December 2010 - Haskell-Cafe

how to write a loop in haskell way
by ender 19 Dec '10

19 Dec '10

Hi all: I'v been learning haskell for several months, and now I'm trying to write some "real word" program in haskell, like finding files under one directory or something My problem is that, I dont know the way of writing a loop in haskell. I searched google and found some code that translate c loop into haskell like this one: > > > I am new to haskell and would look to write a function equivalent > > > to the following loop in C > > > > > > int value = 500000; > > > int part_stack[4]; > > > int *part_ptr = part_stack; > > > for (; value; value /= 10000) > > > *part_ptr++ = value % 10000; > > > > part_stack :: [Int] > > part_stack = [0,50] > > > > Note that I've performed a memoization optimization--this > > makes the code both smaller, faster and easier to read! :P > > Ignore David, he's pulling your leg. Here's the proper translation: > > do > alloca $ \value -> do > poke value (500000::Int) > allocaArray 4 $ \part_stack -> do > alloca $ \part_ptr -> do > poke part_ptr part_stack > let loop = do > val <- peek value > if val == 0 then return () else do > p <- peek part_ptr > poke p (val `rem` 10000) > poke part_ptr (p `plusPtr` 1) > poke value (val `quot` 10000) > loop > loop and I really think that's not a "haskell way", it's just translate c code into haskell code byte by byte My question is: how to translate above c code into haskell in "haskell way" Thanks and BR Ender

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Re: [Haskell-cafe] [Haskell] macports, cabal, cabal-install difficulty
by Antoine Latter 19 Dec '10

19 Dec '10

I don't know about the ld warning, but cabal-install can never know which version of cabal-install is installed, because it doesn't track executables - it only tracks and knows about libraries. Right after the warning is the line: > Installing executable(s) in /Users/matthew/.cabal/bin So it looks like it might have worked :-) You can either add ~/.cabal/bin to your path, or move/soft-link the cabal-install executable to your preferred path. Take care, Antoine On Sat, Dec 18, 2010 at 7:52 AM, Matthew Fairtlough <research(a)fairtlough.net> wrote: > Hello, I'm new to this list and to hackage/cabal/cabal-install and haskell > on the mac generally. I don't dare post on cabal-devel because I'm a > newbie. > > I came to this question because I haven't been able to install the unix > package properly (some packages I want depend on it) but I've posted > separately in haskell-cafe on that. > I've port version 1.9.2 and haskell-platform @2009.2.0.2 (devel, haskell) > > My question: Is cabal currently out-of-the-box compatible with macports? I > seem to have problems with cabal-install, in particular the ld warning and > the fact that cabal doesn't seem to know what version of cabal-install is > installed. > > bash-3.2# cabal info cabal > * Cabal (library) > Synopsis: A framework for packaging Haskell software > Latest version available: 1.10.0.0 > Latest version installed: 1.10.0.0 > > bash-3.2# cabal install cabal-install-0.8.2 > Resolving dependencies... > Configuring cabal-install-0.8.2... > Preprocessing executables for cabal-install-0.8.2... > Building cabal-install-0.8.2... > ....<lots of build information deleted>... > [40 of 40] Compiling Main ( Main.hs, > dist/build/cabal/cabal-tmp/Main.o ) > Linking dist/build/cabal/cabal ... > ld: warning: -read_only_relocs cannot be used with x86_64 > Installing executable(s) in /Users/matthew/.cabal/bin > > bash-3.2# cabal info cabal-install > * cabal-install (program) > Synopsis: The command-line interface for Cabal and Hackage. > Latest version available: 0.8.2 > Latest version installed: [ Unknown ] > > ....and lots more:... > > Homepage: http://www.haskell.org/cabal/ > Bug reports: http://hackage.haskell.org/trac/hackage/ > Description: The \'cabal\' command-line program simplifies the process > of > managing Haskell software by automating the fetching, > configuration, compilation and installation of Haskell > libraries and programs. > Category: Distribution > License: BSD3 > Author: Lemmih <lemmih(a)gmail.com> > Paolo Martini <paolo(a)nemail.it> > Bjorn Bringert <bjorn(a)bringert.net> > Isaac Potoczny-Jones <ijones(a)syntaxpolice.org> > Duncan Coutts <duncan(a)haskell.org> > Maintainer: cabal-devel(a)haskell.org > Source repo: http://darcs.haskell.org/cabal-install/ > Executables: cabal > Flags: old-base, bytestring-in-base > Dependencies: unix >=1.0 && <2.5, Win32 ==2.*, bytestring >=0.9, > base <2.0 || >=3.0, base >=2.0 && <2.2, old-time >=1 && > <1.1, > array >=0.1 && <0.4, containers >=0.1 && <0.4, > random >=1 && <1.1, pretty >=1 && <1.1, > directory >=1 && <1.1, process >=1 && <1.1, base >=3, > base <3, time ==1.1.*, zlib >=0.4 && <0.6, > HTTP >=4000.0.2 && <4001, network >=1 && <3, filepath >>=1.0, > Cabal ==1.8.*, base >=2 && <5 > Cached: Yes > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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DNS Problems at haskell.org
by John Peterson 18 Dec '10

18 Dec '10

Just to clarify - I was the contact for Haskell.org but the email address was no longer being forwarded. I had assumed I had been removed as the contact but unfortunately only the technical contact had been changed. All was finally resolved with the help of Yale, where they resurrected the old email address, and Galois, who fixed things up once they had access to the account at Network Solutions. Everything is now in capable hands - sorry about the problems. John

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Re: [Haskell-cafe] Why is Haskell flagging this?
by michael rice 18 Dec '10

18 Dec '10

Here's (attached) a screen shot of the post I received from you. There's your name after the "From:" at the top. Also, this post is a reply, so your message follows. Was there a mix up? Michael --- On Sat, 12/18/10, Thomas Davie <tom.davie(a)gmail.com> wrote: From: Thomas Davie <tom.davie(a)gmail.com> Subject: Re: [Haskell-cafe] Why is Haskell flagging this? To: chrisdone(a)googlemail.com Cc: "michael rice" <nowgate(a)yahoo.com>, haskell-cafe(a)haskell.org Date: Saturday, December 18, 2010, 2:38 PM On 17 Dec 2010, at 21:44, Christopher Done wrote: On 17 December 2010 18:04, michael rice <nowgate(a)yahoo.com> wrote: =================== f :: [Int] -> IO [Int] f lst = do return lst main = do let lst = f [1,2,3,4,5] fmap (+1) lst The problem is that you are applying fmap to a type IO a. fmap (+1) (return [1,2,3]) But to achieve the behaviour you expect, you need another fmap: fmap (fmap (+1)) (return [1,2,3]) Which can be more neatly written with Conal's semantic editor cominators as (fmap . fmap) (+1) (return [1,2,3]) Of course, I question why the list is put in the IO monad at all here... surely this would be much better return $ fmap (+1) [1,2,3] Finally, that has the wrong type for main... perhaps you meant to print it out? main :: IO ()main = print $ fmap (+1) [1,2,3] Bob

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(re)newcomer
by Matthew Fairtlough 18 Dec '10

18 Dec '10

Hello Haskell-cafers, I used to teach Haskell (and Clean!) at University level but haven't touched Haskell in years and certainly never used it with a Mac. Now I work in publishing and want to experiment with Haskell's web services and see if I can help set up an open-source system for handling ONIX xml data. So I tried to install happstack-server (among several other things) and this depends on unix but cabal can't install unix. Error and diagnostics below as best as I could see to report them. Any tips/pointers much appreciated; I'm not sure if this is a bug or where to report it if it is one... thanks for any help. Matthew. bash-3.2# cabal install unix Resolving dependencies... cabal: cannot configure unix-2.4.1.0. It requires base >=4.2 && <4.4 For the dependency on base >=4.2 && <4.4 there are these packages: base-4.2.0.0, base-4.2.0.1, base-4.2.0.2 and base-4.3.0.0. However none of them are available. base-4.2.0.0 was excluded because of the top level dependency base -any base-4.2.0.1 was excluded because of the top level dependency base -any base-4.2.0.2 was excluded because of the top level dependency base -any base-4.3.0.0 was excluded because of the top level dependency base -any bash-3.2# uname -a Darwin Matthew-Fairtloughs-MacBook-Pro.local 10.5.0 Darwin Kernel Version 10.5.0: Fri Nov 5 23:20:39 PDT 2010; root:xnu-1504.9.17~1/RELEASE_I386 i386 bash-3.2# port version Version: 1.9.2 bash-3.2# port info haskell-platform haskell-platform @2009.2.0.2 (devel, haskell) Description: This is the the Haskell Platform: a single, standard Haskell distribution for every system. The Haskell Platform is a blessed library and tool suite for Haskell distilled from Hackage. Homepage: http://hackage.haskell.org/platform/ Runtime Dependencies: ghc, hs-platform-cgi, hs-platform-fgl, hs-platform-editline, hs-platform-GLUT, hs-platform-haskell-src, hs-platform-html, hs-platform-HUnit, hs-platform-mtl, hs-platform-network, hs-platform-OpenGL, hs-platform-parallel, hs-platform-parsec, hs-platform-QuickCheck, hs-platform-regex-base, hs-platform-regex-compat, hs-platform-regex-posix, hs-platform-stm, hs-platform-time, hs-platform-xhtml, hs-platform-zlib, hs-platform-HTTP, hs-platform-alex, hs-platform-happy, hs-platform-cabal Platforms: darwin License: unknown Maintainers: gwright(a)macports.org bash-3.2# cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library bash-3.2# cabal info base * base (library) Synopsis: Basic libraries Latest version available: 4.3.0.0 Latest version installed: 4.1.0.0 Homepage: [ Not specified ] Bug reports: http://hackage.haskell.org/trac/ghc/newticket?component=libraries/base Description: This package contains the Prelude and its support libraries, and a large collection of useful libraries ranging from data structures to parsing combinators and debugging utilities. License: BSD3 Maintainer: libraries(a)haskell.org Source repo: http://darcs.haskell.org/packages/base/ Flags: integer-simple Dependencies: rts -any, ghc-prim -any, integer-simple -any, integer-gmp -any Documentation: /opt/local/share/ghc-6.10.4/doc/ghc/libraries/base Cached: No Modules: <long list elided>

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Haskell, Step by Step, Tutorial, Developing a Whole Application
by __kaveh__ 18 Dec '10

18 Dec '10

Is there a (or more; the more, the better) tutorial for Haskell, developing a whole application (of any kind: web, windows, console)? I mean something like NerdDinner or MVC Music Store for ASP.NET MVC; Or those whole applications in "in Action" books. Thanks Edit 1: Thanks to all for your replies. I just wanted to insist on "step by step developing a whole application" aspect of what I want; not those tutorials that teach Haskell (they are not bad - They are not this). I believe if one could understand Haskell well and even understand it's underlying mathematical foundations, that (absolutely) does not mean "he can develop an application in Haskell". The problem with Haskell is not failing to understand it (I can use LINQ and even developed a DSL based on LINQ - a LINQ provider - and see how it makes composition tidy) but failing to understand the context (system thinking). I can use same concepts in C# because I "know" the context and it is easy to "feed in" new tools/concepts. In Haskell, one can understand every bit of it (well almost) but there is no context there to fit in! So (I might not be wrong - but who knows!) there is an absolute need for this kind of "tutorial" (I wish there was a better name for this). Edit 2: I believe there is a way to learn and teach Haskell in a neat and straight way (That does not mean I think Haskell is a pragmatic programming language - don't get me wrong; I appreciate it; but start- up barrier with Haskell it too high; It has still a long way ahead to be considered as a common purpose programming language and may never does. But Haskell is definitely father of future programming languages - and also not so future ones!) Note: I have asked this question on stackoverflow (http://goo.gl/ bCFf7) and I will try to sync useful information as much as possible.

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DNS problems at haskell.org?
by Eugene Kirpichov 18 Dec '10

18 Dec '10

Hello. For a couple of friends of mine, hackage.haskell.org happens to resolve to something strange (parked domain), though haskell.org works ok. This might be something to tell to haskell.org admins. Find below an example tracert (messages in Russian have been translated). C:\Program Files (x86)\Far2>tracert hackage.haskell.org Tracing to hackage.haskell.org [209.62.105.19], max hops = 30 1 8 ms 4 ms 4 ms bsr01.nn.ertelecom.ru [91.144.184.69] 2 * * 2 ms net184-77.nn.ertelecom.ru [91.144.184.77] 3 13 ms 9 ms 9 ms NNOV-D2-HQ-XX---1-3-0.499.main.synterra.ru [83.229.187.17] 4 9 ms 29 ms 9 ms 83.229.226.101 5 10 ms 9 ms 9 ms m9-cr01-te4-3.msk.stream-internet.net [195.34.38.37] 6 142 ms 142 ms 142 ms ss-crs-1-be3.msk.stream-internet.net [195.34.53.86] 7 142 ms 142 ms 142 ms mar-crs-1-be6.msk.stream-internet.net [195.34.59.141] 8 143 ms 142 ms 142 ms oct-crs-1-be1.spb.stream-internet.net [195.34.53.194] 9 143 ms 142 ms 142 ms bro-cr01-be3.stk.stream-internet.net [212.188.1.194] 10 66 ms 66 ms 66 ms tct-cr01-te5-1.ams.stream-internet.net [195.34.53.14] 11 142 ms 307 ms 207 ms sd-cr01-te2-1.nyc.stream-internet.net [195.34.59.114] 12 * * * Request timed out 13 179 ms 179 ms 179 ms e8-2.ibr03.dllstx3.networklayer.com [70.87.253.189] 14 190 ms 189 ms 190 ms e1-1.ibr01.hstntx2.networklayer.com [70.87.253.50] 15 190 ms 190 ms 190 ms te2-2.dsr02.hstntx2.networklayer.com [74.55.252.38] 16 184 ms 185 ms 184 ms po2.car3.hstntx2.networklayer.com [74.55.252.106] 17 189 ms 189 ms 189 ms ev1s-209-62-105-19.theplanet.com [209.62.105.19] Trace finished C:\Program Files (x86)\Far2>tracert haskell.org Tracing to haskell.org [78.46.100.180], max hops = 30 1 4 ms 3 ms 2 ms bsr01.nn.ertelecom.ru [91.144.184.69] 2 * 2 ms 2 ms net184-77.nn.ertelecom.ru [91.144.184.77] 3 10 ms 9 ms 9 ms NNOV-D2-HQ-XX---1-3-0.499.main.synterra.ru [83.229.187.17] 4 9 ms 9 ms 11 ms 83.229.226.101 5 10 ms 9 ms 9 ms m9-cr01-te4-3.msk.stream-internet.net [195.34.38.37] 6 55 ms 55 ms 55 ms ss-crs-1-be3.msk.stream-internet.net [195.34.53.86] 7 53 ms 52 ms 52 ms m9-crs-1-be9.msk.stream-internet.net [195.34.59.250] 8 55 ms 55 ms 55 ms bor-crs-1-be1.spb.stream-internet.net [195.34.53.126] 9 52 ms 52 ms 52 ms anc-cr01-po3.ff.stream-internet.net [195.34.53.102] 10 52 ms 52 ms 52 ms anc-cr02-po1.ff.stream-internet.net [212.188.0.122] 11 56 ms 56 ms 56 ms decix2-gw.hetzner.de [80.81.193.164] 12 60 ms 60 ms 60 ms hos-bb1.juniper1.fs.hetzner.de [213.239.240.242] 13 64 ms 61 ms 63 ms hos-tr1.ex3k10.rz12.hetzner.de [213.239.228.139] 14 60 ms 60 ms 62 ms lambda.haskell.org [78.46.100.180] Trace finished -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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Making type-incompatible strategies interchangeable
by Jacek Generowicz 18 Dec '10

18 Dec '10

# Imagine an activity which may be performed either by a computer, or # by a human (alternatively, either locally, or remotely across a # network). From Haskell's type system's perspective, these two will # look completely different (most obviously, the human (or the # network) is wrapped in IO). How can they be made interchangeable ? # To demonstrate what I mean, I offer the following concrete toy # example, in Python. # It's a harness for playing the trivial higher-lower number guessing # game, with interchangeable strategies for either player. In this # example I provide two strategies (Computer / ask Human via IO) for # each role (asker and guesser). # How can this sort of interchangeability of computations which are # conceptually identical, but incompatible from the types perspective, # be expressed in Haskell? from random import randint # A simple game harness. It is given the two players, and mediates the # interaction between them. def game(asker, guesser): feedback = None count = 0 while not feedback == 0: guess = guesser(feedback) feedback = asker(guess) print "Guess: %s, Answer: %s" % (guess, feedback) count += 1 print "Got it in", count # A couple of decorators to smoothe the use of the generators which # are used to implement the players. def hide_send(generator_function): def proxy(*args, **kwds): return generator_function(*args, **kwds).send return proxy def advance(hidden_send_proxy): def proxy(*args, **kwds): send = hidden_send_proxy(*args, **kwds) send(None) return send return proxy # Artificial player who knows the secret @advance @hide_send def higher_lower_asker_C(low=0, high=100): secret = randint(low, high) guess = yield while True: guess = yield cmp(guess, secret) # Artificial player trying to guess the secret @hide_send def higher_lower_guesser_C(low=0, high=100): while True: guess = (low + high) // 2 feedback = yield guess if feedback < 0: low = guess else: high = guess # Interface to human who knows the secret @advance @hide_send def higher_lower_asker_H(): guess = yield # No feedback before first guess while True: print "My guess is", guess print "Please reply with one letter: is my guess (l)ow, (c)orrect or (h)igh ?" guess = yield {'l':-1, 'c':0, 'h':1 }[raw_input()] # Interface to human trying to guess @hide_send def higher_lower_guesser_H(): while True: feedback = yield input("What is your guess? ") print {-1:"Too low.", 0:"Correct!", +1:"Too high."}[feedback] # Given the above preparation, the game can now be played in all 4 # possible permutations of Computer/Human vs. Computer/Human. game(higher_lower_asker_C(), higher_lower_guesser_C()) game(higher_lower_asker_H(), higher_lower_guesser_C()) game(higher_lower_asker_C(), higher_lower_guesser_H()) game(higher_lower_asker_H(), higher_lower_guesser_H())

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Re: [Haskell-cafe] Infinite lists in real world programs
by Yves Parès 18 Dec '10

18 Dec '10

Thanks you all for your links, I will read this. However, Brent, I fail to understand your implementation of Monad... I understand the purpose, but not the algorithm. 2010/12/18 Matthew Sottile <mjsottile(a)mac.com> > Hi- > > This may be of some interest: > > https://github.com/mjsottile/hsworkflow/raw/master/docs/works09paper.pdf > > That describes a simple project that I think is similar to what you are > looking at doing. The code is in that github repository as well, a couple > directories up from the paper. > > https://github.com/mjsottile/hsworkflow/ > > -m > > > On Dec 15, 2010, at 5:52 AM, Yves Parès wrote: > > > Hello Café, > > > > I was wondering if using infinite lists was a viable and efficient > solution in haskell programs (I mean not simple prototypes) : > > I was considering using them to model agents in a hierarchical > multi-agent application for school. > > A list would representate the state of an agent at a step of the program. > > > > Let's say we have two simple agents, one multiplying its input by 2 and > the other dividing it by 4 : > > > > > > agent1 = fmap (*2) > > agent2 = fmap (/4) > > > > allValues = xs where > > ys = agent1 xs > > xs = 100:agen2 ys > > > > main = do > > mapM_ print $ take 100 allValues > > > > > > Of course, in a real program, an agent would rather take a list of > multiple agents (i.e. a list of lists) in input, so that its ouput could > depend on what several agents feed him. > > > > Some could state what I'm trying to do is FRP, and I agree. But it > remains a simple goal so I'd like to keep the program simple, and not go > into the whole complexity of a FRP framework (and I'm working with a > non-haskeller). > > For instance, with my solution, I cannot dynamically connect or > disconnect agents during the runtime, but I'll will not need to do that in > my program. > > Besides, I'd like to try to implement this myself, not use an already > existing framework. > > > > So is it viable or would the use of multiple infinite lists kill the > performances? > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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ByteString or Text version of getDirectoryContents
by Jesse Schalken 18 Dec '10

18 Dec '10

Is there a ByteString or Text version of getDirectoryContents? I am writing a program which scans a filesystem and it runs out of memory using Strings for all the file names, and I would like to avoid the overhead of calling ByteString.pack on the results of getDirectoryContents. I don't mind if it only works on POSIX, but System.Posix.Directory.readDirStream returns a String as well, and looking at how readDirStream is implemented I am left puzzled as it looks like it is using calls to native C functions but I can't find where the C string is turned into a String so I can change it to a ByteString or Text. Thanks, Jesse

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