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Online judge and Haskell?
by Steffen Mazanek 28 Apr '03

28 Apr '03

Hello, it's me again and I have a new mad idea. There are a lot of programming contests out there and unfortunately most (all?) of them do not allow Haskell code till now! I think, this is hard to accept. Why not ask the people which run acm.uva.es/problemset to provide their problem sets, input and output files and the basic judging software and install it at haskell.org? Alternatively this may be a nice project for a student. I think an adaption of this software to permit Haskell code can not be that difficult! Do you think there is a need for such a project? Maybe it could help Haskell to become a part of official programming contests and hence to gain more popularity. I'm sorry to say, that due to another project my free time is very limited so I can not get it going by only myself. Ciao, Steffen -- Haskell: that's where I just curry until fail, unwords any error, drop all undefined, maybe break, otherwise in sequence span isControl and take max $, id: (d:[])

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mmap and IArray slices
by David Roundy 28 Apr '03

28 Apr '03

I had a couple ideas, and I was wondering if they were good ones. One is the idea of a slice function on an IArray: a' = slice a (i,j) giving an array with the elements i..j of array a. The nifty idea would be that this could be done without any memory copy. The catch, of course, would be that you might end up having to retain the entire array a just because you need a little slice of it. However, I think this risk (which should be documented) would be overwhelmed by the increased efficiency (both in space and time) gains that would be possible. The most obvious (to me at least, since I've been working a lot with PackedStrings recently) application would be in substrPS. Using slice, this would be an O(1) operation, rather than an O(n) operation, and would use essentially no memory (at the cost of retaining the mother string). I'm thinking that it may even be possible to make the GC smart enough to get rid of mother strings which are needed by very few children (at least if it's desperate), if array slices were supported at a low enough level... I imagine they'd have to be supported at a very low level. The second (and related) idea is that it would be nice to be able to use mmap on a readonly file to read it into an immutable array. Of course, this works really nicely with array slices, since with the slices the mmapped array wouldn't need to be copied, and could remain backed by the file itself. I imagine being able to read a 100M file into a PackedString, then run linesPS (or possibly a special slice version of linesPS) on it, and have a [PackedString], all of which is backed by the file, so when memory gets tight it can be just thrown away (by the VM system) and then get read in again when I use it. The mmap idea seems even tougher than the slice idea, as it would require (preferably) keeping track of all mmapped regions so when the file is closed they could all be copied over into normal memory and unmapped. Technically they could remain mmapped after the file is closed, but I think that would cause problems if the program then wants to reopen the file for writing. Anyhow, those are just a couple of ideas I had, and I have nowhere near the knowledge if haskell internals to tell if they are workable or not. -- David Roundy http://www.abridgegame.org

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RE: Is this a bug in multithreading code?
by Simon Marlow 28 Apr '03

28 Apr '03

> Nick Name <nick.name(a)inwind.it> > > You are right, [...] > > I further conjecture that more than half of all space leaks can be > identified not by profiling your code but by running the following > command: > > grep '+1' *.hs *.lhs > > I'm half joking but I've found that this one is very common in certain > kinds of code. Good idea! Here's another space-leak-avoiding trick: whenever you write Int as a constructor field, consider writing !Int instead. Same goes for other basic types and tuples. This pre-emptive space leak avoidance helps us in GHC quite a bit. Cheers, Simon

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Arkadasa yardimci olalim
by Dr. Ahmet Yalcin 27 Apr '03

27 Apr '03

Arkadaslar, lütfen dikkate alin, yardimci olalim: Merhaba, cok zor durumda ve baska caresi kalmamis biri olarak hosgörünüze ve insan sevginize siginarak sizden bana cok kisada olsa kulak vermenizi rica edecegim. Ben, 34 yasinda evli ve 4 yasinda olan bir cocuk babasiyim. Aile hayatimizin sorunsuz ve mutlu sekilde devam ederken biricik oglumuz aniden rahatsizlandi. Uzun süren arastirmalar sonucu oglumuz Ali Can'in kalbinde hayati bir fonksiyon bozuklugunun bulundugu doktorlar tarafindan tespit edildi. Oglumuz cok acil sekilde ameliyat olmazsa maalesef hayata gözlerini yumacak. Ancak ne varki ameliyat icin gerekli olan parayi ödememiz bizim icin mümkün gözükmüyor. Bu nedenle uzun ugraslar sonucu iki önemli Internet sirketi ile yaptigimiz görüsme sonucu, bize her mailde gönderecegimiz link'in tiklanmasi durumunda cocugumuzun ameliyat masraflarina 10 Cent (= 0,10 Euro) katkida bulunacaklari yönünde anlasmaya varabildik. Bu dogrultuda sizden rica edecegimiz, sizi sözkonusu sirketlerin sayfalarina yönelten bu linki tiklamaniz. Bu linki tiklayarak sizinde ameliyat masraflarini karsilamamizda katkiniz olabilir. Sizden bu zor durumumuzda istedigimiz sadece bir tik. >> TIKLAYINIZ --> http://partners.webmasterplan.com/click.asp?ref=160073&site=2514&type=text&… Bunun yaninda bu emaili tüm tanidiklariniza gönderirseniz bize son derece yardimci olursunuz. Ne kadar kisi linki tiklarsa, gerekli olan ameliyat masrafini yakalayabilmemiz o kadar olasi olur. Hos görünüz icin tesekkür eder, saygilarimi sunarim. Mehmet Can Yakupoglu.

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Sell:Industrial Permanent Magnets
by L.Z.Mou 26 Apr '03

26 Apr '03

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Re: Is this a bug in multithreading code?
by Nick Name 25 Apr '03

25 Apr '03

Il 22:54, mercoledì 23 aprile 2003, hai scritto: > I conjecture that the problem would go away if you added appropriate > uses of seq or $! to your program so that the value stored in the MVar > was always fully evaluated You are right, the following code works well strictIncM x = return $! x + 1 ... forkIO (loopM (modifyMVar_ v strictIncM)) ... Thanks for your help Vincenzo

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RE: Super Haddock (was Re: Literate Programming)
by Simon Marlow 25 Apr '03

25 Apr '03

Ketil Z. Malde writes: > > "Simon Marlow" <simonmar(a)microsoft.com> writes: > > > Somebody else¹ wrote: > > >> This seems like a good way of transitioning from .lhs to > >> haddock'd .hs. In the long run, I think we (meaning Simon :-) > >> should to extend haddock to take the place of the .lhs style of > >> documenting code. > > > That's an interesting idea. It's not at all what Haddock > was intended > > for, but that's not to say it couldn't be done! > > I'm not sure I would like this. I guess I'm one of (the apparently > very few?) who are using LaTeX lhs style (using \begin/\end{code}). > Would a Haddock replacement give me the same kind of functionality in > producing a nice printable copy? I definitely have grown attached to > having math, footnotes, page headings, sections, and so on. Haddock isn't (intended to be) a literate programming system, so I don't imagine it will replace full LaTeX literate source any time soon. The suggestion is just that it could produce nicely-formatted source code complete with lightly marked-up comments - which I think is a fine idea, and not too difficult to implement using the existing framework. > ¹) As Exchange doesn't leave useful thread information, I'm not quite > sure who. Simon and others, if you must use Exchange, could you take > care to leave an attribution, prefereably including the message-id? I can get proper In-Reply-To headers by avoiding Exchange altogether, but it requires me to select an option each time I send a message, so I often forget (sorry!). I've reported it as a bug in Exchange, but it might be a generation or two before it gets fixed. Cheers, Simon

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Slow program
by Mark Carroll 25 Apr '03

25 Apr '03

One of my GHC programs is running much more slowly than I'd expect, and I wonder if laziness is getting in the way or if it's getting into an infinite loop. How should I diagnose the problem? Tonight I've started with Hat - it seems to install okay, but doesn't seem to have a Control/Monad/Identity.hx file that I need. ): -- Mark

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Haskell Help
by Gyslain Giguère 25 Apr '03

25 Apr '03

Hi! I'm a computer science student whose last assignment (due next wednesday) of the semester is to present and explain a Haskell 98 program I found on the internet. Unfortunately, I can't find anything written in Haskell 98 that can be explained in about 20 minutes, and doesn't use any graphic libraries (since we haven't studied them). So if you have any examples (course exercises, handouts), I would be very grateful! Thanks to all! Gyslain Giguère, Student, Cognitive Computer Science, Université du Québec à Montréal giguere.gyslain(a)courrier.uqam.ca

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Re: Extracting values from several data constructors
by Dean Herington 24 Apr '03

24 Apr '03

[redirected to haskell-cafe] On 24 Apr 2003, Steffen Mazanek wrote: > Hello. > I am sure this question is not new. Even so, please > give me the solution or point me to an old thread. > > How can I implement something like: > > data Test = T1 Int | T2 Int > test::Test->Int > test (_ x) = x > > Or is this impossible? In case of yes, why? > This would save a lot of case differentiations in > my application. Or is there an appropriate compiler option? It can be done, but only for labeled fields. For example: data Test = T1 { test :: Int } | T2 { test :: Int } See section 3.15 of the Haskell report. Dean

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Re: Writing for both State and StateT
by Jon Cast 24 Apr '03

24 Apr '03

Derek Elkins <ddarius(a)hotpop.com> wrote : <snip> > Yes, I've tried it. I didn't mean GHCi doesn't EVER validate the > constraints on input, just that it doesn't when you use :t. So do you mean to use :t & cut & paste the definition? Interesting. A fair bit of work (more than just adding a parameter () to the definition), but I guess it'd work. <snip> Jon Cast

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Is this a bug in multithreading code?
by Nick Name 24 Apr '03

24 Apr '03

Hi all, does someone know why this program works ------------------------ import Concurrent loopM = sequence_ . repeat main = do v <- newMVar 0 forkIO (loopM (modifyMVar_ v (return . (1+)))) -- forkIO (loopM (modifyMVar_ v (return . (1+)))) loopM (readMVar v >>= print) ------------------------- but uncommenting the commented line makes the program fail with "stack overflow" ? Thanks for help Vincenzo

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

Mark Carroll <mark(a)chaos.x-philes.com> wrote: > On Tue, 22 Apr 2003, Jon Cast wrote: > > When you have two identical methods like this, you can try removing > > the type from one and seeing what hugs or ghci thinks the type is. > > Then, you can specialize that if you want. > Unfortunately, in this case, with that function in isolation I get, > Ambiguous type variable(s) `s', `m' > in the constraint `MonadState s m' > arising from use of `get' .... Ah, yes the good old monomorphism restriction. Should have thought about that. The way around this is to make the declaration a function: > next_random_probability () = > do old_rng <- get > let (random_probability, new_rng) = randomR (0, 1) old_rng > put new_rng > return random_probability This will typecheck, giving most general type (MonadState s m, Num a, Random a, RandomGen s) => () -> m a, according to ghci. Then, you can take off the argument type to get the resulting type. <snip> Jon Cast

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

Derek Elkins <ddarius(a)hotpop.com> wrote: > On Tue, 22 Apr 2003 23:05:09 +0100 > Alastair Reid <alastair(a)reid-consulting-uk.ltd.uk> wrote: > > > > > > Unfortunately, in this case, with that function in isolation I get, > > > > > Ambiguous type variable(s) `s', `m' in the constraint > > > `MonadState s m' arising from use of `get' .... > > > > An easy way to get round this is to give a completely bogus type > > signature like: > > > > foo :: a > > > > The compiler will report a mismatch and the error message will tell > > you what it thinks the type is. > > Or the straightforward and simple answer: in GHCi or Hugs, just do :t > f. E.g. :t get ==> (MonadState s m) => m s. GHCi (and I imagine Hugs > too) doesn't try to validate the constraints. Have you tried this? GHCi does indeed typecheck the input (``validate the constraints'' as you put it). It has to, to be a Haskell 98-conforming interpreter. Jon Cast

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

> On Tue, 22 Apr 2003, Derek Elkins wrote: > > I had a similar problem, but that was because I needed to use the > > constructors. MonadState has the methods you are using just write > > your code requiring MonadState as both State and StateT are > > instances. > > next_random_probability :: ( ..., MonadState g m) => m a > (snip) > Works perfectly, thanks very much! That must be the only combination I > didn't try. (-: When you have two identical methods like this, you can try removing the type from one and seeing what hugs or ghci thinks the type is. Then, you can specialize that if you want. > No doubt it'll all be clearer when I better understand what's going on! Jon Cast

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Duda
by Daniela 23 Apr '03

23 Apr '03

Estamos escribiendo un programa en el cual debemos definir la estructura de expresiones regulares y no sabemos como hacerlo en este lenguaje. Alguna idea? Gracias.

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Re: Writing for both State and StateT
by Derek Elkins 22 Apr '03

22 Apr '03

I had a similar problem, but that was because I needed to use the constructors. MonadState has the methods you are using just write your code requiring MonadState as both State and StateT are instances. next_random_probability :: ( ..., MonadState g m) => m a >From: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com> >To: haskell-cafe(a)haskell.org >Subject: Writing for both State and StateT >Date: Tue, 22 Apr 2003 07:48:57 -0400 (EDT) > >At the moment I have code like, > > >next_random_probability :: (RandomGen g, Num a, Random a) => State g a > >next_random_probability = > do old_rng <- get > let (random_probability, new_rng) = randomR (0, 1) old_rng > put new_rng > return random_probability > >next_random_probabilityT :: (RandomGen g, Num a, Random a, Monad m) => >StateT g m a > >next_random_probabilityT = > do old_rng <- get > let (random_probability, new_rng) = randomR (0, 1) old_rng > put new_rng > return random_probability > > >Because these just use get and put which are both from MonadState, is it >possible to write just one next_random_probability with a more general >type signature, maybe mentioning MonadState, or at least two wrappers that >call the same worker function, so that the same code is good for both >State and StateT? > >-- Mark > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963

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Writing for both State and StateT
by Mark T.B. Carroll 22 Apr '03

22 Apr '03

At the moment I have code like, next_random_probability :: (RandomGen g, Num a, Random a) => State g a next_random_probability = do old_rng <- get let (random_probability, new_rng) = randomR (0, 1) old_rng put new_rng return random_probability next_random_probabilityT :: (RandomGen g, Num a, Random a, Monad m) => StateT g m a next_random_probabilityT = do old_rng <- get let (random_probability, new_rng) = randomR (0, 1) old_rng put new_rng return random_probability Because these just use get and put which are both from MonadState, is it possible to write just one next_random_probability with a more general type signature, maybe mentioning MonadState, or at least two wrappers that call the same worker function, so that the same code is good for both State and StateT? -- Mark

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GHC XmlParse
by Christof Lehmann 22 Apr '03

22 Apr '03

Hello, I have a small program with a 'import XmlParse' - statement. During compiling with ghc --make main.hs I get following error message ghc-5.02.2: chasing modules from: main.hs ghc-5.02.2: can't find module `XmlTypes' Ghc finds modules like IO and System. Which option do I need, that ghc finds the Xml... - modules. thanks -- Christof Lehmann VUB Printmedia GmbH Berlin Bonn Dresden Köln Leipzig Chopinstr. 4 TEL: +49 341 960 50 93 04103 Leipzig FAX: +49 341 960 50 92

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splatterhunks
by William Lee Irwin III 22 Apr '03

22 Apr '03

-- This program takes patch filenames as arguments and splits the patch -- files up on a hunk-by-hunk basis. I have found this useful for Linux -- kernel development, and perhaps others might find it useful in -- similar contexts where patches are the primary method of communication. module Main where import System import List main = getArgs >>= mapM_ splatterHunkFile splatterHunkFile file = mapM_ (\(f,ss) -> writeFile f (unlines ss)) . zip (map (((reverse . takeWhile (/= '/') . reverse $ file)++).("-"++).show) [1..]) . splatterHunks =<< readFile file splatterHunks ss = map (map unlines) $ zipWith (\s -> map (s:)) (map head . (splitList "diff") $ lines ss) (map (splitList "@@") . (splitList "diff") $ lines ss) splitList prefix ss = map ($ss) $ zipWith (\m n -> (if n >= 0 then take (n - m) else id) . drop m) (map fst . filter (\(n,s) -> isPrefixOf prefix s) $ zip [0..] ss) (tail (map fst . filter (\(n,s) -> isPrefixOf prefix s) $ zip [0..] ss) ++ [-1])

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idea: chroot :: FilePath -> IO a -> IO a
by David Roundy 21 Apr '03

21 Apr '03

I have the idea that we ought to be able to implement the equivalent of a unix chroot command, but all within the haskell IO monad. So the idea would be that if I want to do some operation that only requires access to, say /var/cache/test, but involves user-provided input which I don't trust (and if I don't want to trust the security of my parsing of the possibly malicious input), I could run: chroot "/var/cache/test" $ complicated_function "/" scary_user_input which would be the equivalent of complicated_function "/var/cache/test" scary_user_input except that every file and directory access (read or write or stat or whatever) would be forced to be in a subdirectory of /var/cache/test. Moreover, calls such as system would not be allowed (that is, they'd throw an isChrootError exception), since such calls of course could not be verified to touch no file outside of /var/cache/test (at least not using only haskell). Actually, there are two ways this could be done. One is the one mentioned above, which is roughly equivalent to a unix chroot. Another (now that I think about it, probably more elegant) would be to implement a restrict_access type of function, which wouldn't change the meaning of any file paths as a chroot does, but would simply cause any access outside of the specified directory to fail. This would have the advantage of being transparent to the writer of complicated_function--and might be extended to allow access to more than one directory (e.g. for a webserver of only static pages maybe /var/www and /etc/myserver/). As I see it, implementing such a function would require support in the IO monad (some of which may already be there?), since it would need to know for each primitive operation which arguments are file paths, plus whether that operation is even legal in a chroot environment. Or perhaps it's the other way around, and every primitive operation would need to support chroot... I know I would find such a function exceedingly comforting, since I wonder from time to time what would happen if users stick weird filenames in the wrong places, or make symlinks to things they should not have access to. And the cool thing is that with haskell's monadic IO, I think it could be done in a very elegant manner, and in such a way that it is natural (and simple) to wrap a chroot around individual operations, giving finer grained control than I can imagine doing any other way. Any ideas whether such a feat would be possible, and if so how hard it would be? -- David Roundy http://www.abridgegame.org

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Re: HI
by bullins39434＠aol.com 18 Apr '03

18 Apr '03

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Re: Parsec question
by Sarat Chandra 17 Apr '03

17 Apr '03

Thanks, "uust" was the missing link. --- Doaitse Swierstra <doaitse(a)cs.uu.nl> wrote: >It is in: > >http://cvs.cs.uu.nl/cgi-bin/cvsweb.cgi/uust/lib/ > >As far as I can see you should be able to check the whole uust tree out, > > Doaitse > > >On donderdag, apr 17, 2003, at 17:06 Europe/Amsterdam, Sarat Chandra >wrote: > >> Doaitse, >> >> I can't find the "Parsing" directory. May be the directory is not >> made visible to public. However, under the "PC" directory, I see >> UU_Parsing.hs and other files (but no examples directory). Can you >> please check it. >> >> Thanks. >> >> --- Doaitse Swierstra <doaitse(a)cs.uu.nl> wrote: >>> Parsec cannot do this. If you want this behaviour check out the >>> Parsing >>> library at cvs.cs.uu.nl (directory lib). In the examples directory you >>> find an example parser for Haskell written using these combinators. At >>> the top level there is also a Tiger compiler that is written with >>> these >>> combinators, >>> >>> Doaitse Swierstra >>> >>> >>> On woensdag, apr 16, 2003, at 12:00 Europe/Amsterdam, Sarat Chandra >>> wrote: >>> >>>> I am using Parsec to write a parser. I would like to know >>>> how parse-errors can be accumulated without stopping at the >>>> first error. I would like to skip the input upto some >>>> synchronization point (for e.g a semi-colon) and continue parsing >>>> from there. Thanks. >>>> >>>> _____________________________________________________________ >>>> Get your own FREE e-mail account at http://www.volcanomail.com >>>> >>>> _____________________________________________________________ >>>> Select your own custom email address for FREE! Get you(a)yourchoice.com >>>> w/No Ads, 6MB, POP & more! >>>> http://www.everyone.net/selectmail?campaign=tag >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> _____________________________________________________________ >> Get your own FREE e-mail account at http://www.volcanomail.com >> >> _____________________________________________________________ >> Select your own custom email address for FREE! Get you(a)yourchoice.com >> w/No Ads, 6MB, POP & more! >> http://www.everyone.net/selectmail?campaign=tag >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe _____________________________________________________________ Get your own FREE e-mail account at http://www.volcanomail.com _____________________________________________________________ Select your own custom email address for FREE! Get you(a)yourchoice.com w/No Ads, 6MB, POP & more! http://www.everyone.net/selectmail?campaign=tag

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Re: Parsec question
by Sarat Chandra 17 Apr '03

17 Apr '03

Doaitse, I can't find the "Parsing" directory. May be the directory is not made visible to public. However, under the "PC" directory, I see UU_Parsing.hs and other files (but no examples directory). Can you please check it. Thanks. --- Doaitse Swierstra <doaitse(a)cs.uu.nl> wrote: >Parsec cannot do this. If you want this behaviour check out the Parsing >library at cvs.cs.uu.nl (directory lib). In the examples directory you >find an example parser for Haskell written using these combinators. At >the top level there is also a Tiger compiler that is written with these >combinators, > > Doaitse Swierstra > > >On woensdag, apr 16, 2003, at 12:00 Europe/Amsterdam, Sarat Chandra >wrote: > >> I am using Parsec to write a parser. I would like to know >> how parse-errors can be accumulated without stopping at the >> first error. I would like to skip the input upto some >> synchronization point (for e.g a semi-colon) and continue parsing >> from there. Thanks. >> >> _____________________________________________________________ >> Get your own FREE e-mail account at http://www.volcanomail.com >> >> _____________________________________________________________ >> Select your own custom email address for FREE! Get you(a)yourchoice.com >> w/No Ads, 6MB, POP & more! >> http://www.everyone.net/selectmail?campaign=tag >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe _____________________________________________________________ Get your own FREE e-mail account at http://www.volcanomail.com _____________________________________________________________ Select your own custom email address for FREE! Get you(a)yourchoice.com w/No Ads, 6MB, POP & more! http://www.everyone.net/selectmail?campaign=tag

2 1

Parsec question
by Sarat Chandra 16 Apr '03

16 Apr '03

I am using Parsec to write a parser. I would like to know how parse-errors can be accumulated without stopping at the first error. I would like to skip the input upto some synchronization point (for e.g a semi-colon) and continue parsing from there. Thanks. _____________________________________________________________ Get your own FREE e-mail account at http://www.volcanomail.com _____________________________________________________________ Select your own custom email address for FREE! Get you(a)yourchoice.com w/No Ads, 6MB, POP & more! http://www.everyone.net/selectmail?campaign=tag

2 1

Say�n haskell-cafe ,Baskuda.Com`da Bebek / Oyuncak Kategorisi A��ld�!
by Baskuda-4233 16 Apr '03

16 Apr '03

1 0

RE: [OT[ proving programs for novices
by David Bakin 16 Apr '03

16 Apr '03

How would this book relate - in subject, difficulty, audience - to: The Science of Programming by Gries? Algebra of Programming by Bird, De Moor, Moor? Thanks! - Dave -----Original Message----- From: Doaitse Swierstra [mailto:doaitse@cs.uu.nl] Sent: Tuesday, April 15, 2003 12:38 PM To: Bill Wood Cc: Doaitse Swierstra; haskell-cafe(a)haskell.org Subject: Re: [OT[ proving programs for novices One might be interested in the newly published book by Roland Backhouse: "Program Construction : Calculating Implementations from Specifications" http://www.wiley.com/remtitle.cgi?0470848820 I quote from the publishers web site: "The ever-increasing dependence of our lives and livelihoods on the correct functioning of computer software means that logic and program correctness are core elements of all good computer science degrees. This book presents both these topics in one self-contained text. The focus of the book is on "correct-by-construction" program design -- the discipline of calculating programs from their specifications. Modern, calculational logic is introduced in combination with key program construction principles, such as the assignment axiom, loop invariants and bound functions. This material is intertwined with motivational discussion, programming examples and challenging problem-solving exercises, bringing the book alive for its intended audience, undergraduates in computer science and mathematics, as well as professional programmers wishing to further develop their programming skills. The book covers the elements of logic and program correctness that form the foundations of further study --- the logical connectives and their algebraic properties, induction, quantifiers and program construction rules. Substantial examples of program construction are included. Many exercises are provided, all with detailed solutions. " Doaitse Swierstra _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

[OT[ proving programs for novices
by Chris Moline 15 Apr '03

15 Apr '03

hi. this isnt a haskell question but i am hoping you will forgive it. i was wondering where i could find books/websites/tutorials on how to construct proofs and how to prove programs. preferably books that are aimed at novices with some programming experience but little math/logic experience. i have paul hudaks hsoe book which has some material on proving properties of programs. sincerely chris moline

4 3

wiki problems
by Isaac Jones 14 Apr '03

14 Apr '03

Greetings, Not sure who to send this to: I noticed on the wiki that the HaskellNewbies page was blank for some reason, and when I attempted to put something there, I got the below error. I tried this over the course of two days just to make sure there wasn't actually an editing conflict. "Editing Conflict Another author has modified this page since you checked it out. So that you won't interfere with the other author's work, you need to reload, re-edit, and submit again. Sorry!" peace, isaac

3 3

linesPS is not analagous to lines
by David Roundy 14 Apr '03

14 Apr '03

I've just switched my code to use PackedStrings rather than Strings for file contents, and ran into what appears to be a bug in the Data.PackedString module in ghc 5.04.3. unlines is the inverse of lines, since lines converts empty lines into empty strings. linesPS, however, ignores empty lines. :( It seems like it would be a good idea to either fix this (which might not be a good idea, if existing code depends on its current behavior), or add a note in the documentation that it isn't analagous to lines, so that other people won't make the same mistake based on its similar name. My example program that demonstrates this: import Data.PackedString main = do temp <- return $ packString "hello\nworld\n\nI\njust\nskipped\n\n a line\n" putStr $ "The unlines of linesPSed version:\n\n" putStr $ unlines $ map unpackPS $ linesPS temp putStr $ "The unlines of linesed version:\n\n" putStr $ unlines $ lines $ unpackPS temp -- David Roundy http://www.abridgegame.org

2 1

(no subject)
by Eileen Head 13 Apr '03

13 Apr '03

I have two version of factorial as follows: > fac = 1.0: zipWith (*) [1.0 ..] fac > fact = 1.0: [ f | f <- zipWith (*) [1.. ] fact ] Could some explain why does facBad run out of stack memory while fact work fine? i.e. ... > take 5 fact [1.0,1.0,2.0,6.0,24.0] ...> take 5 facBad [1.0 ERROR - Control stack overflow Thanks

2 1

Re: Combining Monads
by Dominic Steinitz 13 Apr '03

13 Apr '03

Your program doesn't seem to have survived the transmission but try http://www.cse.ogi.edu/~mpj/pubs/springschool.html or http://www.cse.ogi.edu/~mpj/pubs/composing.html. For the theory, look at Triples, Toposes and Theories by Barr & Wells. I think it's chapter 7 but I haven't got it in front of me. Dominic Steinitz ----- Original Message ----- From: <haskell-cafe-request(a)haskell.org> To: <haskell-cafe(a)haskell.org> Sent: Saturday, April 12, 2003 5:01 PM Subject: Haskell-Cafe digest, Vol 1 #660 - 3 msgs > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-admin(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Combining monads (Mark T.B. Carroll) > 2. Re: Combining monads (Scott Turner) > 3. Re: Combining monads (Glynn Clements) > > --__--__-- > > Message: 1 > Date: Sat, 12 Apr 2003 09:52:58 -0400 (EDT) > From: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com> > To: haskell-cafe(a)haskell.org > Subject: Combining monads > > This message is in MIME format. The first part should be readable text, > while the remaining parts are likely unreadable without MIME-aware tools. > Send mail to mime(a)docserver.cac.washington.edu for more info. > > --481034957-302199926-1050155578=:555 > Content-Type: TEXT/PLAIN; charset=US-ASCII > > Hello! I am trying and failing to use one monad in another. A tiny example > is enclosed, where I try to use IO to help along a State-ful computation. > Is the real mistake in do_next? Is there a minimal rewrite that will get > this working that I can use as a template for other things? Or, is there a > good tutorial somewhere on this type of thing? Any pointers are much > appreciated! > > -- Mark > > --481034957-302199926-1050155578=:555 > Content-Type: TEXT/PLAIN; charset=US-ASCII; name="Test.hs" > Content-Transfer-Encoding: BASE64 > Content-ID: <Pine.LNX.4.44.0304120952580.555(a)cicero.aetion.internal> > Content-Description: Example of incompetence > Content-Disposition: attachment; filename="Test.hs" > > aW1wb3J0IENvbnRyb2wuTW9uYWQuU3RhdGUNCmltcG9ydCBEYXRhLkxpc3QN > Cg0KZ2V0X2xlbmd0aCA6OiBTdHJpbmcgLT4gSU8gSW50DQoNCmdldF9sZW5n > dGggbmFtZSA9IA0KICAgIGRvIGNvbnRlbnRzIDwtIHJlYWRGaWxlIG5hbWUN > CiAgICAgICByZXR1cm4gKGxlbmd0aCAobGluZXMgY29udGVudHMpKQ0KDQp1 > cGRhdGVfYmVzdF93aXRoIDo6IChTdHJpbmcsIEludCkgLT4gU3RhdGVUIChT > dHJpbmcsIEludCkgSU8gKCkNCg0KdXBkYXRlX2Jlc3Rfd2l0aCAoY3VycmVu > dF9uYW1lLCBjdXJyZW50X2NvdW50KSA9DQogICAgZG8gKGJlc3RfbmFtZSwg > YmVzdF9jb3VudCkgPC0gZ2V0DQogICAgICAgaWYgY3VycmVudF9jb3VudCA+ > IGJlc3RfY291bnQgDQoJICB0aGVuIHB1dCAoY3VycmVudF9uYW1lLCBjdXJy > ZW50X2NvdW50KQ0KCSAgZWxzZSByZXR1cm4gKCkNCg0KZG9fbmV4dCA6OiBT > dHJpbmcgLT4gU3RhdGVUIChTdHJpbmcsIEludCkgSU8gKCkNCg0KZG9fbmV4 > dCBuYW1lID0gDQogICAgZG8gY291bnQgPC0gZ2V0X2xlbmd0aCBuYW1lDQog > ICAgICAgdXBkYXRlX2Jlc3Rfd2l0aCAobmFtZSwgY291bnQpDQoNCmxvbmdl > c3RfZmlsZSA6OiBbU3RyaW5nXSAtPiBJTyAoU3RyaW5nLCBJbnQpDQoNCmxv > bmdlc3RfZmlsZSBuYW1lcyA9DQogICAgZXhlY1N0YXRlVCAoc2VxdWVuY2Vf > IChtYXAgZG9fbmV4dCBuYW1lcykpICgibm9uZSIsIDApDQoNCm1haW4gPSBk > byAobmFtZSwgY291bnQpIDwtIGxvbmdlc3RfZmlsZSBbIi9ldGMvaG9zdG5h > bWUiLCAiL2V0Yy9udHAuY29uZiIsICIvZXRjL3RpbWV6b25lIl0NCgkgIHB1 > dFN0ckxuICgiTG9uZ2VzdCBmaWxlIGlzICIgKysgbmFtZSArKyAiIHdoaWNo > IGlzICIgKysgc2hvdyBjb3VudCArKyAiIGxpbmVzIGxvbmciKQ0K > --481034957-302199926-1050155578=:555-- > > --__--__-- > > Message: 2 > From: Scott Turner <p.turner(a)computer.org> > To: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com>, haskell-cafe(a)haskell.org > Subject: Re: Combining monads > Date: Sat, 12 Apr 2003 09:14:45 -0500 > > On Saturday 12 April 2003 09:52, Mark T.B. Carroll wrote: > > Hello! I am trying and failing to use one monad in another. > > Is there a minimal rewrite that will get this working ... ? > Yes. Just add lift. > do count <- lift $ get_length name > > > --__--__-- > > Message: 3 > From: Glynn Clements <glynn.clements(a)virgin.net> > Date: Sat, 12 Apr 2003 15:26:03 +0100 > To: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com> > Cc: haskell-cafe(a)haskell.org > Subject: Re: Combining monads > > > Mark T.B. Carroll wrote: > > > Hello! I am trying and failing to use one monad in another. A tiny example > > is enclosed, where I try to use IO to help along a State-ful computation. > > Is the real mistake in do_next? Is there a minimal rewrite that will get > > this working that I can use as a template for other things? Or, is there a > > good tutorial somewhere on this type of thing? Any pointers are much > > appreciated! > > Combining monads isn't straightforward. There is a paper on the topic, > at: > > http://cm.bell-labs.com/cm/cs/who/wadler/topics/monads.html#combining-monads > > However, you can't really combine the IO monad in the way that the > above paper describes. > > The most direct way to fix your program is to use an IORef instead of > a state monad, e.g. > > > import IORef > > > update_best_with :: IORef (String, Int) -> (String, Int) -> IO () > > update_best_with ref (current_name, current_count) = do > > (best_name, best_count) <- readIORef ref > > when (current_count > best_count) $ do > > writeIORef ref (current_name, current_count) > > > do_next :: IORef (String, Int) -> String -> IO () > > do_next ref name = do > > count <- get_length name > > update_best_with ref (name, count) > > > longest_file :: [String] -> IO (String, Int) > > longest_file names = do > > ref <- newIORef ("none", 0) > > mapM_ (do_next ref) names > > readIORef ref > > A more elegant solution would be to generate a list of filename/length > pairs (e.g. using mapM), then just use maximumBy to select the > longest, e.g. > > > get_length' :: String -> IO (String, Int) > > get_length' name = do > > length <- get_length name > > return (name, length) > > > longest_file :: [String] -> IO (String, Int) > > longest_file names = do > > pairs <- mapM get_length' names > > return $ maximumBy cmp pairs > > where cmp (_, l1) (_, l2) = compare l1 l2 > > -- > Glynn Clements <glynn.clements(a)virgin.net> > > > --__--__-- > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > End of Haskell-Cafe Digest

2 1

Combining monads
by Mark T.B. Carroll 12 Apr '03

12 Apr '03

Hello! I am trying and failing to use one monad in another. A tiny example is enclosed, where I try to use IO to help along a State-ful computation. Is the real mistake in do_next? Is there a minimal rewrite that will get this working that I can use as a template for other things? Or, is there a good tutorial somewhere on this type of thing? Any pointers are much appreciated! -- Mark

4 3

writing compiler in haskell
by Debanjan Chatterjee 11 Apr '03

11 Apr '03

Hi I am doing my Masters in Computing. The master's course is a conversion course for non-computing graduates. And, the only language taught in the course is Java. My undergraduate degree is in accounting. I would like to ask whether with my background I can learn Haskell and as well write a compiler for C-- in Haskell in three months for my dissertation. Is this a feasible project within the time frame? I would also like to mention that I have never written a compiler before. I would grateful if anyone can advise me on this matter. Sincerely Yours Debanjan

3 2

Re: Threading monads
by Jon Cast 11 Apr '03

11 Apr '03

Jan-Willem Maessen <jmaessen(a)MIT.EDU> wrote: > Actually, though the operations of most monads constrain evaluation > order, an interesting property of the identity monad is that it should > not do so. This observation has led me to think long and hard about > the distinction between monadic and non-monadic computations in > Haskell (admittedly to no real conclusion). Why would you do this? I don't see what conclusion you could come to that would involving discarding the ID monad, so you'd still need to distinguish between different monads. OTOH, the do-notation or other monadic notations don't really add much to the ID monad. So, a non-monadic syntax would still be desirable for this very common special case. So, what could you possibly think would change? Jon Cast

2 1

Re: Threading monads
by Jon Cast 11 Apr '03

11 Apr '03

Mark Carroll <mark(a)chaos.x-philes.com> wrote: > I could give an example, actually, which might make things > clearer. Take one of my unfoldM's: > unfoldM f base = > do fb <- f base > case fb of > Nothing -> return [] > Just (a, b) -> do rest <- unfoldM f b > return (a : rest) > Now, using the Identity monad I can write a wrapper for non-monadic > f's: > myUnfoldr f b = > runIdentity (unfoldM (return . f) b) > which works the same as a regular unfoldr. With a monadic function > and a wrapper that uses the Identity monad, I can offer both monadic > and non-monadic interfaces to the unfolding functionality that is only > implemented once. > Is this a good design idea? Are there better ones? Is myUnfoldr > necessarily less efficient than a "normal" unfoldr that doesn't have > monads underneath constraining evaluation order? Monads don't necessarily constrain the order of evaluation. The ID monad, in particular, is perfectly lazy. So, it doesn't place any more constraints on the order of evaluation than ordinary Haskell does. Furthermore, (although I can't check this atm) I suspect if you ask a Haskell compiler to specialize unfoldM for ID, you'll get a version that acts internally almost exactly like unfoldr---and that produces a myUnfoldr implementation precisely equal to unfoldr. Jon Cast

1 0

Threading monads
by Mark T.B. Carroll 10 Apr '03

10 Apr '03

Occasionally I have a non-monadic function (call it "x") that takes a function (call it "y") as an argument and uses it for stuff. Then, one day I inevitably want to pass it a y that needs to do something monadically: e.g. when I wanted unfoldM instead of unfoldr. So, I have to rewrite x accordingly. If I want to write an x that is open to taking a monadic y but doesn't require it to be monadic, should I maybe write it monadically and provide a wrapper that puts non-monadic y's in the Identity monad? Might this wrapping approach cause less efficient code than writing an alternative non-monadic x for non-monadic y's? Am I making any sense at all? (-: -- Mark

3 2

unfoldM
by Mark T.B. Carroll 10 Apr '03

10 Apr '03

I wanted a monadic unfoldr and couldn't see one so I thought I'd share two possibilities I'd been playing with: unfoldM_a :: Monad m => (b -> Maybe (m (a, b))) -> b -> m [a] unfoldM_a f base = case f base of Nothing -> return [] Just result -> do (a, b) <- result rest <- unfoldM_a f b return (a : rest) unfoldM_b :: Monad m => (b -> m (Maybe (a, b))) -> b -> m [a] unfoldM_b f base = do fb <- f base case fb of Nothing -> return [] Just (a, b) -> do rest <- unfoldM_b f b return (a : rest) test_case_a "" = Nothing test_case_a word@(_:xs) = Just (do a <- print (length word) return (a, xs)) test_case_b "" = return Nothing test_case_b word@(_:xs) = do a <- print (length word) return (Just (a, xs)) main = unfoldM_a test_case_a "foo" putStr "----\n" unfoldM_b test_case_b "foo" -- Mark

2 1

announcing darcs
by David Roundy 09 Apr '03

09 Apr '03

(My apologies for a possibly off-topic post, but I don't know where else I might contact interested haskellers.) For anyone who might be interested, I've written a revision control system which is in haskell (called darcs), which is licensed under the GPL. It contains a few bits of code that you might find useful, such as an interface to libcurl (for lazily reading files over http or ftp), an LCS function and code for opening a pipe to sendmail as a Handle. Of course, I'd welcome anyone who wants to contribute to darcs itself. the darcs webpage is http://www.abridgegame.org/darcs/ darcs is currently unix-only (counting MacOS X as unix). A port to windows would be a fair amount of work, but probably would mostly be straightforward (mostly dealing with slashes versus backslashes and getting libcurl to work under windows). -- David Roundy http://www.abridgegame.org/darcs/

2 1

state of FRP
by Isaac Jones 09 Apr '03

09 Apr '03

Greetings, I'm interested in learning more about Functional Reactive Programming since I want to be a better designer of Haskell programs. I don't want to use it for animation or GUIs in particular, but as a general design paradigm. With some digging, one discovers www.haskell.org/frp which has some pointers to papers on frp, but each of them seems to describe something new or domain specific wrt frp. I'd love advice about what order to read these papers in. Is there any more general introduction to frp for a working programmer? In learning Monads, I found standard libraries like Control.Monad.State to be extremely instructive since they encapsulate the basic operations that are available. (I wrote some documentation for this module a few weeks back to share what I learned. Its in CVS now.) So I'm curious about frp libraries. haskell.org/frp mentions an frp library that will exist one day. The manual looks interesting and this seems like it would be a good place to start, but Yale FRP is, alas, not yet available. Further poking reveals www.haskell.org/afrp which has an available snapshot, but no documentation. Any advice out there on how to go about learning frp? Should I see what I can glean from using functional reactive animation or GUI libraries? peace, isaac

2 1

IOHCC Results?
by Ashley Yakeley 07 Apr '03

07 Apr '03

Anyone know? I didn't submit anything but I am curious... http://iohc.mgoetze.net/ -- Ashley Yakeley, Seattle WA

2 1

comonads, io
by peter＠premise.demon.co.uk 06 Apr '03

06 Apr '03

I came across an interesting paper "Codata and comonads in Haskell" by Richard Kieburtz, and some other work by Alberto Pardo on comonads. Kieburtz proposes an OI comonad, as an alternative to (or something alongside?) the IO monad. Just for reasons of symmetry, it's natural to wonder if they aren't as pervasive in functional programs as monads. Frustratingly, I can't seem to grasp the intuition. I'm aware that the state-monad s->(s,a) has a dual state-in-context comonad (s,s->a). How does the side-effect manifest itself? The (ineffective) hints the authors give revolve around the idea that side-effects "derive" from the context of a program. Any other hints? Are comonads especially appropriate for programming systems that react to changes in their environment? Is an object of OI type something like a program to react to events? I can't make the slightest sense of Kieburtz's OI co-monad, with commands like coGetChar :: OI Handle -> Char. Peter Hancock

4 6

creating MIME mail
by David Roundy 05 Apr '03

05 Apr '03

Any ideas how to easily create MIME mail? A brief look at haskell.org didn't show any libraries that seemed useful for this. Basically I'd like to send information from my program (in the form of a couple of attachments to an email) which can then be extracted on the other end. So I guess I'd like to be able to both create _and_ parse MIME email. The reason for it being email is so the receiver can easily check it out before piping it into my program. -- David Roundy http://www.abridgegame.org

1 0

RE: simulating dynamic dispatch
by Simon Peyton-Jones 04 Apr '03

04 Apr '03

(Taking this off to haskell-cafe) It's not unreasonable for it to be ambiguous. The argument 'f' says *forall b*. The cast will cast to a *specified* type. But you aren't specifying the type. If you passed a less polymorphic 'f' it might be ok. Simon | -----Original Message----- | From: Hal Daume III [mailto:hdaume@ISI.EDU] | Sent: 21 March 2003 22:29 | To: Simon Peyton-Jones | Cc: oleg(a)pobox.com; haskell(a)haskell.org | | Excellent paper! I have started using some of the techniques already! | | But, back to the real question :). I now no longer think such a thing is | possible. Essentially what I want (given 'cast :: Typeable a, Typeable b | => a -> Maybe b' [1]), is to write something like: | | test :: forall a r . Typeable a => | (forall b . (Typeable b, Foo b) => b -> r) -> | a -> Maybe r | test f a = liftM f (cast a) | | but then 'b' is ambiguous. I'm not sure there is any way around | this. Can someone help? | | - Hal | | [1] is there any reason why parens are requires around class | contexts? i've always found this odd... | | -- | Hal Daume III | hdaume(a)isi.edu | "Arrest this man, he talks in maths." | www.isi.edu/~hdaume | | On Fri, 21 Mar 2003, Simon Peyton-Jones wrote: | | > You might also find the 'cast' function in Section 3 of "Scrap your | > boilerplate" useful. | > http://research.microsoft.com/~simonpj/papers/hmap/ | > I'm not certain, but it has the right smell. | > Simon | > | > | -----Original Message----- | > | From: oleg(a)pobox.com [mailto:oleg@pobox.com] | > | Sent: 21 March 2003 04:19 | > | To: hdaume(a)ISI.EDU; haskell(a)haskell.org | > | Subject: Re: simulating dynamic dispatch | > | | > | | > | > i'm hoping to be able to simulate a sort of dynamic dispatch based | > on | > | > class instances. | > | | > | It seems you want to dispatch based not on a type but on the | > | constraint of a type. | > | | > | You code almost worked. Here's the a bit updated and working version. | > | | > | class Foo a where { foo :: a -> Bool } | > | class Bar a where { bar :: a -> Bool } | > | | > | data FB = forall a . Foo a => MkFoo a | forall a . Bar a => MkBar a | > | | > | instance Foo FB where | > | foo (MkFoo x) = foo x | > | | > | instance Bar FB where | > | bar (MkBar x) = bar x | > | | > | -- some instances for the test | > | instance Foo Int where | > | foo x = x == 0 | > | | > | instance Bar Char where | > | bar x = x == 'a' | > | | > | | > | test x = case x of | > | (MkFoo a) -> Just $ foo a | > | (MkBar a) -> Just $ bar a | > | -- _ -> Nothing | > | | > | | > | -- *Main> test $ MkFoo (0::Int) | > | -- Just True | > | -- *Main> test $ MkFoo (10::Int) | > | -- Just False | > | -- *Main> test $ MkBar 'a' | > | -- Just True | > | -- *Main> test $ MkBar 'b' | > | -- Just False | > | | > | _______________________________________________ | > | Haskell mailing list | > | Haskell(a)haskell.org | > | http://www.haskell.org/mailman/listinfo/haskell | >

1 0

GISTalk - New GIS Forum!
by Tim Johnson 03 Apr '03

03 Apr '03

WaterTalk, StormTalk, and SewerTalk http://www.haestad.com/frm_gtk_030321 You're invited to join GISTalk, a new online forum for the Geographic Information System (GIS) community. GISTalk follows the success of our WaterTalk, StormTalk, and SewerTalk forums, providing GIS professionals a place to ask questions and raise issues regarding geographic and geospatial information systems and receive feedback from GIS experts across the world. GISTalk is your one-stop source for GIS-related information. With over 10,000 engineers active on the forums, you're sure to find answers and opinions for virtually any project you are working on. Professionals interested in joining any or all of the discussion lists can subscribe by visiting the link below... http://www.haestad.com/frm_gtk_030321 This message is intended for civil engineers and water resource professionals. If it has reached haskell-cafe(a)haskell.org in error, reply to this message with a subject line of "stop". LSID: 12274-2076416

1 0

Tokenizing Strings
by jamesd＠mena.org.au 02 Apr '03

02 Apr '03

I have a string that needs to be split/tokenized based on a delimiter. This can easily be accomplished using 'break' if the delimiter is only 1 character (i.e. break isSpace "this is a string"), but I can't see any way of using this for a delimiter with multiple characters. in this case, I have a string containing multiples fields seperated by *two* blank lines (\n\n). I can't just break on the newline character, as single newline characters can be found inside each field. any idea how I can do this without too much hassle? -James (jamesd(a)mena.org.au)

4 4

how to use POSIX function utime?
by David Roundy 28 Mar '03

28 Mar '03

I'm trying to figure out how to use the POSIX function utime. I'm pretty much stumped at the moment, largely because I can't figure out how to do anything useful with ClockTime variables. I can't see any way to make use of GHC.Posix.utime, as it gives a binding to the utime function, but the argument is of type Ptr (), and I can't see how to make this to a pointer to the contents of a pair of ClockTimes. I had thought that I could just write my own utime wrapper in C, but I'm still stumped on getting anything out of a ClockTime! The problem is that I would like my code to be portable, so I can't assume a ClockTime is a UInt32 (or that a time_t is a 32 bit int). Any ideas how I can get around this? The only idea I have at the moment is to define a MyClockTime which is always a UInt64 and then do in my C wrapper I could cast back and forth from time_t to an int64 type. Ugh. -- David Roundy http://www.abridgegame.org

2 2

Re: how to track down infinite loop?
by David Roundy 26 Mar '03

26 Mar '03

On Wed, Mar 26, 2003 at 07:40:30AM -0800, Andy Moran wrote: > On Wednesday 26 March 2003 07:23 am, David Roundy wrote: > > I would try using Debug.Trace.trace. It takes a string and an expression, > and returns the expressions while outputting the string to stderr (I > think). You can choose your own mnemonics to prevent the truncation > problem. Thanks for the hint! I like trace, strongly reminiscent of the good old technique of sprinkling printfs around. :) It turned out the problem was running sequence on an infinite list of type [Maybe a]. In retrospect obvious, but sort of hard to notice (especially since the infinite list is created in a different module from the sequence call. Fortunately, in this case it is safe to just truncate the list to some large size (since the function in question is only used in the quickChecking code, and not in the actual code). -- David Roundy http://civet.berkeley.edu/droundy/

1 0

how to track down infinite loop?
by David Roundy 26 Mar '03

26 Mar '03

Hello. I've got an infinite loop (I think) which I recently introduced to my code (the visible symptom is stack space overflow), and am wondering if there is an easier way to figure out where it might be. The problem being that I've made a rather large number of changes (all of which are mutually dependent). In the past, I've used ghc's heap profiling to track this sort of thing down, as I can pretty easily trace from that what functions were called in the infinite loop. The problem I'm now having is that the string describing the call stack is truncated too much, so I can't see who the real culprit is. I know the best thing would be to go over all my changes with a fine-toothed comb, but it would be nice to have an idea where to look. Actually, it's not all that much new code, but I usually find it hard to see a bug that I've written myself... Any ideas for tricks to see where a program is looping indefinitely? I'm sure I can track down this bug pretty easily, but is seems like this is something one really ought to be able to do... -- David Roundy http://civet.berkeley.edu/droundy/

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RE: Asynchronous exceptions and "resume"
by Simon Marlow 26 Mar '03

26 Mar '03

> Hi all, is there a way, or is it planned to, or has anyone published > articles on... resuming from asynchronous exceptions? > > I mean: it would be useful there was a > > suspend :: ThreadID -> IO () > > where the result is the remaining computation of the other > thread, wich > one could forkIO again, or simply run in the current thread. This is fairly easy to implement in GHC. Simple call Schedule.c:raiseAsync() with NULL for the exception, and make it return the final closure after it has stripped the thread's stack. Entering that closure should restart the thread's computation exactly where it left off. If the thread was blocked on something (eg. takeMVar), then it should restart the blocking operation. The types aren't quite right: the closure you get back will behave like an application of unsafePerformIO, so you will need to wrap it using eg. Control.Exception.evaluate to get an IO action. I'm sure there are plenty of cool uses for this... Cheers, Simon

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