- Haskell-Cafe - Haskell.org

Renaming a package on Hackage
by Jeff Heard 25 Dec '08

25 Dec '08

How does one go about renaming a package on Hackage? I finally figured out a name for Thingie, and I'd like to change the package name to reflect that, so that future versions look to the new package and not the old one.

3 2

Time for a new logo?
by Don Stewart 25 Dec '08

25 Dec '08

I noticed a new haskell logo idea on a tshirt today, http://image.spreadshirt.net/image-server/image/configuration/13215127/prod… Simple, clean and *pure*. Instead of the "we got lots going on" of the current logo. Any graphic designers want to try some variations on this theme of purity? A new year, and a new mature logo... -- Don

57 113

Defining a containing function on polymorphic list
by Raeck Zhao 24 Dec '08

24 Dec '08

I am trying to define a containing function to see if a value is one of the elements within a list which is polymorphic, but failed with the following codes: > contain :: a -> [a] -> Bool > contain x [] = False > contain x (y:ys) = if x == y then True else contain x ys it seems that the problem is the 'operator' == does not support a polymorphic check? Any way can solve the problem? or any alternative solution to achieve the purpose? Thanks! Raeck _________________________________________________________________ It’s the same Hotmail®. If by “same” you mean up to 70% faster. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_…

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Type classes vr.s functions
by Brian Hurt 24 Dec '08

24 Dec '08

So, style question for people, if I can. I have a certain problem- basically, I have a bunch of functions which need a special function, of type a -> Foo say. And a bunch of other functions which can define that function on some type of interest, and then what to call the first batch of functions. I can do this either by defining a type class, something like: class Fooable a where toFoo :: a -> Foo or I can simply have all the functions which need a toFoo take an extra agrument. Performance really isn't that important here, so it's really a matter of style- which approach would people prefer in this case? Brian

4 4

understanding enumerator/iteratee
by Jason Dusek 24 Dec '08

24 Dec '08

So, it looks Iteratee takes a "step" on the resource -- whatever it is -- and Enumerator manages the resource and sequences the steps of the Iteratee. The Enumerator, then, defines our way of managing a particular resource -- how to take a step, how to close it, &c. -- while the Iteratee describes a computation that computes an output and also tells us when to free the resource. Is that a correct interpretation? I find the material on Enumerator and Iterator to be vague (or at least not very concrete). I have seen code for a random access Iteratee/Enumerator pair, as well. In that case, the role of the Iteratee is to provide the "next step" to take -- one of forward <n>, backward <n>, close? -- _jsn

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Can I build and install GHC 6.10.1 without previous installed ghc
by Wang, Chunye (NSN - CN/Beijing) 24 Dec '08

24 Dec '08

Hi All, I have our own Linux distribution installed in my server. it is based on RedHat 9.0. Because it is the server, I can not update it to any other linux distribution Here is my situation, I have no machine with any version of ghc installed. This is what I did % wget http://haskell.org/ghc/dist/6.10.1/ghc-6.10.1-src.tar.bz2 % wget http://haskell.org/ghc/dist/6.10.1/ghc-6.10.1-src-extralibs.tar.bz2 % tar -jxf ghc-6.10.1-src-extralibs.tar.bz2 %tar -jxvf ghc-6.10.1-src.tar.bz2 % cd ghc-6.10.1 % sh boot grep: packages: No such file or directory Booting . Booting libraries/base Booting libraries/directory /usr/share/aclocal/libgcrypt.m4:23: warning: underquoted definition of AM_PATH_LIBGCRYPT run info '(automake)Extending aclocal' or see http://sources.redhat.com/automake/automake.html#Extending-aclocal /usr/share/aclocal/ao.m4:9: warning: underquoted definition of XIPH_PATH_AO Booting libraries/editline Booting libraries/network Booting libraries/old-time Booting libraries/process Booting libraries/regex-posix Booting libraries/time Booting libraries/unix % ./configure checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu Canonicalised to: x86_64-unknown-linux checking for ghc... no checking for ghc-pkg matching ... no checking for ghc-pkg... no checking whether ghc has editline package... no checking for nhc... no checking for nhc98... no checking for hbc... no checking for ld... /usr/local/bin/ld configure: error: GHC is required unless bootstrapping from .hc files. I know http://hackage.haskell.org/trac/ghc/wiki/Building/Prerequisites said that GHC 6.6 is required to build and install the GHC 6.10.1 So I try to follow the instructions http://hackage.haskell.org/trac/ghc/wiki/Building/Porting. Because I don't have any ghc installed in any machine. I have to use the "Porting GHC to a new platform " %./configure --enable-hc-boot --enable-hc-boot-unregisterised checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu Canonicalised to: x86_64-unknown-linux checking for ghc... no checking for ghc-pkg matching ... no checking for ghc-pkg... no checking whether ghc has editline package... no checking for nhc... no checking for nhc98... no checking for hbc... no checking for ld... /usr/local/bin/ld checking for path to top of build tree... ./configure: line 2733: -v0: command not found ./configure: line 2737: utils/pwd/pwd: No such file or directory configure: error: cannot determine current directory I think it is because the utils/pwd/pwd.hs is not compiled and linked into the ``pwd''. So I create a bash version of pwd #!/bin/bash pwd And m make it executeble, chmod +x utils/pwd/pwd Then I try to run the ./configure --enable-hc-boot --enable-hc-boot-unregisterised again %./configure --enable-hc-boot --enable-hc-boot-unregisterised again %cd includes %make %cd .. Finnally, I found I still need ghc is intalled on the host machine. %perl --version This is perl, v5.8.5 built for x86_64-linux-thread-multi Copyright 1987-2004, Larry Wall Perl may be copied only under the terms of either the Artistic License or the GNU General Public License, which may be found in the Perl 5 source kit. Complete documentation for Perl, including FAQ lists, should be found on this system using `man perl' or `perldoc perl'. If you have access to the Internet, point your browser at http://www.perl.com/, the Perl Home Page. %gcc --version gcc (GCC) 4.1.2 Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %make --version GNU Make 3.80 Copyright (C) 2002 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %autoconf --version autoconf (GNU Autoconf) 2.59 Written by David J. MacKenzie and Akim Demaille. Copyright (C) 2003 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %automake --version automake (GNU automake) 1.9.2 Written by Tom Tromey <tromey(a)redhat.com>. Copyright 2004 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %sed --version GNU sed version 4.1.2 Copyright (C) 2003 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, to the extent permitted by law. %diff --version diff (GNU diffutils) 2.8.1 Copyright (C) 2002 Free Software Foundation, Inc. This program comes with NO WARRANTY, to the extent permitted by law. You may redistribute copies of this program under the terms of the GNU General Public License. For more information about these matters, see the file named COPYING. Written by Paul Eggert, Mike Haertel, David Hayes, Richard Stallman, and Len Tower. %python -V Python 2.3.4 Best Regards Chunye Wang <chunye.wang(a)nsn.com>

4 7

monad constraint + record update
by Peter Padawitz 24 Dec '08

24 Dec '08

I'd like to define a monad Set for types in the class Eq. But how can the arguments of Set be constrained when Set is defined as an instance of Monad? instance Eq a => Monad Set where ... obviously cannot work. Is there a standard update function for fields in data types, something that OO programmers do with assignments like obj.attr := value ? Peter

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intercalate and (byte)strings
by wman 24 Dec '08

24 Dec '08

I encountered the following code : -- B == Data.ByteString ; L == Data.ByteString.Lazy contents' = B.intercalate B.empty $ L.toChunks contents with a previously unencountered function intercalate. A quick google query later i knew that it's just intersperse & concat nicely bundled and started wondering why anybody would do this, as simple contents' = B.concat $ L.toChunks contents would do (probably nearly) the same. The only thing I am able to come up with is that it somehow helps streamline the memory usage (if it has some meaning). Is there some reason to use intercalate <empty> <list> instead of concat <list> (probably when dealing with non-lazy bytestrings) ? Thx, wman.

4 4

Detecting system endianness
by Mauricio 24 Dec '08

24 Dec '08

Hi, Is there some way I can check the endianness of the machine my haskell code is running in? Thanks, Maurício

8 12

Is this related to monomorphism restriction?
by Maurício 24 Dec '08

24 Dec '08

Hi, Why isn't the last line of this code allowed? f :: (TestClass a) => a -> Integer f = const 1 a = (f,f) g = fst a The only thing I can think about is monomorphism restriction, but it's allowed (or even the third line would not be accepted). Is there something I could read to understand that? Thanks, Maurício

6 8

Parsec question
by Erik de Castro Lopo 24 Dec '08

24 Dec '08

Hi all, I'm rather new to Haskell and I'm diving right into the deep end writing a parser using Parsec. In particular I'm using Text.ParserCombinators.Parsec.Language to do some of the heavy lifting and have this: import qualified Text.ParserCombinators.Parsec.Language as L import qualified Text.ParserCombinators.Parsec.Token as T lexer = T.makeTokenParser L.emptyDef { L.identStart = letter <|> char '_', L.identLetter = alphaNum <|> char '_', .... identifier :: CharParser st String identifier = T.identifier lexer and now I need to parse things "this.that.the.other". I'd like to have a function with the following signature: qualifiedIdentifier :: CharParser st [ String ] which should return [ "this", "that", "the", "other" ] and write it in terms of identifier and thats where I'm stuck. Anyone care to whack me with the cluestick? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "If you don't have freedom as a principle, you can never see a reason not to make an exception." -- Richard Stallman.

3 6

Re: [Haskell-cafe] Re: [Haskell] ANN: HLint 1.0
by Claus Reinke 24 Dec '08

24 Dec '08

>>> Well, sort of. Ok, we can parse that. Let's assume a variable x holds >>> the output of :show modules as a String. We call lines on it, then map >>> words on it, do a !! 2 on it, and we get ["Util.hs,", "Recorder.hs,", >>> "Game.hs,", "Monadius.hs,", "Demo.hs,"]. Chuck in a map (filter (\= >>> ',')), and we get a good list. We can turn the list into a string >>> suitable for hlint with a quick unwords. >>> >>> So our long sought after command becomes ':def hoogle (\_ -> return $ >>> ":! " ++ (unwords $ map (filter (\= ',')) $ (map words $ lines x) !! >>> 2))'. But wait, how do we get 'x'? How do we call :show modules inside >>> a Haskell expression? >> The first url includes a link to a .ghci mini-tutorial (section 4) that, >> among other things, implements >> :redir <var> <cmd> -- execute <cmd>, redirecting stdout to <var> > Perhaps my cold has fogged my head too much, but I'm not sure how > :redir would help. I could do :redir var "hlint .", but that's as > unsatisfactory as :! "hlint ." You were asking about getting the output of ':show modules' into a variable 'x', so that you can process it further. ':redir x :show modules' should do just that. There is another example command for implementing ':edit' this way (by now a native ghci command). Claus

1 0

example editrc
by brian 24 Dec '08

24 Dec '08

This is making ghci with editline nicer for me. It says to use the vi key mapping, tab for completion, C-l to clear the screen, and jj to go into vi command mode. $ cat ~/.editrc bind -v bind \\t rl_complete bind ^L ed-clear-screen bind jj vi-command-mode

1 0

Initializing GHC from Python
by Ron de Bruijn 24 Dec '08

24 Dec '08

Hi, We have just published a small article on how one can initialize GHC from Python, with only optional use of C. You can read it at http://gamr7.com/blog/?p=65 . Best regards, Ron de Bruijn

2 1

New Haskell logo contest
by Fritz Ruehr 24 Dec '08

24 Dec '08

I think this logo contest is a great idea. I submitted my "classy Haskell" logo from the merch. page, but I have to admit I like some of the other ones on the submission page a whole lot. Hey, *here's* an idea: maybe whoever wins the logo contest has to take over management of the Haskell merchandise page on CafePress! :) -- Fritz Ruehr

2 1

cabal, multi-file applications
by Cetin Sert 24 Dec '08

24 Dec '08

Hi, I get the following error message: cetin@unique:~/lab/test/qths/hnm$ make configure runhaskell Setup.hs configure Configuring HNM-0.2... cetin@unique:~/lab/test/qths/hnm$ make build runhaskell Setup.hs build Preprocessing library HNM-0.2... Preprocessing executables for HNM-0.2... Building HNM-0.2... demo3.hs:4:7: Could not find module `HNM.WLAN': Use -v to see a list of the files searched for. make: *** [build] Error 1 when I try to build the following program: http://sert.homedns.org/hs/hnm/ http://sert.homedns.org/hs/hnm/hnm.cabal How can I tell in my cabal file that wlan.hs should be built first than settings.hs than demo3.hs? Best Regards, Cetin Sert

3 2

[Byte8] <-> ByteString
by Galchin, Vasili 23 Dec '08

23 Dec '08

Hello, I have been reading through Data->ByteString. What is the is most elegant and efficient way to map/unmap [Byte8] <-> ByteString? Thanks, Vasili

3 3

Cabal Install & Links to Source from Haddock Docs
by R Hayes 23 Dec '08

23 Dec '08

Is there a way I can get Haddock Docs WITH links to source (local) from modules installed with "cabal install xxx"? Getting the docs themselves is pretty easy by changing either ~/.cabal/ config or using --enable-documentation. Automatically generating the source (colourised or not) and integrated links eludes me. -r

3 3

Re: understanding enumerator/iteratee
by oleg＠okmij.org 23 Dec '08

23 Dec '08

Artyom Shalkhakov wrote > I would say that it [iteratee] just tells us how to react to various forms of > input. :) This is much like the function you pass to foldr. Precisely. To sum up all elements of Data.Map, we do Map.fold (+) 0 mp to sum up all elements of a set we do Set.fold (+) 0 st ditto for any other foldable data structure. Clearly the function that sums the current element with the accumulator, (+), doesn't know or care from which collection the elements are coming from. The initial seed, 0, is again unaware of the collection. Iteratee is indeed the function that you pass to fold (combined with the seed for practical reasons). One may conceptually consider iteratee to be a pair of the function to feed to fold, and the initial seed (the accumulator in the above example). That achieves the separation of concerns: fold (aka, enumerator) has the intimate knowledge of the collection and how to get to the next element; iteratee knows what to do with the current element.

1 0

Takusen
by Günther Schmidt 23 Dec '08

23 Dec '08

Hi all, where can I find some sample code or other examples to familiarize me with Takusen? Does Takusen use safe ccall only, or does it also use ccall unsafe? Günther

2 1

A hell of a question
by Andrzej Jaworski 23 Dec '08

23 Dec '08

I just want to say Hello to let you know that there are some serious entities watching you besides monads and FBI:-) There has been a hell of a discussion recently about logos, languages and religion and I want to add to this. First let me disassociate Haskell from Taoism which to may taste has left us in an unhealthy climate. It suffices to say that Taoism is a school of clever trics and cute aphorisms but without the slightest attempt to explain or generalize let alone produce an abstract idea or a system. That is why its wisdom is non transferable in spite of majority of humans desending from it. Haskell on the contrary is a minority school that implements abstract ideas for problem solving in the most transferable way to date, so that other languages look into it for their share. But don't worry, thay will choke becouse it is them who practice Taoizm. Playing too many tricks will eventually trick them, even if some are powerful enough to brainwash dicent professors to preach interoperability or the like. Every viable complexity needs a single underlying concept to survive, including you and the universe. Microsoft and the like excluding;-) Haskell has all that: consistency, transparency and self-contained concept. However Haskell is also somehow asynchronic with the Bible as it is condemned to perpetual purity only to be saved from it via monads, which according to Spinoza "are arranged by God in a perfect order which ascends to God, the supreme monad!!!". Thus we can look at Haskell's purity as a kind of harmless attempt to play God by means of incapsulating the world only to ignor it. This could somehow put it in the same boat with the devil. They both prefer your brain to Turing machin:-) I would leave with the eternal question "what the hell Haskell is?" and with my Santa Close emphasising Haskell's purity and my simplicity. Here: http://haskell.org/sitewiki/images/7/79/WCF.Andrzej.Jaworski.gif Have fun, -Andrzej Jaworski

4 5

OpenAL
by brian 23 Dec '08

23 Dec '08

Does the example code at http://articles.bluishcoder.co.nz/Haskell/OpenAL work for anyone? I added some putStrLns to see the device and context, and get $ ./Main2 Device (ALCdevice 0x08a744c8) Context (ALCcontext 0x08aabda0) AL lib: alSource.c:2291: alcDestroyContext(): 1 Source(s) NOT deleted AL lib: alBuffer.c:1097: exit() 2 Buffer(s) NOT deleted and no sound. I had more complex code using OpenAL working some time ago, but the sound was crackly because my distribution used the original OpenAL implementation. Now that people are switching to OpenAL Soft, Haskell's OpenAL seems to be acting strangely. I mailed Sven in September, but didn't hear back.

1 0

"Rewrite thunk action" rule?
by Peter Todd 23 Dec '08

23 Dec '08

I have a program with this data structure: data Element = Element { elementOrigin :: V, elementSubs :: [Element] } and this important bit of code that operates on it: transform :: T -> Element -> Element transform t e = Element { elementOrigin = tmulv t (elementOrigin e), elementSubs = map (transform t) (elementSubs e) } Where V is a vertex and T is a matrix transformation. The following is true: transform a (transform b e) == transform (a * b) e I have read about rewrite rules, such would efficiently rewrite the obvious transformations. However, given that the program will be applying *many* transformations to very deep and wide Element trees, and applying those transformations at many levels, I don't see how the optimizer would be able to handle all cases, for instance a thunk getting left over from perhaps evaluation prior to some IO function. FWIW here's an example "tree-builder" I'm using to test with: mkElems 0 _ = Element { elementOrigin = V 0 0, elementSubs = [] } mkElems depth width = Element { elementOrigin = V 0 0, elementSubs = replicate width $ transform (rotation (pi / (fromIntegral width))) $ transform (translation $ V 1 1) $ mkElems (depth - 1) width } with depth = width = 5 If I could somehow arrange for the transform function to detect when it is being applied to a unevaluated thunk, and then modify the thunk in place, that would basically be the behavior I need. Any suggestions? -- http://petertodd.org 'peter'[:-1]@petertodd.org

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Re: [Haskell-cafe] A hell of a question
by Andrzej Jaworski 23 Dec '08

23 Dec '08

Arnaud Bailly kindly exposed my mistake: I cited Leibniz Monadology attributing it to Spinoza. Call me idiot but it wasn't thoughtless mistake, I really mixed up Spinoza's concept of 'modes' with Leibniz's concept of 'monad'. But it is because of my laziness rather than foolishness (at least this time). My argument is that in his final writings Leibniz's 'monads' encompass reality as perceptions and emanate from God as thought emanates from the mind, which is exactly what Spinoza's 'modes' are about. So monads and modes being equal I prefer Spinoza as a patron for Haskell for his purity of cause-effect treatment. Arnold adds another Haskell-Spinoza nicety, I hope he won't mind my including his all letter. Thank you Arnold, it is encouraging to find guys like you. Philosophy is not my thing but I believe every man should tackle it as exercise. Hello Andrzej, Thanks a lot for your bit of rationalism among all these devotions :-) I think however that you are confusing Spinoza with Leibniz in the following assertion. Monads are a concept invented by the latter, together with the famous "the best possible world ever" mocked by Voltaire in "Candide". The comparison between Haskell and spinozism is rather interesting though, especially when one considers that Spinoza's "Ethic" is based on the idea that the ultimate goal of one self is to increase its power to live, an affect which it calls "Joy". Thinking of Haskell as a way to increase one's joy and one's power is a nice thought. Regards, Arnaud Bailly

1 0

Re: [Haskell] ANN: HLint 1.0
by Gwern Branwen 23 Dec '08

23 Dec '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 So has anyone figured out how to integrate hlint with ghci? I've tried several approaches. Using hlint as a preprocessor ( http://www.haskell.org/ghc/docs/latest/html/users_guide/options-phases.html… ) doesn't work because the pre-processor needs to take 3 args. Even if one writes a wrapper shell script along the lines of 'cp $2 $3 && hlint $2', this still won't work. It apparently breaks when you load a file which requires another file. I am not sure why. I then thought to redefine :load and :reload in ghci; this would work very well with Emacs and Yi, since they tell ghci to :load and :reload buffers. Turns out, you're not allowed to overwrite :load and :reload. You can do :def reload whatever to your heart's content, but the new version never takes. (Not even if defined in .ghci.) Alright. A :hlint would probably be enough. I can always edit Yi/Emacs to follow a :reload/:load with a :hlint, after all. And aren't there shell command primitives already? So I set out, and I realize: I can't simply settle for some version of :! "hlint .", because the working directory rarely changes - if one opens ghci in ~/, and loads a file elsewhere, :pwd reveals one to still be in ~/. And a 'hlint .' could take a very long time indeed in ~/. Well, alright. I just need the file name of the module I'm working with. Surely there is an easy way to do that. And indeed, :show modules gives me what I'm looking for: *Recorder Control.Monad Data.Char Data.List> :show modules Util ( Util.hs, interpreted ) Recorder ( Recorder.hs, interpreted ) Game ( Game.hs, interpreted ) Monadius ( Monadius.hs, interpreted ) Demo ( Demo.hs, interpreted ) Well, sort of. Ok, we can parse that. Let's assume a variable x holds the output of :show modules as a String. We call lines on it, then map words on it, do a !! 2 on it, and we get ["Util.hs,", "Recorder.hs,", "Game.hs,", "Monadius.hs,", "Demo.hs,"]. Chuck in a map (filter (\= ',')), and we get a good list. We can turn the list into a string suitable for hlint with a quick unwords. So our long sought after command becomes ':def hoogle (\_ -> return $ ":! " ++ (unwords $ map (filter (\= ',')) $ (map words $ lines x) !! 2))'. But wait, how do we get 'x'? How do we call :show modules inside a Haskell expression? I have carefully looked over http://haskell.org/haskellwiki/GHC/GHCi#Using_GHCi and http://www.haskell.org/ghc/docs/latest/html/users_guide/ghci-commands.html and my conclusion is that you can't. You can't do a let x = :show modules, there is no function which will take ":show modules", and so on. :functions can accept Haskell output, but it's a one-way barrier. It's no good writing Haskell functions which need information from the :functions. Which all leaves me where I started - no ghci integration with hlint. Any ideas? - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAklNNSIACgkQvpDo5Pfl1oJwWgCfZJ7nwkHfsZymIRass0ewmhFE en4AnjmQam3V8Go6pGoLTRsp5zZEzAms =nmJm -----END PGP SIGNATURE-----

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A hell of a question
by Andrzej Jaworski 22 Dec '08

22 Dec '08

Hi Luke, When neurosurgeons split the brain into left and right hemisphere cutting it along corpus callosum the patient will talk to you with his left half and right half independently - each time unaware what his other half was talking about a moment earlier. I believe Zen emulates such split on a microscopic scale. We all go thru similar state when we conceive an original idea - if we do not write this down or formulate it immediately the discovery will loose its punch and sense of depth. The best documented and most prominent example is Hegel's discovery of dialectical logic - after extensive writing about that at some point he honestly admits loosing the original concept. But if not for his western oververbosity we might not today use the term 'naive' set theory. Every serious mathematician touching foundations is today perfectly aware of the price we pay for formulating things. But I am unaware of any virtue of trying to clap with one hand;-) Regards, -Andrzej

1 0

haskell for windows mobile 6 on Intel PXA270 processor
by Constantine Potapov 22 Dec '08

22 Dec '08

Hi All. I'm looking for haskell interpreter/compiler for windows mobile 6 (I have an Asus A696 PDA with intel PXA270 processor). Somebody head about any haskell implementation for this platform? -- With best regards, Constantine Potapov.

1 0

Re: [Haskell] ANN: HLint 1.0
by Neil Mitchell 22 Dec '08

22 Dec '08

Hi Lennart, > It would be nice if HLint didn't suggest things that it will object to > in the next round. > Like > > LLVM/Core/CodeGen.hs:176:1: Eta reduce > Found: > applyArgs f g = apArgs 0 f g > Why not: > applyArgs f = apArgs 0 f The idea is to specify things one step at a time, so the user learns from the hints, as well as fixing them. However, in the case of eta reduction, doing several reductions at a time does make sense. I'll add it to the todo list. Thanks Neil

1 0

Removing/Uninstalling cabal packages ?
by Laurent Giroud 22 Dec '08

22 Dec '08

Hi, I have been doing a few experiments with cabal packages lately and I wish to uninstall these to return to a cleaner package base. However, there doesn't seem to be a "cabal uninstall" command and reading the documentation at http://www.haskell.org/ghc/docs/latest/html/Cabal/index.html does not tell much about how to do that manually. Note that what I wish to do is both delete the installed files and the cabal database so that the uninstalled packages does not show anymore when running "cabal list --installed". Would anyone know how to do that ? Thanks in advance. Regards, Laurent -- Laurent Giroud ml(a)niaow.com

3 2

RE: [Haskell-cafe] Trouble with FFI on Windows2000...
by Serge LE HUITOUZE 22 Dec '08

22 Dec '08

Hi Kyra (I tried to email you directly, but, for some reason, my mail bounced...), Hi Haskellers, > > dlltool -d cproj1.def -l libcproj1.a > > ghc --make testFFI_2.hs -optl-lcproj1 -optl-L. > > [...] > > This leads to an error box with message: > > "DLL (null) not found on specified path" > > dlltool -d cproj1.def -l libcproj1.a -D cproj1.dll Thanks very much for this answer, short and effective (an ideal one)! Just an additional question, just for curiosity: Does anyone know why this "-D" option doesn't appear in the following two pages? - http://www.haskell.org/haskellwiki/GHC/Using_the_FFI#Setting_up_your_build_… - http://www.haskell.org/haskellwiki/GHC:FAQ#GHC_on_Windows (parag. 1.4.3.4) Is this because the behavior I got is Win2K specific (hence very few people run into in 2008)? Thanks again Kyra for your precious help. --Serge

1 0

Yampa vs. Reactive
by Tony Hannan 22 Dec '08

22 Dec '08

Hello, Can someone describe the advantages and disadvantages of the Yampa library versus the Reactive library for functional reactive programming, or point me to a link. Thanks, Tony P.S. It is hard to google for Yampa and Reactive together because "reactive" as in "function reactive programming" always appears with Yampa

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ANN: llvm-0.4.0.1
by Lennart Augustsson 22 Dec '08

22 Dec '08

We have now released version 0.4.0.1 of the Haskell LLVM bindings. (This release that is quite incompatible with the old 0.0.2 release.) LLVM is a virtual machine and the bindings allow you to generate code for this virtual machine. This code can then be executed by a JIT or written to a file for further processing by the LLVM tools. The LLVM bindings has two layers. You can either use the low level bindings that is just the same as the C bindings for the LLVM. This level is quite unsafe as there is very little type checking. The recommended way is a high level binding (somewhat less complete) which eliminates many errors by leveraging the Haskell type system. A simple example, generating code for a function that adds two numbers and then calling it from Haskell: import Data.Int import LLVM.Core import LLVM.ExecutionEngine cgplus :: CodeGenModule (Function (Int32 -> Int32 -> IO Int32)) cgplus = createFunction InternalLinkage $ \ x y -> do r <- add x y ret r main = do ioplus <- simpleFunction cgplus let plus = unsafePurify ioplus print $ plus 20 22 Enjoy! Bryan O'Sullivan Lennart Augustsson

1 0

Records and associated types
by Taru Karttunen 22 Dec '08

22 Dec '08

Hello What is the correct way to transform code that uses record selection with TypeEq (like HList) to associated types? I keep running into problems with overlapping type families which is not allowed unless they match. The fundep code: class Select rec label val | rec label -> val instance TypeEq label label True => Select (Label label val :+: rest) label val instance (Select tail field val) => Select (any :+: tail) field val And a conversion attempt: class SelectT rec label where type S rec label instance TypeEq label label True => SelectT (Label label val :+: rest) label where type S (Label label val :+: rest) label = val instance (SelectT tail field) => SelectT (any :+: tail) field where type S (any :+: tail) field = S tail field which fails with: Conflicting family instance declarations: type instance S (Label label val :+: rest) label -- Defined at t.hs:19:9 type instance S (any :+: tail) field -- Defined at t.hs:23:9 How is it possible to get the TypeEq constraint into the type family? Attached is a complete example that illustrates the problem. - Taru Karttunen

3 2

ANN: Data.List.Split
by Brent Yorgey 22 Dec '08

22 Dec '08

I am pleased to announce the release of Data.List.Split, which provides a wide range of strategies and a unified combinator framework for splitting lists with respect to some sort of delimiter. It strives to be flexible yet simple. If you've ever wished there was a simple 'split' function you could grab from Data.List, this is the package for you---no matter which of the seventeen* slightly different ways to split a list you actually need! Just use one of the prepackaged splitting strategies, or build your own with the combinator library. Get it from Hackage: cabal install split http://hackage.haskell.org/cgi-bin/hackage-scripts/package/split Get the source: darcs get http://code.haskell.org/~byorgey/code/split Get excited: http://byorgey.wordpress.com/2008/12/21/datalistsplit/ -Brent * Actual number may vary.

1 0

Logos of Other Languages
by Ashley Yakeley 22 Dec '08

22 Dec '08

I browsed around a bit for logos from other languages... Python http://www.python.org/images/python-logo.gif The "snake pair" is visually interesting while still remaining simple. The typeface is unusual and yet clean and humanistic. The logo is only slightly marred by the "TM". Overall, elegant and appealing. A. Caml http://caml.inria.fr/styles/modern/title-en.gif This is actually more of a heading than a logo: it continues to the right edge of the page to close the oval. The typeface is readable, but the text is a bit verbose, spelling out "The Caml Language" with a visual nod to its ML roots. B+. Ruby http://www.ruby-lang.org/images/logo.gif An ordinary book typeface with an ordinary picture of an ordinary ruby. And the strapline "A Programmer's Best Friend" is vague and uninspired. Bland, but at least inoffensive. C. Perl http://upload.wikimedia.org/wikipedia/en/e/e0/Programming-republic-of-perl.… I'm not sure if this is "the Perl logo" or "O'Reilly's Perl logo", but it's ugly, busy, difficult to read and a bit obscure. The title "PROGRAMMING REPUBLIC OF" also lends an air of snotty pretension. D. Apple Dylan http://osteele.com/projects/images/Dylan.logo-thumb.png Arty, abstract, vaguely Hermitian, but a bit corporate. Easy to make a website favicon from, but the actual word "Dylan" is not strictly included. A-. All of these get one thing right that the current and most of the proposed Haskell logos do not: they don't make any reference to the syntax of the language itself. Doing so seems to miss the point of a logo: it's supposed to appeal visually, rather than semantically. So I'd like to see some submissions that don't use lambdas. -- Ashley Yakeley Seattle, WA

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Re: [Haskell] ANN: Hoogle with more libraries
by Neil Mitchell 21 Dec '08

21 Dec '08

Hi Bob, > 1. Searching using a package name that isn't all lower case results in > nothing (e.g. (a -> b) -> f a -> f b +InfixApplicative gives no results, > while (a -> b) -> f a -> f b +infixapplicative gives 2). Yes, if you do +InfixApplicative it assumes you mean only in the module InfixApplicative, not the package. Module is upper case, package starts with lowercase. > 2. Searching with the +package syntax seems to remove all other possible > results. Perhaps it would be nice to show results from the default library > set below. Add +default to search the default too. > If you get the package name wrong (i.e. specify a package that hoogle can't > see), it would be nice for it to report something like google does -- maybe > you meant xyz. Definately! The whole area needs an overhall. Thanks Neil

1 0

Windows vs. Linux x64
by Bartosz Wójcik 21 Dec '08

21 Dec '08

Hi Everybody, while working on my resent project I've noticed that my code seems to be faster under Windows than under Linux x64. More exactly this was an AI game evaluator that ran on given parameters. There was no IO performed. I've run 3 lots of test on both systems and stored some figures. It was physicaly the same PC. 1st lot WinXP total time = 27.18 secs (1359 ticks @ 20 ms) total alloc = 5,788,242,604 bytes (excludes profiling overheads) Linux total time = 34.44 secs (1722 ticks @ 20 ms) total alloc = 11,897,757,176 bytes (excludes profiling overheads) 2nd lot WinXP total time = 63.96 secs (3198 ticks @ 20 ms) total alloc = 13,205,507,148 bytes (excludes profiling overheads) Linux total time = 80.76 secs (4038 ticks @ 20 ms) total alloc = 27,258,694,888 bytes (excludes profiling overheads) 3rd lot WinXP total time = 207.10 secs (10355 ticks @ 20 ms) total alloc = 44,982,716,780 bytes (excludes profiling overheads) Linux total time = 267.58 secs (13379 ticks @ 20 ms) total alloc = 92,307,482,416 bytes (excludes profiling overheads) I've used the same compile and runtime options for both. I've tried to run with -H option, but this didn't improve anything. Is this common behaviour? Does anybody know what can be the reason? regards, Bartek

7 11

Line noise
by Andrew Coppin 21 Dec '08

21 Dec '08

I hang out on another forum that is populated by various kinds of computer geeks. There's a fair few programmers in there, as well as nerds of every degree. And yet, every time I post anything written in Haskell, everybody complains that it "looks like line noise". What do "normal" programmers find so puzzling about Haskell? You might guess some of the following: - High-order functions. - Lambda functions. - Algebraic data types. - Lazy evaluation. - Recursion. - Monads. Actually, none of these things were mentioned. The things people have *actually* complained to me about are: - Haskell expressions are difficult to parse. - Several standard library elements have unhelpful names such as "elem", "Eq" and "fst". (My favourite has to be "Ix". I mean, WTH?) - Several standard library functions have names which clash badly with the usual meanings of those names - e.g., "break", "return", "id". - Variable names such as "x" and "f" aren't fabulously helpful to lost programmers trying to find their way. If you take a "typical" lump of C or Java code, there's so much redundancy and noise in it that you can *usually* figure out vaguely what's going on, even if it doesn't completely make sense. When presented with a typical Haskell fragment... uh... well, good luck with that. Let's take Haskell's most celebrated example: qsort [] = [] qsort (x:xs) = qsort (filter (x <) xs) ++ [x] ++ qsort (filter (x >=) xs) To a novice, it's not actually real clear what the arguments to qsort are. You and I know that function application binds much more tightly than (++), but a novice isn't going to know that. You might also not immediately realise that "xs" is also an argument to filter, not just (x <) or whatever. The people I spoke to also seemed pretty confused about the usage of (.) and ($), even after I explained it a few times. Several people also voiced being puzzled about Haskell's layout rules. I'm not sure what we *do* with all this data, but I found it interesting so I thought I'd share. ;-) I've spent the last few years trying to convert a few people to The Haskell Way(tm), but so far I haven't succeeded in the slightest. I just get yelled at for pointing white noise. Heh. Oh well!

23 30

Trouble with FFI on Windows2000...
by Serge LE HUITOUZE 21 Dec '08

21 Dec '08

Hi there! I'm trying to FFI-bind a MS Visual dll (under Windows2000) to a Haskell program. I have this little C(++) dll: ******* FILE inc.h: START ******* extern "C" { int pten(int i); } ******* FILE inc.h: END ******* ******* FILE inc.cpp: START ******* #include "inc.h" int pten(int i) { return i+10; } ******* FILE inc.cpp: END ******* ******* FILE cproj1.def: START ******* LIBRARY cproj1 EXPORTS pten ******* FILE cproj1.def: END ******* And this little Haskell program: ******* FILE testFFI_2.hs: START ******* {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign.C -- get the C types foreign import ccall unsafe "pten" c_pten :: CInt -> CInt plus10 :: Int -> Int plus10 i = fromIntegral (c_pten (fromIntegral i)) main = print (map plus10 [1..10]) ******* FILE testFFI_2.lhs: END ******* Using information found at paragraph 1.4.3.4 of http://www.haskell.org/haskellwiki/GHC:FAQ#GHC_on_Windows I do the following: > dlltool -d cproj1.def -l libcproj1.a > ghc --make testFFI_2.hs -optl-lcproj1 -optl-L. This seems fine, since it produces a "testFFI_2.exe". However, executing it, I get a MSWindows error box with a message that looks like (approximate translation): "DLL (null) not found on specified path" At first, I didn't have the "LIBRARY" directive in the ".def" file, so I thought that was the reason for the "(null)" appearing in the message. But adding said directive didn't change anything... [I've checked that my MSVisual dll was in current dir, and that this dir was in the search paths showed in the error box] Can anyone enlighten me? Thanks in advance. --Serge

2 1

Graham Hutton's calculator example for win32
by Ahn, Ki Yung 21 Dec '08

21 Dec '08

In Graham Hutton's "Programming in Haskell" there is an interactive calculator example using ANSI code to implement the UI on the terminal. This example doesn't work on MS Windows XP or other MS OSes based on NT kernel, since their command line does not support ANSI very well. But, thanks to ansi-terminal on Hackage, I was able to extract minimal code from the package to make a win32 version of the calculator example in Hutton's book. calculatorWin32.lhs and Win32ANSI.hs is an implementation for win32. I extracted the win32 console API bindings for setting cursor positions from ansi-terminal project and put them in Win32ANSI.hs. I had to put this in a separate file because I had an issue with ghci. To run this, I had to compile the console API bindings with ghc first and then run ghci as follows C:\> ghc -c Win32ANSI.hs C:\> ghci calculatorWin32.lhs Without compiling the object code, ghci cannot find the proper link for win32 console API FFI bindings. C:\> ghci calculatorWin32.lhs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 3] Compiling Parsing ( Parsing.lhs, interpreted ) [2 of 3] Compiling Win32ANSI ( Win32ANSI.hs, interpreted ) During interactive linking, GHCi couldn't find the following symbol: GetConsoleScreenBufferInfo@8 This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs(a)haskell.org Is this a bug or a natural behavior of ghci? This is strange to me since ghci finds the proper link for the functions in the other C libraries such as getch in conio.h. In addition, I am attaching a patched calculator.lhs which works for Unix/Linux on both GHC 6.8.x and GHC 6.10.1. The one currently on the book homepage only works for GHC 6.8.x but not GHC 6.10.1. This is due to the bug fix of hSetBuffering in GHC 6.10.1. To run these calculator example you will also need Parsing.lhs from the book hompage. http://www.cs.nott.ac.uk/~gmh/Parsing.lhs -- Ahn, Ki Yung

1 0

Haskell as a religion
by Hugo Pacheco 21 Dec '08

21 Dec '08

I just found this on the web. Do you like the description of Haskell? It is not that far from true :P http://www.aegisub.net/2008/12/if-programming-languages-were-religions.html -- www.di.uminho.pt/~hpacheco

12 14

Beginner's TH question
by Jeff Heard 21 Dec '08

21 Dec '08

Two things... can I add fields to records using Template Haskell, like: data T = T { $fields, myfield :: Field, ... } I assume the answer there is no, and then what's wrong with this? I get: Illegal instance declaration for `UIState t' (All instance types must be of the form (T a1 ... an) where a1 ... an are type *variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `UIState t' In the expression: [d| instance UIState t where { setSizeY v a = setSizeY v . uist $ a setSizeX v a = setSizeX v . uist $ a setDrawing v a = setDrawing v . uist $ a setKey v a = setKey v . uist $ a .... } |] In the definition of `deriveUIState': deriveUIState uist t = [d| instance UIState t where { setSizeY v a = setSizeY v . uist $ a setSizeX v a = setSizeX v . uist $ a setDrawing v a = setDrawing v . uist $ a .... } |] in this module: -# LANGUAGE TemplateHaskell #-} module Graphics.Rendering.Thingie.TH where import Language.Haskell.TH import Graphics.Rendering.Thingie.UIState import qualified Graphics.Rendering.Thingie.BasicUIState as S deriveUIState uist t = [d| instance UIState t where mousePosition a = S.mousePosition . uist $ a mouseLeftButtonDown a = S.mouseLeftButtonDown . uist $ a mouseRightButtonDown a = S.mouseRightButtonDown . uist $ a mouseMiddleButtonDown a = S.mouseMiddleButtonDown . uist $ a mouseLeftButtonClicked a = S.mouseLeftButtonClicked . uist $ a mouseRightButtonClicked a = S.mouseRightButtonClicked . uist $ a mouseMiddleButtonClicked a = S.mouseMiddleButtonClicked . uist $ a mouseWheel a = S.mouseWheel . uist $ a keyCtrl a = S.keyCtrl . uist $ a keyShift a = S.keyShift . uist $ a keyAlt a = S.keyAlt . uist $ a key a = S.key . uist $ a drawing a = S.drawing . uist $ a sizeX a = S.sizeX . uist $ a sizeY a = S.sizeY . uist $ a setMousePosition v a = setMousePosition v . uist $ a setMouseLeftButtonDown v a = setMouseLeftButtonDown v . uist $ a setMouseRightButtonDown v a = setMouseRightButtonDown v . uist $ a setMouseMiddleButtonDown v a = setMouseMiddleButtonDown v . uist $ a setMouseLeftButtonClicked v a = setMouseLeftButtonClicked v . uist $ a setMouseRightButtonClicked v a = setMouseRightButtonClicked v . uist $ a setMouseMiddleButtonClicked v a = setMouseMiddleButtonClicked v . uist $ a setMouseWheel v a = setMouseWheel v . uist $ a setKeyCtrl v a = setKeyCtrl v . uist $ a setKeyShift v a = setKeyShift v . uist $ a setKeyAlt v a = setKeyAlt v . uist $ a setKey v a = setKey v . uist $ a setDrawing v a = setDrawing v . uist $ a setSizeX v a = setSizeX v . uist $ a setSizeY v a = setSizeY v . uist $ a |]

3 2

I hate constructing strings with ++
by Thomas Hartman 21 Dec '08

21 Dec '08

I hate constructing strings with ++. Especially icky when you start dealing with escape characters and stuff. cabal install HStringTemplateHelpers ..... import Text.StringTemplate.Helpers putStrLn $ render1 [("name",name)] "Why, hello, $name$" just thinkin out loud :) (originally at http://www.reddit.com/r/programming/comments/7khkd/the_fun_of_programming_a…)

2 2

"non-functions like unsafePerformIO are not technically part of the haskell language!"
by Jason Dusek 21 Dec '08

21 Dec '08

From an old thread: > non-functions like unsafePerformIO are not technically part > of the haskell language! How is this true, exactly? -- _jsn |...an old thread.| http://www.nabble.com/Re%3A-Re%3A-Parsers-are-monadic--p11390440.html

4 4

Re: [Haskell] ANN: HLint 1.0
by Gwern Branwen 20 Dec '08

20 Dec '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 On Sat, Dec 20, 2008 at 4:55 AM, Neil Mitchell wrote: > Hi, > > I am pleased to announce HLint, a tool for making suggestions to > improve your Haskell code. Previously this tool was called Dr Haskell > and depended on a working installation of Yhc - both of those have now > changed. HLint requires GHC 6.10*, and to install simply type: > > cabal update && cabal install hlint > > If you haven't yet installed the cabal command, instructions are here: > http://ghcmutterings.wordpress.com/2008/11/10/bootstrapping-cabal-install/ > > As an example of what HLint does, running "hlint darcs-2.1.2" over the > latest stable release of darcs gives 385 suggestions, including: > > darcs-2.1.2\src\CommandLine.lhs:46:1: Use a string literal > Found: > [' ', '\t', '"', '%'] > Why not: > " \t\"%" > > darcs-2.1.2\src\CommandLine.lhs:49:1: Eta reduce > Found: > quotedArg ftable > = between (char '"') (char '"') $ quoteContent ftable > Why not: > quotedArg = between (char '"') (char '"') . quoteContent > > darcs-2.1.2\src\CommandLine.lhs:94:1: Use concatMap > Found: > concat $ map escapeC s > Why not: > concatMap escapeC s > > To see all the hints in a nice interactive document visit > http://www-users.cs.york.ac.uk/~ndm/hlint/hlint-report.htm > (recommended if you are thinking of trying out hlint) > > All necessary links, including a manual, hackage links, bug tracker > and source code can be found from the tool website: > http://www-users.cs.york.ac.uk/~ndm/hlint/ > > Acknowledgements: Niklas Broberg and the haskell-src-exts package have > both been very helpful. The darcs users mailing list gave many good > suggestions which I've incorportated. > > Please direct any follow up conversations to haskell-cafe@ > > Thanks > > Neil > > * Why GHC 6.10? View patterns. Hi, Neil. Thanks for the tool; it found some worthwhile changes in my projects. But a few points: 1) How does one actually use the CLI tool? You didn't say. It *seems* that one just feeds it a list of random filepaths, so on Mueval, say, the command would be 'hlint main.hs Mueval/*.hs' (and not, say, 'hlint mueval.cabal'). But I'm not actually sure. 2) I think I found a parsing bug. One line in Mueval/Interpreter.hs runs: > fmap (take n exceptionMsg ++) $ render' (n-length exceptionMsg) s which gives the error: > Mueval/Interpreter.hs:145:59: Parse failure, Parse error > No relevant suggestions Adding spaces between 'n' and 'length', so it reads: > fmap (take n exceptionMsg ++) $ render' (n - length exceptionMsg) s lets hlint parse and suggest for it. - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAklM+kwACgkQvpDo5Pfl1oKDhwCfRgQutbvr9QbKvbLJlbhdla2Z absAniWC+ksXHhfbunsdOsnmG69lppFZ =X9uv -----END PGP SIGNATURE-----

2 1

Re: [Haskell] ANN: HLint 1.0
by Neil Mitchell 20 Dec '08

20 Dec '08

Hi > I noticed that you convert point-wise into point-free. > Perhaps you could add some point-free transformations to remove redundancy > in certain cases. > Is that a goal of the library? It does some transformations of th at nature, but the idea isn't to remove redundancy, its to make the code clearer. Do you have any suggestions for things it could easily do to remove redundancy? It already does a few relating to if expressions. > a suggestion for a rule: dollar eta: f $ x -> f x That can easily be added. > Why not: > ("TAG " `isPrefixOf` just_name pinfo) That's an open bug: http://code.google.com/p/ndmitchell/issues/detail?id=109 In general, most of the hints are "replace this with this" - evaluating the new bit (even to remove easy constants) is beyond it for the moment. Some people theorised it might be too hard, non-terminating, or incorrect. As it happens this rule only fires when "TAG " is either a string literal or a list literal, and when the number is a constant, so its always safe and the guard could always be eliminated. > 1) How does one actually use the CLI tool? You didn't say. It *seems* > that one just feeds it a list of random filepaths, so on Mueval, say, > the command would be 'hlint main.hs Mueval/*.hs' (and not, say, 'hlint > mueval.cabal'). But I'm not actually sure. The filenames are either filenames, or directories which are recursively searched, i.e cd mueval && hlint . is probably sufficient. I should document it more clearly in the manual. Thanks Neil

2 1

Re: [Haskell] ANN: HLint 1.0
by Martijn van Steenbergen 20 Dec '08

20 Dec '08

Hi Neil, This is awesome, thank you. :-) It found a 'use liftM', 'use on' and three eta-reduces in Yogurt. It seems like the line numbers could be a bit more accurate: ./Network/Yogurt/IO.hs:54:3: Use liftM Found: rec >>= return . (c :) Why not: liftM (c :) rec Where the code is: 50 -- Waits for input, but once the first character is read, waits 51 -- no longer than the specified number of ms before giving up. 52 hGetImpatientLine :: Handle -> Int -> IO String 53 hGetImpatientLine h patience = rec where 54 rec = do 55 c <- hGetChar h 56 if c == '\n' 57 then return [c] 58 else do 59 b <- hWaitForInput h patience 60 if b 61 then rec >>= return . (c:) 62 else return [c] I imagine it could have told me to look at line 61 right away. Thanks, Martijn.

4 4

Re: [Haskell] ANN: HLint 1.0
by Hugo Pacheco 20 Dec '08

20 Dec '08

Hi again, I have run HLint against my libraries and found lots of suggestions, mainly excessive brackets. This is what I suggest from my experience: . a suggestion for a rule: dollar eta: f $ x -> f x. some parser errors I encountered: import Data.Generics hiding ((:*:)) type instance Rep Id x = x Cheers, hugo On Sat, Dec 20, 2008 at 10:21 AM, Hugo Pacheco <hpacheco(a)gmail.com> wrote: > I noticed that you convert point-wise into point-free.Perhaps you could > add some point-free transformations to remove redundancy in certain cases. > Is that a goal of the library? > > Cheers, > hugo > > > On Sat, Dec 20, 2008 at 9:55 AM, Neil Mitchell <ndmitchell(a)gmail.com>wrote: > >> Hi, >> >> I am pleased to announce HLint, a tool for making suggestions to >> improve your Haskell code. Previously this tool was called Dr Haskell >> and depended on a working installation of Yhc - both of those have now >> changed. HLint requires GHC 6.10*, and to install simply type: >> >> cabal update && cabal install hlint >> >> If you haven't yet installed the cabal command, instructions are here: >> http://ghcmutterings.wordpress.com/2008/11/10/bootstrapping-cabal-install/ >> >> As an example of what HLint does, running "hlint darcs-2.1.2" over the >> latest stable release of darcs gives 385 suggestions, including: >> >> darcs-2.1.2\src\CommandLine.lhs:46:1: Use a string literal >> Found: >> [' ', '\t', '"', '%'] >> Why not: >> " \t\"%" >> >> darcs-2.1.2\src\CommandLine.lhs:49:1: Eta reduce >> Found: >> quotedArg ftable >> = between (char '"') (char '"') $ quoteContent ftable >> Why not: >> quotedArg = between (char '"') (char '"') . quoteContent >> >> darcs-2.1.2\src\CommandLine.lhs:94:1: Use concatMap >> Found: >> concat $ map escapeC s >> Why not: >> concatMap escapeC s >> >> To see all the hints in a nice interactive document visit >> http://www-users.cs.york.ac.uk/~ndm/hlint/hlint-report.htm >> (recommended if you are thinking of trying out hlint) >> >> All necessary links, including a manual, hackage links, bug tracker >> and source code can be found from the tool website: >> http://www-users.cs.york.ac.uk/~ndm/hlint/ >> >> Acknowledgements: Niklas Broberg and the haskell-src-exts package have >> both been very helpful. The darcs users mailing list gave many good >> suggestions which I've incorportated. >> >> Please direct any follow up conversations to haskell-cafe@ >> >> Thanks >> >> Neil >> >> * Why GHC 6.10? View patterns. >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell >> > > > > -- > www.di.uminho.pt/~hpacheco > -- www.di.uminho.pt/~hpacheco

2 1

Re: [reactive] problem with unamb -- doesn't kill enough threads
by Conal Elliott 20 Dec '08

20 Dec '08

(I'm broadening the discussion to include haskell-cafe.) Andy -- What do you mean by "handling all thread forking locally"? - Conal On Thu, Dec 18, 2008 at 1:57 PM, Andy Gill <andygill(a)ku.edu> wrote: > Conal, et. al, > I was looking for exactly this about 6~9 months ago. I got the suggestion > to pose it as a challenge > to the community by Duncan Coutts. What you need is thread groups, where > for a ThreadId, you can send a signal > to all its children, even missing generations if needed. > > I know of no way to fix this at the Haskell level without handling > all thread forking locally. > > Perhaps a ICFP paper about the pending implementation :-) but I'm not sure > about the research content here. > > Again, there is something deep about values with lifetimes. > > Andy Gill > > > On Dec 18, 2008, at 3:43 PM, Conal Elliott wrote: > > I realized in the shower this morning that there's a serious flaw in my > unamb implementation as described in > http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice. > I'm looking for ideas for fixing the flaw. Here's the code for racing > computations: > > race :: IO a -> IO a -> IO a > a `race` b = do v <- newEmptyMVar > ta <- forkPut a v > tb <- forkPut b v > x <- takeMVar v > killThread ta > killThread tb > return x > > forkPut :: IO a -> MVar a -> IO ThreadId > forkPut act v = forkIO ((act >>= putMVar v) `catch` uhandler `catch` > bhandler) > where > uhandler (ErrorCall "Prelude.undefined") = return () > uhandler err = throw err > bhandler BlockedOnDeadMVar = return () > > The problem is that each of the threads ta and tb may have spawned other > threads, directly or indirectly. When I kill them, they don't get a chance > to kill their sub-threads. > > Perhaps I want some form of garbage collection of threads, perhaps akin to > Henry Baker's paper "The Incremental Garbage Collection of Processes". As > with memory GC, dropping one consumer would sometimes result is cascading > de-allocations. That cascade is missing from my implementation. > > Or maybe there's a simple and dependable manual solution, enhancing the > method above. > > Any ideas? > > - Conal > > > _______________________________________________ > Reactive mailing list > Reactive(a)haskell.org > http://www.haskell.org/mailman/listinfo/reactive > > >

4 6

OT: representations for graphs
by Bayley, Alistair 20 Dec '08

20 Dec '08

(OT, but I'm hoping some of you might have some ideas on this anyway...) I was wondering what alternative representations there are for graphs, or maybe if there might be a way to derive one/some from some insightful observations. For the purposes of storing and exmaining (querying) graphs in an SQL database. For example, a tree (so, a specialised sub-class of graph) can be represented by a three models, that I know of: - adjacency-list (the most common) - materialised-path (a denormalisation of adjacency-list) - nested-sets Nested-sets (and materialised-path) works for trees because the graph is - directed (so we know which nodes are parents or children) - acyclic (there's a definite root, and leaves) - every child has a single parent (so no diamond shapes - does this property have a name?) Nested-sets works well with SQL databases for querying, at the expense of updates. Adjacency-list is easier to update, but some queries suck, or are downright impossible in normal SQL. We can use the adjacency-list model for graphs too, but it has the same query deficiencies as for trees. I'd like some sort of alternative to adjacency-list, like nested-sets, that would work better for querying at the expense of updates. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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