/17a78c120ae26514 Cheers, Olivier.

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-O2 bug in GHC 6.8.1?
by Brad Clow 21 Nov '07

21 Nov '07

I upgraded from GHC 6.6.1 to 6.8.1 and around that time I noticed that the output from an app I am working on changed. I have distilled the code down to the following example that produces different output depending on whether it is compiled with -O2 or not: main = do let (T x) = read "T 3" print $ f x print $ g x data T = T Int deriving (Read, Show) f x = truncate $ 2000 / fromIntegral ((x * 25) + 10) g :: Int -> Int g x = f x Here is the transcript from a shell (example code is saved in test.hs): $ ghc --make test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 23 23 $ rm test.hi test.o test $ ghc -O2 --make test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 23 24 It is a little disconcerting that -O2 changes the output. :-) Regards brad -- www.scoodi.com Contribute things you don't need and find things you do, all within your local community.

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Re: [Fwd: [Haskell-cafe] Fun with Cabal on Windows! [Stream fusion for Hackage]]
by Andrew Coppin 21 Nov '07

21 Nov '07

Duncan Coutts wrote: > On Mon, 2007-11-19 at 19:40 -0600, J. Garrett Morris wrote: > >>> Reproduced on mine too, I think I always have added that to the PATH as well. >>> >> Same here, I've also always had to add gcc-lib to the PATH. >> > > I've pushed what I think is a fix. It now works for me on windows. > > Mind you it always did for me because it was finding ld.exe from mingw's > path. Sigh. > > Can someone grab Cabal HEAD and make sure it works for them too please. > Just tested this with Duncan over IRC. Seems to be working just fine in Cabal HEAD and GHC 6.8.1. Finds ld.exe just fine, no PATH twiddling required. Nice work.

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will the real quicksort please stand up? (or: sorting a > million element list)
by Thomas Hartman 21 Nov '07

21 Nov '07

It has been noted in a few places that the 2 line quicksort demo in the Introduction section of the haskell wiki http://www.haskell.org/haskellwiki/Introduction isn't a "real" quicksort, and wouldn't scale well with longer lists. Interested, and wanting to put my recently learned test data generating skills to the test, I put this to the test with a little suite for various versions of quickcheck I found lying around. My patience extends to <3 minutes for waiting for a result, which for my tests was about the amount needed for a million integer long list. This was definitely faster for the "treesort" below than the naive quicksort from the wiki. (Prelude sort, which I think is mergesort, just blew the stack.) I wonder if there are "tricks" or lore that could be applied to get better results or insight. t. {-import Control.Monad (when) import Control.Monad.ST import Data.Array.ST import Data.Array.IArray import Data.Array.MArray import System.IO.Unsafe import Control.Monad -} import Test.QuickCheck hiding (test) import System.Random import Data.List {- $ time ghci -e 'test treesort 6' quicksort.hs # a "real" quicksort, according to roconnor 1000000 real 2m18.702s user 1m3.296s sys 0m0.453s $ time ghci -e 'test qs1 6' quicksort.hs # "naive" (from the wiki intro to haskell, not a "real" quicksort 1000000 real 4m18.174s user 1m57.615s sys 0m0.665s $ time ghci -e 'test sort 6' quicksort.hs # mergesort from the prelude *** Exception: stack overflow real 0m13.634s user 0m3.805s sys 0m0.446s hartthoma@linuxpt:~/ProjectRepos/learning/quicksort> -} -- For interactive development, I wound up working in hugs +S +I, -- because ghci in emacs tended to lock up -- hugs just segfaults, but at least the computer doesn't lock. -- by the way, is there a way to get ghci to behave similarly to hugs? (restricted memory mode?) -- t1 and t2 are about the same speed for n = 4, got bored of waiting for n great. -- naive, "not really" quicksort according to recent reddit discussion. test sortf n = genlist n >>= return . ( length . sortf ) -- naive qs1 [] = [] qs1 (x:xs) = qs1 (filter (< x) xs) ++ [x] ++ qs1 (filter (>= x) xs) -- roconnor claims that "real" quicksort in haskell is really treesort. -- from http://programming.reddit.com/info/2h0j2/comments --I'm talking about a deforested version of tree sort. In tree sort you build a binary search tree by adding the head of the list to the root of the tree, and then building binary search trees for those items less than the root, and those items greater than the root. qs2 l = treeSort l treeSort l = foldTree (\x lft rht -> lft++[x]++rht) [] (mkTree l) -- After building the search tree, it is turned into a list by folding \x lft rht -> lft++[x]++rht. foldTree f g Leaf = g foldTree f g (Node x l r) = f x (foldTree f g l) (foldTree f g r) mkTree [] = Leaf mkTree (x:xs) = Node x (mkTree (filter (<= x) xs)) (mkTree (filter (x <) xs)) -- If you deforest this algorithm (removing the intermediate tree structure) you are left with -- treeSort' [] = [] -- treeSort' (x:xs) = treeSort' (filter (<= x) xs) ++ [x] ++ treeSort' (filter (x <) xs) -- for testing genlist n = rgen ( vector $ 10^n ) :: IO [Int] rgenIntList = rgen (arbitrary :: Gen [Int]) :: IO [Int] rgen gen = do sg <- newStdGen return $ generate 10000 sg gen data Tree a = Leaf | Node a (Tree a) (Tree a) --- 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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user error when using Text.Regex.PCRE
by Olivier Boudry 21 Nov '07

21 Nov '07

Hi all, I'm getting a strange user error when working with Data.ByteString.Char8 and Text.Regex.PCRE. Error I get is: CustomerMaster: user error (Text.Regex.PCRE.ByteString died: (ReturnCode 0,"Ptr parameter was nullPtr in Text.Regex.PCRE.Wrap.wrapMatch cstr")) The part of the code causing problems is at the end of this e-mail. The complete code is attached. I'm working at cleansing customer addresses for a ERP system data migration. The failing function is supposed to split some bytestrings using a list of regular expressions (split on words like suite, building, attention, etc...) stored in the "tags" binding. I use a foldl' to try each regular expression on each address (addr is a list of address part, can be seen as a list of bytestring values). The funny and weird thing is that when I have only one regular expression in tags, it works. When I have more than one regular expression in tags it fails (user error) the first time one of the regular expression matches. A match replaces the current address part with 3 new parts, before, match, after. The next regular expression in the tags list will test the new before and after parts. I presume it's the testing of those newly created parts that causes the problem. Any idea about what could go wrong here? Thanks, Olivier. === code === -- | Split unparsed address parts on keywords (suite, building, doors, etc...) splitOnTags :: AddressState splitOnTags = do addr <- get put $ foldl' f addr tags where f a t = concatMap (split t) a split (T {partType = p, regex = r}) a@(AP X v) | BS.null y = [a] | otherwise = [AP X x, AE p, AP X z] -- new address parts where (x, y, z) = match r v split _ a = [a] -- | Regex and part type representing tags used for splitting addresses tags :: [Tag] tags = [T AT (makeRegexOpts compCaseless execBlank "\\b(?:ATTN|ATTENTION|C/O)\\b[[:punct:]]?") (BS.pack "ATTN: ") ,T PB (makeRegexOpts compCaseless execBlank "\\b(?:(?:P\\.?\\s?O\\.?\\s*)BOX|C\\.P\\.|POSTFACH)\ \[[:punct:]]?\\s+(?:NO.\\s+)?") (BS.pack "PO BOX: ") ,T BLDG (makeRegexOpts compCaseless execBlank "\\b(?:BLDG|BUILDING|HANGAR)[[:punct:]]?") (BS.pack "BLDG: ") ,T DOCK (makeRegexOpts compCaseless execBlank "\\bDOCK[[:punct:]]?") (BS.pack "DOCK: ") ,T STE (makeRegexOpts compCaseless execBlank "\\b(?:SUITE|STE|APT|ROOM)[[:punct:]]?\ \\\s+(?:NO[[:punct:]]?)?") (BS.pack "STE: ") ,T UNIT (makeRegexOpts compCaseless execBlank "\\bUNIT[[:punct:]]?") (BS.pack "UNIT: ") ,T FLOOR (makeRegexOpts compCaseless execBlank "FLOOR") (BS.pack "FLOOR: ") ,T DOORS (makeRegexOpts compCaseless execBlank "\\bDOORS?[[:punct:]]?") (BS.pack "DOORS: ")]

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ANNOUNCE: Important bug fix for regex-pcre ByteStrings.
by ChrisK 21 Nov '07

21 Nov '07

Greetings, There are new version 0.82 and 0.93 of regex-posix. If you use regex-posix with Data.ByteString then you should upgrade to obtain a fix for a crash error. There are new version of regex-pcre available on hackage and the two darcs repositories: http://darcs.haskell.org/packages/regex-pcre/ for "stable" version 0.82 and http://darcs.haskell.org/packages/regex-unstable/regex-pcre/ for "unstable" version 0.93 The new version regex-pcre-0.82 replaces the broken version 0.81 The new version regex-pcre-0.93 replaces the broken version 0.92 I apologize for the error. I took the address of the ByteString to give to the pcre library by using unsafeUseAsCStringLen. I had not appreciated that this can return a null pointer for an empty ByteString. My code caught this null pointer and reported it, however, and so a good error message sent in by a user allowed me to quickly find and hopefully fix it. For version 0.92 the error also applied to Lazy ByteStrings. In case anyone is curious, the fixed code checks the pointer returned by unsafeUseAsCStringLen and replaces a null pointer to a non-null pointer to a private private bytestring "myEmpty" with the length set to 0 by "trim": > {-# INLINE asCStringLen #-} > asCStringLen :: ByteString -> (CStringLen -> IO a) -> IO a > asCStringLen s op = B.unsafeUseAsCStringLen s checked > where checked cs@(ptr,_) | ptr == nullPtr = B.unsafeUseAsCStringLen myEmpty (op . trim) > | otherwise = op cs > myEmpty = B.pack [0] > trim (ptr,_) = (ptr,0) This fix is motivated by desiring to ensure asCStringLen never passes a null pointer to "op" and that no memory copy operations are used. The safe "useAsCString(Len)" do not appear capable of returning a null pointer. Only "unsafeUseAsCString(Len)" on an empty bytestring might return a null pointer. -- Chris

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Re: [Haskell] Recruiting functional programmers
by Mattias Bengtsson 20 Nov '07

20 Nov '07

> Interested in recruiting Haskell programmers from Chalmers/Gothenburg > university? As an experiment, I am planning a recruitment event here in > December-see www.jobs-in-fp.org for how to take part. > Just checked my calendar and all according to Murphy's law this had to be the weekend when i'm in Stuttgart on vacation. Will there be more events if this one turns out successful? /Mattias --

1 0

Translations and Haskell
by Felipe Lessa 20 Nov '07

20 Nov '07

Hello, I'd like to start a project using Gtk2Hs and one thing is concerning me: what's the current approach on writing portable and translatable GUI programs in Haskell? I couldn't find anything on some quick searchs on haskell.org, hackage and google. If there aren't any libraries available, are there any suggestions on how to create one? Would it be too much job for someone without experience with gettext to write bindings and create and extractor like xgettext (or modify it)? Maybe TH can help? I don't know, I'm just throwing out some random ideas =). Cheers, -- Felipe.

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RFC: demanding lazy instances of Data.Binary
by Nicolas Frisby 20 Nov '07

20 Nov '07

I've noticed a few posts on the cafe, including my own experience, where the spine-strictness of the Binary instance for lists caused some confusion. I'd like to suggest an approach to preventing this confusion in the future, or at least making it easier to resolve. Having decided that it is indeed appropriate for some standard instances to be strict by default [1], I think it would be beneficial to standardize an approach for expressing that a lazy instance is expected. I propose the following newtype be added to Data.Binary. A demonstration immediately follows. > newtype Lazily a = Lazily { unLazily :: a } > -- example instance > instance Binary a => Binary (Lazily [a]) where > -- lazy get and put Now > [1..] = (unLazily . decode . encode . Lazily) [1..] instead of > _|_ = (decode . encode) [1..] This email is a request for comments on this concept. I think it is a minimal way of expressing the intent that the serialisation be lazy [2]. Please share any concerns or suggestions. I'll submit a patch once the discussion is complete... or if it becomes inactive ;) Thanks for your time, Nick 1 - One solution is to make all standard Binary instances lazy wherever possible, but I presume that in most cases it's not needed and the compactness gained through default strictness (such as the [] instance) is quite significant. 2 - A more involved option is to recognize that serialisation always maps to a sequence, and so create another standard data type and class. > data Chunks a = Empty | Chunk !a (Chunks a) -- a la lazy bytestrings > instance Binary a => Binary (Chunks a) where > -- lazy put and get > class Chunky a chunk | a -> chunk where > toChunks :: a -> Chunks chunk > fromChunks :: Chunks chunk -> a All of this machinery, however, may prematurely emphasize problem-specific concerns. Thus it obfuscates the point: we just want sufficient laziness. Whatever ends up happening, the Chunks data type may be a route so common for Lazily instances that it makes sense to provide it in a library (? http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict-0.2)

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HTTP actions & proxy server
by Jim Burton 20 Nov '07

20 Nov '07

How would I go about converting the little get program at http://darcs.haskell.org/http/test/get.hs to use a proxy server? I tried adding a call to setProxy like this but it doesn't work: get :: URI -> IO String get uri = do browse $ setProxy (Proxy "myproxy:80" Nothing) eresp <- simpleHTTP (request uri) resp <- handleE (err . show) eresp case rspCode resp of (2,0,0) -> return (rspBody resp) _ -> err (httpError resp) where showRspCode (a,b,c) = map intToDigit [a,b,c] httpError resp = showRspCode (rspCode resp) ++ " " ++ rspReason resp Thanks. -- View this message in context: http://www.nabble.com/HTTP-actions---proxy-server-tf4815272.html#a13775608 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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binary operator modifiers
by Tim Newsham 20 Nov '07

20 Nov '07

I would love to have the ability to define binary operator modifiers. For example: f \overline{op} g = liftM2 op f g f \overleftarrow{op} n = liftM2 op f (return n) n \overrightarrow{op} g = liftM2 op (return n) f \widehat{f} x = liftM f x so that for example you could define functions like: (*3) \overline{+} (/2) and 3 \overrightarrow{+} \widehat{read} getContents Obviously you could write this out the long way: liftM2 (3+) $ liftM read getContents or go through the trouble of defining a bunch of binops f <+> g = liftM2 (+) f g n +> g = return n <+> g f <+ n = f <+> return n read' = liftM read (*3) <+> (/2) 3 +> read' getContents but doing this for more than one or two operators gets tedious quickly... Is there any way in Haskell to modify binops in this way and still be able to use them infix? If not, has anyone considered supporting strange syntaxes like this? Tim Newsham http://www.thenewsh.com/~newsham/

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Re: [Haskell-cafe] Properties of monads
by Radosław Grzanka 20 Nov '07

20 Nov '07

2007/11/19, Ryan Ingram <ryani.spam(a)gmail.com>: > I am pretty sure that the constructor MaybeT is exactly what you are looking > for. > > newtype MaybeT m a = MaybeT (m (Maybe a)) > > therefore > > MaybeT :: m (Maybe a) -> MaybeT m a > I swear I tried it. I swear.. Why didn't it worked than and it works now?? ;) Thank you very much for a tip. :) Thanks, Radek (who can go sleep now with clear mind). -- Codeside: http://codeside.org/ Przedszkole Miejskie nr 86 w Lodzi: http://www.pm86.pl/

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Properties of monads
by Radosław Grzanka 20 Nov '07

20 Nov '07

Hello, I am writing some toys programs to learn and try to apply Monads properties (without success, I must say). Although I spent half a day on this code: http://hpaste.org/3957 I couldn't simplify (shorten) getStrip function. After reading "Doing it with class" ( http://www.haskell.org/all_about_monads/html/class.html ) I had an impression that I could "collapse" cases using some Monads properties. But maybe I misunderstood something. Can anyone look at it and give me a pointers?? (I don't mind if the code becomes unreadable a bit.) Thank you, Radek.

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