- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] What on earth
by PR Stanley 21 Sep '07

21 Sep '07

but ebook usually means a PDF which would be all right if the text didn't contain specialist symbols. The publishers weren't very helpful when I contacted them. According to the "lady" on the phone they didn't even have the latex source code. I'll probably try again. Hopefully this time I'll get someone more reasonable to deal with. Cheers, Paul At 00:51 21/09/2007, you wrote: >According to >http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780511292187, >it is available as an ebook. Perhaps if you contact them and explain >that you have the paper version, they will send you an electronic copy >of it? > >Rodrigo > >On 20/09/2007, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > tfoldr (-) 1 [4,3] > > = 4-3-(-1) > > = 2 > > A similar thing happens with foldr (/) 2 [8,2] = 8.0 > > I thought foldr \oplus v [] = v > > Any ideas? > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] What on earth
by Miguel Mitrofanov 21 Sep '07

21 Sep '07

> tfoldr (-) 1 [4,3] > = 4-3-(-1) > = 2 Erm? You mean "foldr"? foldr (-) a [x,y,z,t,w] == x - (y - (z - (t - (w - a)))) So, foldr (-) 1 [4,3] == 4 - (3 - 1). > A similar thing happens with foldr (/) 2 [8,2] = 8.0 8/(2/2) = 8/1 = 8 > I thought foldr \oplus v [] = v Yeah, that's right.

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Haskell guards
by Victor Nazarov 20 Sep '07

20 Sep '07

I still can't remember how guards are treated in Haskell. Here is the code snippet in question: foo a | a == 1 = 6 foo a | a == 2 = 7 foo a = 8 Would Haskell fall through to the third alternative if a is not equal to 1 or 2. I know that this can be rewritten more sanely, but I just consider guard behavior. What would be the value of (foo 3)? -- vir http://vir.comtv.ru/

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length defined with foldr
by PR Stanley 20 Sep '07

20 Sep '07

Hi length = foldr (. n . 1 + n) 0 Any idea how to define length with foldr. The above definition doesn't make much sense. Many thanks, Paul

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Spot the difference!
by PR Stanley 20 Sep '07

20 Sep '07

Hi \_ n -> 1 + n \_ -> (\n -> 1 + n) The outcome seems to be identical. is there a substantive difference between the two definitions? The second one seems to be more expressive of the currying principle. Any thoughts? Thanks, Paul

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Call for Abstracts for 3rd SAPLING Meeting, November 12, 2007
by Manuel M T Chakravarty 20 Sep '07

20 Sep '07

Call for Abstracts for 3rd SAPLING Meeting, November 12, 2007 Sydney Area Programming Language INterest Group http://www.comp.mq.edu.au/plrg/sapling.html 10am-4:30pm, November 12, 2007 Engineering Theatre, Building 32 (Engineering) Australian National University Invited Speaker --------------- Prof William M Waite, University of Colorado, Boulder, USA About SAPLING ------------- SAPLING aims to promote and develop academic research in the Programming Languages field within the Sydney area, including nearby regional centres such as Canberra, Newcastle and Wollongong. Important Dates: ---------------- - Submission deadline: Oct 15, 2007 - Notification: Oct 19, 2007 - Workshop: November 12, 2007 SAPLING Meetings ---------------- SAPLING meetings are held twice a year to allow programming language academics, researchers and students to present and discuss their research in an informal setting. The third meeting will be held at the Australian National University. SAPLING talks are generally of 20 minutes duration with substantial periods of time available for discussion. Talks can present completed work, outline work in progress, demonstrate a system, or provide a short tutorial on a topic of general interest. Submissions for talks should be in the form of an abstract no longer than one A4 page in plain text, PostScript or PDF format. The abstract should be emailed to Tony Sloane (asloane AT ics.mq.edu.au) by close of business on October 15, 2007. As far as possible, selection of presenters is non-competitive, with all interested qualified speakers given an opportunity to talk, within the time constraints of a one day meeting. Preference will be given to topics of general interest. No proceedings will be published but the talk abstracts and links to related material will be published on the SAPLING website. There is no registration fee to attend this meeting. Participants will be responsible for their own lunch costs. Accommodation ------------- For non-Canberra residents, some accommodation suggestions are available on the web site for this meeting: http://www.comp.mq.edu.au/~asloane/pmwiki.php/SAPLING/SAPLING072 Steering Committee: ------------------- Anthony Sloane, Macquarie (Chair), asloane AT ics.mq.edu.au Manuel Chakravarty, UNSW, chak AT cse.unsw.edu.au Clem Baker-Finch, ANU, Clem.Baker-Finch AT anu.edu.au

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wxhaskell package for ubuntu feisty, amd64
by Iván Pérez Domínguez 20 Sep '07

20 Sep '07

I'm trying to install wxhaskell in ubuntu feisty, with ghc-6.6. After several days of trial and error and a few hours trying to change all types in my program to gtk2hs types, I'm tired. Does anyone have a .deb for wxhaskell that works with ghc-6.6?

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What on earth
by PR Stanley 20 Sep '07

20 Sep '07

Hi tfoldr (-) 1 [4,3] = 4-3-(-1) = 2 A similar thing happens with foldr (/) 2 [8,2] = 8.0 I thought foldr \oplus v [] = v Any ideas? Thanks, Paul

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Re: [Haskell-cafe] length defined with foldr
by PR Stanley 20 Sep '07

20 Sep '07

>>Still fighting with the botched OCR? >>I am, as a matter of fact! Is that okay with you? *smile* > > > >>length = foldr (\_ n -> 1 + n) 0 >> >>-- >>brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com >>system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu >>electrical and computer engineering, carnegie mellon university KF8NH

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Why isn't pattern matching lazy by default?
by Peter Verswyvelen 20 Sep '07

20 Sep '07

I'm experimenting with functional reactive programming for creating simple 2D/3D video games and interactive apps, trying to develop my own version of it from scratch, for learning Haskell. I got stuck with an endless loop when trying to split a stream into a pair of two streams (a kind of reactive if/then/else). Luckily I first read the Haskell School of Expression so I remembered that pattern matching is not lazy and this could be the cause, which it was (I had to replace (x:xs) by ~(x:xs)) I could also fix the problem by not using pattern matching at all, using head/tail calls instead. Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. Thanks, Peter

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Re: [Haskell-cafe] Missing Symbol (2)
by PR Stanley 19 Sep '07

19 Sep '07

The book is very well written. I did try to follow the wikibook thingummajig but found the English a bit trying. Cheers Paul t 07:43 19/09/2007, you wrote: >Paul, > >The Hutton book has gotten great reviews, but pouring over a faulty >OCRd version can only be counterproductive. I highly recommend Yet >Another Haskell Tutorial[1]. After that you can check out all the >other wonderful online resources (starting here [2]). At that point a >few OCR errors won't be able to stand in your way! > >Best wishes on you Haskell explorations, >Joel > >[1] http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf >[2] http://haskell.org/haskellwiki/Learning_Haskell >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell] Functor ((,) a)
by Ross Paterson 19 Sep '07

19 Sep '07

On Wed, Sep 19, 2007 at 10:24:50AM +0200, Janis Voigtlaender wrote: > BTW, what would have been the easiest way for me to find this out on my > own? Somehow, I would have hoped to be able to navigate to the appropriate > point from the mentioned place in the online docs, where it is stated that > the instance in question exists. Usually the instance for C (T a1 ... an) should be in either the module defining C or the one defining T (so if you import both you have it); if T is from the Prelude, then the instance should be together with C. But Control.Monad.Instances is a special case, because exists to hold instances of Haskell 98 classes for H98 types that are not defined in H98. Putting them in the proper place would break compatibility with H98.

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Missing Symbol (2)
by PR Stanley 19 Sep '07

19 Sep '07

Hi Here's another one: f [] = v f (x:xs) = x . f xs The "." is supposed to denote a generic operator - f [] maps to some value v and applied to non-empty list applies head to ? operator. Thanks, Paul

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Re: [Haskell] downtime for monk ({darcs, hackage, cvs}.haskell.org) Monday from 3pm UTC
by Ian Lynagh 19 Sep '07

19 Sep '07

On Wed, Sep 19, 2007 at 09:16:56AM +0100, Bayley, Alistair wrote: > > From: haskell-bounces(a)haskell.org > > [mailto:haskell-bounces@haskell.org] On Behalf Of Ian Lynagh > > > > The machine called monk ... > > will be down from 3pm UTC on Monday 24th for an OS and RAM upgrade. > > Do you have any estimate for how long the upgrades will take? That depends on how many things go wrong. I'll send an e-mail when we believe it's all up and working again. Thanks Ian

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(win)hipe for Haskell?
by Peter Verswyvelen 19 Sep '07

19 Sep '07

During a googling session, I can across (Win)HIPE, a visualization program for the functional language HOPE. See http://dalila.sip.ucm.es/~cpareja/winhipe IMHO a similar tool would be a nice for learning/teaching Haskell; does that exist, or something else that comes close? Thanks, Peter

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Re: [Haskell-cafe] Missing Symbol (2)
by PR Stanley 19 Sep '07

19 Sep '07

It's probably \oplus then. The font size according to my trusty screen reader is 11 point which is not particularly large. Still, if anybody else finds it annoying I'll reduce it to 9pt. Paul At 13:21 19/09/2007, you wrote: >This looks like foldr, specifically: >f = foldr (.) v > >so really, there's no way of telling what the operator could be, other >than that: >(.) :: a -> b -> b >where >f :: [a] -> b >v :: b > >BTW, I find that most of your emails are in a very large font size. Do >other people notice this? It's not very nice to read. Gmail's 'Show >Original' feature tells me that the HTML version of your message >contains <font size=3> near the start. > >On 19/09/2007, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > Here's another one: > > f [] = v > > f (x:xs) = x . f xs > > The "." is supposed to denote a generic operator - f [] maps to some > > value v and applied to non-empty list applies head to ? operator. > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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transparent parallelization
by Thomas Girod 19 Sep '07

19 Sep '07

Hi there. Beeing rather new to the realm of Haskell and functional programming, I've been reading about "how is easier it is to parallelize code in a purely functional language" (am I right saying that ?). My knowledge of parallelization is also very weak, but I've been thinking about this and I have a question. Let's say we take a small piece of code, like the quicksort algorithm. > qsort [] = [] > qsort (x:xs) = qsort lesser ++ [x] ++ qsort greater > where lesser = filter (<x) xs greater = filter (>=x) xs (actually I didn't test this code, this is just for the example) Here, computing "lesser" and "greater" are two disjoint actions - we don't need the result from one to compute the other, and haskell does not allow one to alter data so that would change the behaviour of the other. So I guess those could be run in parallel. Would it be fairly simple to automatically determine parts of code that can be run in parallel, and parts that cannot (at least in non-monadic code) ? So the final question is : if it is possible to automatically define segments of code that can be run in parallel, is [insert compiler name here] compiling this code as a one thread thing, or as a multithreaded version ? I guess on single processor machines, it is more efficient to do it as a single thread - but with those "many-cores" processors that are coming, it probably won't be true anymore. Sorry if this post is blowing off open doors (I guess this doesn't mean anything in english but it sounds cool) ... Regards Thomas Girod

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Re: [Haskell] Functor ((,) a)
by Bas van Dijk 19 Sep '07

19 Sep '07

On 9/19/07, Janis Voigtlaender <voigt(a)tcs.inf.tu-dresden.de> wrote: > BTW, what would have been the easiest way for me to find this out on my > own? The following is probably not the easiest way: I keep a copy of the sources of GHC and the libraries [1] on my disk. When I want to search for something I simply do a grep. In emacs you also have a grep-find function which performs a grep in the background and shows the result in a new buffer. The results are hyperlinked so you can just click on a result and emacs will open the respected file and jump to the found string. In your case you do a 'M-x grep-find' followed by the search string: "instance Functor ((,)". This will result in: ./Control/Monad/Instances.hs:19:instance Functor ((->) r) where ./_darcs/pristine/Control/Monad/Instances.hs:19:instance Functor ((->) r) where I think you can get rid of the annoying _darcs... by following: [3]. I haven't tried that though. (Note I'm on Linux. I don't know if this works in Windows. It probably does with Cygwin.) Of course the disadvantage of this approach is that you have to know exactly what you are looking for. Then there's also Hoogle [3] with which you can search for names or types. However in your case this wouldn't have helped. regards, Bas [1] http://hackage.haskell.org/trac/ghc/wiki/Building/GettingTheSources [2] http://wiki.darcs.net/DarcsWiki/HintsAndTips#head-c189bd259a3d141796ebb4f8d… [3] http://haskell.org/hoogle

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help me ! who has vty ?
by clisper 19 Sep '07

19 Sep '07

who has vty for haskell? I can't find the right one so who can mail one to me ? thanks 2007-09-19 clisper

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Re: [Haskell] Functor ((,) a)
by Matthew Brecknell 19 Sep '07

19 Sep '07

Janis Voigtlaender: > What do I have to import to get the Functor ((,) a) instance? (redirected to haskell-cafe) Control.Monad.Instances, believe it or not.

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Re: [Haskell] Swapping parameters and type classes
by Simon Marlow 19 Sep '07

19 Sep '07

Ian Lynagh wrote: > On Tue, Sep 18, 2007 at 03:48:06PM +0100, Andrzej Jaworski wrote: >> Responding to Simon Peyton-Jones' reminder that this is a low-bandwidth list I >> was obscure and commited a blunder. >> >> This one and many other threads here are started undoubtedly by experts [sorry >> guys:-)] and coffee brake should work for them, but on numerous occasions >> threads here spawn beginner type questions. So, my thinking was that it is >> perhaps against the tide trying to stop them. >> Why not to make the list Haskell a first contact general practitioner? Then >> creating e.g. "Announcements & Challenge" or "Announcements & ask guru" list >> could take the best from "Haskell" but also would make it less front line and >> thus more elitist, which should imply the manner by itself. > > I proposed renaming > haskell@ -> haskell-announce@ > haskell-cafe@ -> haskell@ > in http://www.haskell.org/pipermail/haskell-cafe/2007-July/028719.html I suggested the same thing at the time we created haskell-cafe, but the concensus was in favour of haskell-cafe. At the time I didn't think it would work - for a long time, the number of subscribers to both lists was almost the same - but now I have to admit I think haskell-cafe is a big win for the community. The -cafe extension gives people the confidence to post any old chatter without fear of being off-topic, and I'm sure this has helped the community to grow. Those of us who grew up with Usenet (RIP) are more at home with the foo/foo-announce split, and it's certainly quite conventional for mailing-list naming too, but on the whole I don't think doing things differently has really done us any harm and it may well have been a stroke of genius :-) Cheers, Simon

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Re: [Haskell-cafe] Missing Symbol
by PR Stanley 19 Sep '07

19 Sep '07

> > Hi > > What symbol would you replace the "?" with: > > the identity function > > id x = x? > >I don't think I'd have anything where the ? symbol is. > >id x = x works just fine. Ah, that's interesting because there's a "." to the right of x in my scanned doc. Still, if no symbol is necessary, so much the better! Cheers, Paul

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Missing Symbol
by PR Stanley 19 Sep '07

19 Sep '07

Hi What symbol would you replace the "?" with: the identity function id x = x? My scanned copy of the Hutton book contains some rather interesting characters. In fact, the OCR program has placed a "." in place of most symbols - e.g. ->, <- ^, <=, \=, etc. Fortunately, I'm able to guess the correct operator in the majority of the cases. Cheers, Paul

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ANN: hstats-0.1
by Marshall Beddoe 19 Sep '07

19 Sep '07

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hstats-0.1 I've just released hstats, a statistical computing module for the Haskell language. Current functionality includes: mean, median, mode, range, standard/average deviation, variance, iqr, kurtosis, skew, covariance, and correlation. I have plans on adding more rank correlation functions, histograms & chi square tests. darcs repo: http://www.sftank.net/code/hstats

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Building "production stable" software in Haskell
by Peter Verswyvelen 19 Sep '07

19 Sep '07

The way I see it as a newcomer, Haskell shifts the typical imperical programming bugs like null pointers and buffer overruns towards "space/time leaks", causing programs that either take exponentially long to complete, stack overflow, or fill up the swap file on disc because they consume gigabytes of memory. Other bugs I found are incomplete pattern matches at runtime, but I already got an email of how to fix this using an external tool, although it would be nice if this is part of the compiler/linker itself. How well and how can a Haskell program be tested to make sure it does not cause these space/time bugs? What tools are typically used? Thanks, Peter

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yi-gtk
by clisper 19 Sep '07

19 Sep '07

who knows how to compile yi-gtk? i tried,but it told me mine miss gtk. 2007-09-18 clisper

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Type-Marking finite/infinte lists?
by Joachim Breitner 19 Sep '07

19 Sep '07

Hi, today while mowing the lawn, I thought how to statically prevent some problems with infinte lists. I was wondering if it is possible to somehow mark a list as one of finite/infinite/unknown and to mark list-processing functions as whether they can handle infinte lists. For example, it is known that a list produced by repeat or cycle is always infinte, that foldl can’t handle infinte lists (while foldr might). length should never be called on an infinte list, and map’s output is infinite iff the input list is. So the compiler might use this information to emit errors like “line 42: program will not terminate: length applied to infinite list” or warnings “line 23: termination not guaranteed: foldl applied to possible infinte list”. I guess you can’t know it for all lists (halting problem...), but for those where it is possible, it would be nice to tell. Or am I missing something? Can this be implemented using some guru type system hacking? Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: joachimbreitner(a)amessage.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Re: [Haskell] Blocked STM & GC question
by Jules Bean 18 Sep '07

18 Sep '07

Simon Marlow wrote: > Ashley Yakeley wrote: >> If I have a thread that's blocked on an STM retry or TChan read, and >> none of its TVars are referenced elsewhere, will it get stopped and >> garbage-collected? >> >> I have in mind a pump thread that eternally reads off a TChan and >> pushes the result to some function. If the TChan is forgotten >> elsewhere, will the permanently blocked thread still sit around using >> up some small amount of memory, or will it be reaped by the garbage >> collector? > > In this case, your thread should receive the BlockedIndefinitely exception: > > http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exceptio… > > > If the system is idle for a certain amount of time (default 0.3s, change > it with the +RTS -I option) a full GC is triggered, which will detect > any threads that are blocked on unreachable objects, and arrange to send > them the BlockedIndefinitely exception. Including MVars? Your quoted text suggests 'Yes' but the docs you link to suggest 'No'. Jules

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unique id for data types
by Barney Hilken 18 Sep '07

18 Sep '07

In order to make my records system practically useable, I need a type family > type family NameCmp n m which totally orders datatypes. More precisely, it should return one of the following types: > data NameLT = NameLT > data NameEQ = NameEQ > data NameGT = NameGT for each pair of datatypes n & m, according to whether n < m, n = m, or n > m in some global ordering. This ordering needs to be independent of the context, so it can't be affected by whatever imports there are in the current module. What I want to know is: does GHC give datatypes any global id I could use to generate such an ordering? Would fully qualified names work? Secondly (assuming it's possible) how easy would it be for me to write a patch to add NameCmp to GHC? Where in the source should I start looking? Thanks, Barney.

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Observations from ListLike
by John Goerzen 18 Sep '07

18 Sep '07

Hi folks, I just announced ListLike[1] on haskell(a)haskell.org. Rather than repeat that announcement here, I'd like to make a few observations that came out of the development of this program: * I wrote extensive QuickCheck cases for this and wrapped them in HUnit for better display and future extensibility. Two bugs in the GHC base libraries were discovered as a result (one has been fixed already in HEAD, one not) * Hugs programs that use cpphs can't use ByteString * It would be really nice if QuickCheck supported I/O and some version of HUnit's TestLabel to generate hierarchical names when failures occur. * It would also be much nicer if QuickCheck defined Arbitrary instances for Word8 and Char. On Hugs, QuickCheck also lacks an instance of Maybe, but GHC has it for some reason. * Testing functions like foldl in Hugs, where QuickCheck is supplying the list and the function, cause Hugs to crash with stack errors. Testing some functions over ByteStrings cause Hugs to crash complaining of being out of ForeignPtrs. I have yet to figure out how to fix either problem; raising the heap size doesn't help. * I had some cases in String.hs[2] where Hugs did not permit me to define the default functions within the class declaration. However, I could define the functions outside, with the exact same type signature as they'd have inside, and within the class declaration, just say something like "words = myWords" to refer to the external declaration. GHC worked fine both ways. It seems that Hugs has a number of issues with code like ListLike that uses some more advanced Haskell features, even though Hugs nominally supports them. If you look through my Mercurial changelogs, you'll see "fix blah for hugs" messages all over. Which is too bad, because Hugs seems more portable (especially to embedded devices) than GHC. [1] http://software.complete.org/listlike [2] http://software.complete.org/listlike/browser/src/Data/ListLike/String.hs

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Israel Haskell Programmers
by B K 18 Sep '07

18 Sep '07

Hello, Are there any Haskell Hackers on this mailing list who live in Israel? I am interested in starting an Israel Haskell User Group.

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Dynamic thread management?
by Hugh Perkins 18 Sep '07

18 Sep '07

Haskell/FP seems to have solved the hardest bit of threading, which is making it obvious which bits of a program are parallelizable, and which are not. Remains to actually parallelize out the programs. Am I being naive or is this trivial? There has been a lot of talk about parallelizing out a program statically, at compile time, but this would seem to suffer from the halting problem? The only way to know how long a function, or one of it's sub-functions, will take to run is to actually run it? Is there some reason why we cant just start a function running in a single thread, whilst running profiling, then after a while we check which bits of the function are taking time to run, and are parellizable, and we parallelize those out? This sounds reasonably trivial and workable? Basically, "parallizable" in a Haskell context means, as a first approximation, any map, foldr, foldl or derivatives, and any independent let assignments, then we can always add in extra parallizable cases later. Profiling is already built into Haskell AFAIK? so the profiling information is already available? Thoughts? Is there some reason why such an approach has been disregarded/is harder than it sounds? (By the way, can someone provide me a non-google link to the SPJ video talking about nested data parallism? google video is unavailable from my current location, and although I did watch this video once, it was on my second day of doing Haskell, a few weeks ago, so much of it was incomprehensible to me ;-) )

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Re: [Haskell] Swapping parameters and type classes
by Bas van Dijk 18 Sep '07

18 Sep '07

On 9/16/07, Mads Lindstrøm <mads_lindstroem(a)yahoo.dk> wrote: > Hi all > > If I have this type: > > data Foo a b = ... > > and this class > > class Bar (x :: * -> *) where ... > > I can imagine two ways to make Foo an instance of Bar. Either I must > "apply" the 'a' or the 'b' in (Foo a b). Otherwise it will not have the > right kind. To "apply" the 'a' I can do: > > instance Bar (Foo a) where ... > > But what if I want to "apply" the 'b' ? How do I do that ? > > > Greetings, > > Mads Lindstrøm > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > (Redirecting to Haskell-Cafe) I think the easiest way to make it work is with a newtype wrapper around 'Foo a b' that swaps the arguments, like Brent mentioned. Without it you need some kind of type level lambda abstraction. Something like: instance Bar (\x -> Foo x b) where ... But this is not valid Haskell. cheers, Bas

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Records: Examples
by Barney Hilken 18 Sep '07

18 Sep '07

> {-# LANGUAGE TypeFamilies #-} Hi Justin, thanks for your interest. Hope this helps! > module Examples where > import Records To get started, you need to define your labels. They are just singleton datatypes: > data FirstName = FirstName deriving (Show, Eq, Ord) > data Surname = Surname deriving (Show, Eq, Ord) > data Address = Address deriving (Show, Eq, Ord) > data PhoneNo = PhoneNo deriving (Show, Eq, Ord) you can define as many as you like. Next you have to define the order on fields. At the moment you have to do this by hand, but I hope to get ghc to do this automatically: > type instance NameCmp FirstName FirstName = NameEQ > type instance NameCmp FirstName Surname = NameLT > type instance NameCmp FirstName Address = NameLT > type instance NameCmp FirstName PhoneNo = NameLT > type instance NameCmp Surname FirstName = NameGT > type instance NameCmp Surname Surname = NameEQ > type instance NameCmp Surname Address = NameLT > type instance NameCmp Surname PhoneNo = NameLT > type instance NameCmp Address FirstName = NameGT > type instance NameCmp Address Surname = NameGT > type instance NameCmp Address Address = NameEQ > type instance NameCmp Address PhoneNo = NameLT > type instance NameCmp PhoneNo FirstName = NameGT > type instance NameCmp PhoneNo Surname = NameGT > type instance NameCmp PhoneNo Address = NameGT > type instance NameCmp PhoneNo PhoneNo = NameEQ Now we are ready to play! To define records, use (=:) and (+:) > barney = FirstName =: "Barney" +: Surname =: "Hilken" +: > Address =: "Horwich" +: PhoneNo =: "697223" You can use as many or as few of the fields as you like, and you can write them in any order, but trying to use a field twice in the same record will give you a (rather incomprehensible) type error. > justin = Surname =: "Bailey" +: FirstName =: "Justin" +: Address =: "Somewhere" To extract the value at a field use (.:) > myPhone = barney.:PhoneNo To delete part of a record, use (-:) > noCallers = barney -: Address To update existing fields in a record, use (|:) > barney' = barney |: Address =: ((barney .: Address) ++ ", UK") The power of the records system is that these five operators, =: +: .: -: |: are Haskell polymorphic functions. So you can define functions like > livesWith p q = p |: Address =: (q .: Address) which returns p, but with its Address field changed to that of q. Note that this function works on any records p and q with Address fields, whatever other fields they may have. You can even define functions parametrised by field names: > labelZip n m = zipWith (\x y -> n =: x +: m =: y) then 'labelZip FirstName Surname' is a function which takes two lists and returns a list of records: > names = labelZip FirstName Surname ["Barney", "Justin"] ["Hilken", "Bailey"] of course, labelZip isn't restricted to the four labels we defined earlier, it works on anything. The system is strongly typed, so record errors (such as missing or duplicated fields) are caught at compile time. There are type operators (:=:), (:+:), (:-:), (:.:) corresponding to the record operators, and classes `Contains`, `Disjoint`, `Subrecord` which allow you to express conditions on types. Unfortunately, the type system sometimes decides that a function has a different type from the one you expect, and won't accept the header you want to give it. More experience with the system is needed before we can say whether this is a problem. Barney.

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Re: [Haskell] Records
by Justin Bailey 18 Sep '07

18 Sep '07

On 9/16/07, Barney Hilken <b.hilken(a)ntlworld.com> wrote: > Now that I have a version of ghc with type classes, I have had a go > at implementing records based on the ideas I mentioned on this list a > few months ago. The code of my first attempt is available at http:// > homepage.ntlworld.com/b.hilken/files/Records.hs That's pretty interesting stuff, but it's hard to see how ti works without some examples. Would you want to include some in your comments? I do appreciate the thorough documentation otherwise, though. Justin p.s. replied via haskell-cafe.

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Cross-compiling of GHC
by L.Guo 18 Sep '07

18 Sep '07

Hi all: How to build a GHC which can run in a embed linux system ? I have toolchain for the targer system. Using which, I can compile on PC and run the program on the embed system. Is this means, if I change CC env-var to the toolchain compiler and compile GHC manually, I can get a program for the embed system ? I have read Building/Porting for GHC[1], that does not mention anything about how to build a GHC on a target machine using a cross-compiler like xxx-linux-gcc. *or* What do I need to do besides the stages that page mentioned ? Regards [1] http://hackage.haskell.org/trac/ghc/wiki/Building/Porting -------------- L.Guo 2007-09-17

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Re: [Haskell] Linear Equation Solver Using Arrays
by Andrew Wagner 17 Sep '07

17 Sep '07

(Replying on haskell-cafe) Let me start with a disclaimer: I haven't looked at your code extensively. That said, the feeling that I get from it is one of listening to a non-native speaker. The things in your program are obviously not completely inaccurate, or they wouldn't have type-checked. And, presumably, you've already done some testing, so it probably produces the right answers. Similarly, there are many things in English that you _could_ say, which would be entirely grammatically correct, but which native speakers would never say. For example, in your code, I noticed a pattern which I've just recently learned: <code> do ... tmp <- readArray scaleInfo j writeArray scaleInfo imax tmp </code> Now, if you were to de-sugar the last 2 lines, it would look something like this: <code> readArray scaleInfo j >>= \tmp -> writeArray scaleInfo imax tmp </code> ...but as soon as you see that lambda expression, you should (hopefully) say "oh, that can be written more easily: <code> readArray scaleInfo j >>= writeArray scaleInfo imax </code> Of course, a native speaker will probably not even think about the intermediate lambda expression, but the point is, this is an idiom. And, really, this is a very tiny example. Your entire program is written in a very imperative style. "Do this, then do that, then do the other thing." And this is to be expected. After all, you said that you were directly porting the algorithm from an imperative language. However, if you're going to do this idiomatically, you really need to think very differently. I suggest reading and pondering this well-known blog entry for more ideas about the differences between the approaches of functional and imperative programming: http://blogs.nubgames.com/code/?p=22 Sorry, I don't have more specific suggestions, but I suspect this is what most people thought when they read your code: it's hard to make specific suggestions, when you're really approaching the problem in a non-idiomatic way. On 9/16/07, Xiao-Yong Jin <xj2106(a)columbia.edu> wrote: > Dear Haskellers, > > My thanks to people on the irc channel, especially `int-e'. > With their help, I managed to write a linear equation solver > using STUArrays. > > > It is basically a direct rewrite of a C version, which > adopts Crout's LU decomposition method with implicit > pivoting, in chapter 2.3 of the book Numerical Recipes. > > I must admit that the code is in a really bad style, > somewhat ugly, because of the limit of my Haskell knowledge. > The code is attached in this email for your amusement. I > would be really thankful if you can give me any kind of > comments, idea, improvement, criticism, because I sincerely > want to make the code better. > > Best regards, > Xiao-Yong > -- > c/* __o/* > <\ * (__ > */\ < > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > > >

2 1

Parsec Scanner Question
by Jude 17 Sep '07

17 Sep '07

Haskell Folks, I have an existing Parsec CharParser parser that I would like to extend to include line continuation support. When there's a backslash-newline combination anywhere in the token stream, I'd like to remove it so it's not read by the rest of the parser, even if its in the middle of a keyword. Currently I'm doing this with a separate "preprocess" function that weeds out all the '\\\n' strings from the input before passing it to the parser. The problem with this is it messes up the line position when there's an error below the '\\\n' point, since the newline is never seen by the parser. So I've been trying to figure out how to handle this better with Parsec. I think I need something that acts just like CharParser, except it skips over the backslash-newline tokens, It looks like section 2.11 in the Parsec manual (Advanced: separate scanners") covers this, but I'm not having much luck turning it into working code. Could someone who's written their own scanner spell out how it's done in a little more detail ? Thanks ! - jude

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Data.Binary Endianness
by Adrian Neumann 16 Sep '07

16 Sep '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 I try to write a Windows Bitmap File using Data.Binary, but I have some trouble. For example, the internet states, that the magic number, that puts 'BM' in the first two bytes of the file is 19778. But when I > put (19778::Word16) I get 'MB' instead. I read on the german Wikipedia, that bmp uses little endian encoding, but Data.Binary uses big endian. Obviously this can't work. Is there a way to switch endianness? Or at least a function that converts between the two? I looked at the Data.Binary Haddock documentation, but found nothing like that. Thanks in advance, Adrian -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFG5Q1Z11V8mqIQMRsRA/V6AJoC/R2k5rz0LjQI4iGoi79zLYZmgwCdEx/g cZOR5jseUcQnc6+I09N+xj8= =K5ww -----END PGP SIGNATURE-----

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unknown RTS option: -N2
by Gregory Propf 16 Sep '07

16 Sep '07

I've built a program with the -threaded option using ghc. This option is supposed to link your program to the threaded runtime with support for multicore CPUS (mine is a dual core). The program pukes with the message in the subject line when I try to use the -N option to tell it to use both CPUs. I'm reasonably sure that I actually have the threaded runtime because initially I got a runtime message from the program about how GTK is single threaded and can only be used with the threaded runtime using runtimeunsafeInitGUIForThreadedRTS instead of the standard initGUI. I made the change in the code and now the program runs but still does not know the -N RTS option. This problem seems to affect all my programs. It is as if the threaded runtime is there but doesn't know any of its special flags. This is the line I'm compiling the program with. It's a life game clone in case you're curious. Compiler is ghc-6.4.2 on 64 bit AMD dual core running Gentoo Linux. - Greg ghc -threaded -package gtk -package glade -o hlife hlgtk.o Hlife.o ____________________________________________________________________________________ Check out the hottest 2008 models today at Yahoo! Autos. http://autos.yahoo.com/new_cars.html

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Re: [Haskell] question about a failure to generalize
by Tim Chevalier 16 Sep '07

16 Sep '07

[replying to haskell-cafe] On 9/15/07, Norman Ramsey <nr(a)eecs.harvard.edu> wrote: > Dear Haskellers, > > I've had a Haskell program rejected for reasons I don't understand. > Here's the relevant bit of code; the problem is that I expected the > type-inference engine to generalize the abbreviation 'fold' to an > overloaded function, but it doesn't---to make the code work, I had to > expand 'fold' into 'foldRegsUsed' everywhere it appears. I'm baffled > because 'fold' isn't mutually recursive with anything, so I would have > expected ordinary Hindley-Milner style inference to generalize it to > something of the type > > UserOfLocalRegs a => (b -> LocalReg -> b) -> b -> a -> b > > But that's not what happens. This code fails to compile because the > compiler is willing to use 'fold' at only one type (CmmExpr as it happens): > > class UserOfLocalRegs a where > foldRegsUsed :: (b -> LocalReg -> b) -> b -> a -> b > > instance UserOfLocalRegs Middle where > foldRegsUsed f z m = middle m > where middle (MidComment {}) = z > middle (MidAssign _lhs expr) = fold f z expr > middle (MidStore addr rval) = fold f (fold f z addr) rval > middle (MidUnsafeCall tgt _ress args) = fold f (fold f z tgt) args > middle (CopyIn _ _formals _) = z > middle (CopyOut _ actuals) = fold f z actuals > fold = foldRegsUsed > > > What rule of the language have I overlooked? > Monomorphism restriction? Replacing fold with foldRegsUsed would work because there's a type signature for foldRegsUsed. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "...There is no mystery; there is only paradox, the incontrovertible union of contradictory truths." -- Edward Abbey

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Sequencing Operations in a Monad
by Dominic Steinitz 15 Sep '07

15 Sep '07

> If you arrange the types to try to do all the operations inside the IO > monad you can't chain together more than 1 binary operation. eg. > > do > S <- A + B > Z <- Q * S > > vs > > do > S <- Q * (A + B) > > Are there any suggestions for this dilemma? Am I using the wrong monad for > this task? I'm not sure if this is what you are asking but isn't liftM2 or some variant what you need? Dominic.

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haskell on llvm?
by brad clawsie 15 Sep '07

15 Sep '07

has anyone ever considered using llvm as a infrastructure for haskell compilation? it wold seem people are looking at building frontends for scheme, ocaml, etc. i don't know if an alternate backend is appropriate, but it would seem to be an interesting way to aggregate the best thinking for various optimizations over a more diverse group of developers.

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Is "take" behaving correctly?
by PR Stanley 15 Sep '07

15 Sep '07

Hi take 1000 [1..3] still yields [1,2,3] I thought it was supposed to return an error. Any ideas? Thanks, Paul

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Memory leak or wrong use of Array ?
by L.Guo 15 Sep '07

15 Sep '07

Hi MailList Haskell-Cafe: I am tring to solve Project Euler problem 70. And write some code. (will at the end of this mail) And, I run the code in GHCi. The problem is that, when the input is 1,000,000, it works fine, when the input is up to 10,000,000, the memory GHCi used increase very fast and did not stop. Is this a memory leak ? or, is there some mis-understand about array ? Regards -------------- -- Mudules : import Data.Array.IO import Foreign ( unsafePerformIO ) -- Codes : p070_solve = putStrLn . show $ solutionOf 10000000 where isPerm a b = sort (show a) == sort (show b) phis n = unsafePerformIO $ do arr <- newArray (2,n) (False,1/1) :: Fractional t => IO (IOArray Int (Bool,t)) mapM_ (sieve arr n) [2..n] factors <- getElems arr return . map (\(n,(b,f)) -> (n,floor $ toRational n*f)) $ zip [2..n] factors where sieve arr ubound p = do (b,o) <- readArray arr p if b then return () else mapM_ (update arr (toRational p)) . takeWhile (<=ubound) $ iterate (+p) p update arr p i = do (_,o) <- readArray arr i writeArray arr i (True,o*(p-1)/p) solutionOf = snd . minimum . map (\(n,phi)->(toRational n / toRational phi,n)) . filter (uncurry isPerm) . phis -------------- L.Guo 2007-09-14

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ANN: HPDF 1.1
by alpheccar 14 Sep '07

14 Sep '07

I have just released the version 1.1 of HPDF. You can get it on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF-1.1 In this release, I am experimenting with typesetting. -- Christophe.

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Clarification Please
by PR Stanley 14 Sep '07

14 Sep '07

Hi Taken from chapter 6, section 8 of the Hutton book on programming in Haskell: 5. Using merge, define a recursive function msort :: (Ord a) => [a] -> [a] that implements merge sort, in which the empty list and singleton lists are already sorted, and any other list is sorted by merging together the two lists that result from sorting the two halves of the list separately. : Hint: first define a function ¬halve :: [a] -> [([a], [a])] ¬that splits a list into two halves whose length differs by at most one. Create a halve function - okay, that's fairly straightforward. The rest, I'm afraid, is a little obscure. I'm not looking for the solution; I'd like to work that out for meself. However, I'd really appreciate some clues as to the general structure of the algorithm. Much obliged, Paul

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getting crazy with character encoding
by Andrea Rossato 14 Sep '07

14 Sep '07

Hi, supposed that, in a Linux system, in an utf-8 locale, you create a file with non ascii characters. For instance: touch abèèè Now, I would expect that the output of a shell command such as "ls ab*" would be a string/list of 5 chars. Instead I find it to be a list of 8 chars...;-) That is to say, each non ascii character is read as 2 characters, as if the string were an ISO-8859-1 string - the string is actually treated as an ISO-8859-1 string. But when I print it, now it is displayed correctly. I don't understand what's wrong and, this is worse, I don't understand what I should be studying to understand what I'm doing wrong. After reading about character encoding, the way the linux kernel manages file names, I would expect that a file name set in an utf-8 locale should be read by locale aware application as an utf-8 string, and each character a unicode code point which can be represented by a Haskell char. What's wrong with that? Thanks for your kind attention. Andrea Here the code to test my problem. Before creating the file remember to set the LANG environmental variable. Something like: export LANG="en_US.utf8" should be fine. (Check your available locales with "locale -a") import System.Process import System.IO import Control.Monad main = do l <- fmap lines $ runProcessWithInput "/bin/bash" [] "ls ab*" putStrLn (show l) mapM_ putStrLn l mapM_ (putStrLn . show . length) l runProcessWithInput cmd args input = do (pin, pout, perr, ph) <- runInteractiveProcess cmd args Nothing Nothing hPutStr pin input hClose pin output <- hGetContents pout when (output==output) $ return () hClose pout hClose perr waitForProcess ph return output

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MonadGL - Partitioning effects without giving up type inference
by Jules Bean 14 Sep '07

14 Sep '07

The OpenGL bindings which come bundled with ghc are a really great example of how even an "almost-literal" port of a C API can still be easier to work with in haskell than it is in C, because of the benefits of type inference and powerful abstractions. Even the ability to mapM_ is a tool to make C programmers envious, and there are useful combinators like preservingMatrix to guarantee pairing of pushes and pops. Because the bindings are ported over using the FFI, all the GL calls are in the IO monad. GL is build around state machines, so it's not at all surprising to end up in some kind of state monad. However, we find that the type system is not powerful enough to distinguish between myActionWhichOnlyMakesGLCalls :: IO () and myActionWhichMixesGLAndIO :: IO () It would be much nicer if the type-system could distinguish the two. One case in point is that an arbitrary IO action can modify IORefs, and it would be nice to have actions whose type guaranteed that they didn't do that. It's fairly simple to imagine something like the following: > {-# OPTIONS -fglasgow-exts #-} (extensions are only for deriving (Monad), it's not important) > > newtype GL a = GL { runGL :: IO a } deriving (Monad) > > unsafeIOToGL :: IO a -> GL a > unsafeIOToGL = GL The intention here of course is that we export 'runGL' which is safe, having type GL a -> IO a, but don't export unsafeIOToGL. Then we have lots of functions which are imported via the FFI and end up with IO types, here is a trival example: > _foo :: IO () > _foo = putStrLn "OpenGL!" And we embed them into the GL monad. No other module can corrupt our GL monad because we don't export unsafeIOToGL. > foo :: GL () > foo = unsafeIOToGL _foo As far as it goes, this technique is absolutely fine. We end up being able to write actions entirely in the GL monad: *Main> :t do { foo ; foo ; foo } do { foo ; foo ; foo } :: GL () ...as well as actions which mix general IO and GL calls : *Main> :t do { runGL foo ; putStrLn "Not a GL call" ; runGL foo } do { runGL foo ; putStrLn "Not a GL call" ; runGL foo } :: IO () The point of this message is actually to get rid of those annoying 'runGL' calls. When writing an IO action I want to be able to freely intermix IO and GL calls. When writing a GL-only action, I want to only use GL calls. And I want the type system to enforce that; and ideally, infer it too. So we define a type-class for "monad which can perform GL" : > class Monad m => MonadGL m where > runMonadGL :: m a -> IO a > embedGL :: GL a -> m a And we write an instance for IO: > instance MonadGL IO where > runMonadGL = id > embedGL = runGL Now we are able to bind our FFI call _foo slightly differently: > foo' :: MonadGL m => m () > foo' = embedGL foo This is interesting because, although we know that IO is in fact the only instance of MonadGL, there might in principle be others. (For example, GL is itself an instance of MonadGL if you put runMonadGL = runGL and embedGL = id). The type signature for foo' guarantees that it will run in *any* MonadGL, and therefore can't use any IO-specific effects, only the GL ones. Now we get the automatic type inference we want: *Main> :t do { foo' ; foo' ; foo' } do { foo' ; foo' ; foo' } :: (MonadGL t) => t () This only performs GL actions, no IO. *Main> :t do { foo' ; putStrLn "Normal" ; foo' } do { foo' ; putStrLn "Normal" ; foo' } :: IO () The single IO call here forces the type to IO, but we are not required to put noisy 'runGL's in front of every GL call. This technique is quite scalable in that you can have any number of MonadFoos representing different librarys with different kinds of state, and (as long as you don't mind the modest blow-up in type signature size) you get, for an arbitrary action, a type signature which pins down precisely what kinds of side-effect the action can have. It would, however, be a real pain to run through all the 'foreign' calls in the rather large GL library and add appropriate wrappers of the form 'embedGL . unsafeIOToGL'. Definitely a job for an automated tool. Incidentally, I don't believe this technique has any performance implication at all. The newtypes are all erased at compile time. Any comments? I'm sure this has been shown before but I don't remember where. Jules

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Vital for real Haskell?
by Peter Verswyvelen 14 Sep '07

14 Sep '07

I showed Vital (http://www.cs.kent.ac.uk/projects/vital) to some teachers at my university and they where really enthousiastic. IMHO the Haskell community needs something like this, but for *real* Haskell (preferable with extensions), and not using Java... Is any work being done on something like this? Thanks, Peter

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