- Haskell-Cafe - Haskell.org

Importance of MonadRandom
by Yitzchak Gale 08 Feb '07

08 Feb '07

I would like to point out the importance of Cale Gibbard's MonadRandom, beyond what is currently mentioned on its wiki page: http://www.haskell.org/haskellwiki/New_monads/MonadRandom This monad makes it possible to write functions that use randomness without having to specify in advance whether the source of randomness will be a pure pseudorandom number generator, as in System.Random, or physical randomness via the IO monad, such as your operating system's source of physical randomness, or random.org, or a hardware random generator. Before use of MonadRandom becomes widespread - and I think it ought to - I would like to suggest a change to the interface. (I mentioned this once to Cale on #haskell, but I didn't say what change I meant.) Currently, the members of the MonadRandom class mimic the members of the Random class in System.Random. I think it would be better if instead they mimicked the members of RandomGen. Like this: \begin{code} class (Monad m) => MonadRandom m where nextR :: m Int splitR :: m (m ()) rangeR :: m (Int, Int) getR :: (forall g . RandomGen g => g -> a) -> m a \end{code} The extra function getR provides access not only to the member functions of Random, but to any function that generates random variables of any type. You would use getR random, getR $ randomR (a, b), etc. instead of getRandom, getRandomR (a, b), etc. Provide a default method for getR as follows: \begin{code} getR f = do r <- nextR (lo, hi) <- rangeR return $ f $ TrivialGen r lo hi data TrivialGen = TrivialGen Int Int Int instance RandomGen TrivialGen where next (TrivialGen r _ _) = r genRange (TrivialGen _ lo hi) = (lo, hi) split _ = undefined \end{code} We would use the default method of getR for MonadRandom instances of things like DevRandom, DevURandom, RandomDotOrg, etc. For the Rand and RandT instances we provide explicit methods: \begin{code} -- For RandT: getR f = RandT $ gets f --For Rand: getR =Rand $ getR f \end{code} I think this is better for several reasons: o We anyway need getR for general random variables o We could lose precision getting other random variables via getRandom in the case where genRange /= (minBound, maxBound) o I think it is a better semantic fit Regards, Yitz

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nested maybes
by Martin DeMello 08 Feb '07

08 Feb '07

I have a Data.Map.Map String -> (Layout, [String]) as follows: type Anagrams = [String] type Cell = (Layout, Anagrams) type WordMap = Map.Map String Cell exists str wmap = let a = Map.lookup (sort str) wmap in case a of Nothing -> False Just x -> case (find (== str) (snd x)) of Nothing -> False _ -> True the existence test looks ugly - any more compact way to write it? martin

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haskell and releasing resources
by Fawzi Mohamed 08 Feb '07

08 Feb '07

I am just coming to haskell, and I wrote a simple command to get some input from a pdf file I just wanted the output of the command so I did something like import System.Process (runInteractiveCommand) import IO (hGetContents) -- | returns the text of the first page of the pdf at the given path, needs pdftotext getTextOfPdf :: String -> IO String getTextOfPdf pdfPath = do (inp,out,err,pid) <- runInteractiveCommand ("pdftotext -l 1 "+ +pdfPath++" -") return (hGetContents out) I don't care about error handling, if something goes wrong it is ok to hang or crash, but knowing unix I wondered if this would do the right thing or if it would create a zombi process. I was about to ask, but then I thought "let's test it", and sure enough the zombi stays there. I tried to even to allocate more than one, wait, I even managed to exhaust the resources of my machine... So here is what I would have liked to happen: when the pid gets garbage collected it tries to wait for the process, and if that fails the pid stays around longer and will try to wait later. Too difficult? I don't know, but it is what I had expected from haskell. Failing that I would have expected clear hints that one should wait for external processes in the documentation, and I found none. So what is the way out? I could do a forkIO and wait for the process but then I wonder do I have to wait for the thread, or dead thread become zombi? I that is the case then the only way out would be to give back also the pid and make the waiting responsibility of the caller, not very nice, but probably the real solution. I have seen that in missingh there is a pipeout example but it seems to me that you are still responsible to wait for the process (with ensureSuccess). Maybe the correct thing is not being able to ignore the return code of the process.... but now I am becoming suspicious of other things, for example a file handle, you need to close it or you can expect that it will be closed when the handle is garbage collected? Are there other places where you need to pay attention to be sure that you are releasing the resources you acquired? I suppose problems can come only when an external resource is involved, or not So 1) documentation should specify if one should do some specific action to free resources, can someone fix this? 2) is there a clean (haskell;) way to deal with this? 3) other places apart external processes where you have to pay attention? What is your wisdom... thanks Fawzi

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Re: [Haskell-cafe] IO is not a monad
by Aaron McDaid 07 Feb '07

07 Feb '07

Hi, Apologies for referring to this old thread... I rearranged the code a little bit while experimenting but retained the same behaviour: Prelude> let f = undefined :: Int -> IO Int Prelude> (f 3 >> f 3) `seq` 42 42 Prelude> (f 3) `seq` 42 *** Exception: Prelude.undefined I think this makes it a little simpler for me to see the issue here. Simply, 'undefined >> undefined' is a bit more defined than simply 'undefined'. Just like 'undefined:undefined' is at least a non-empty list; which can be matched by (_:_) for example. This explains the differing behaviour of the two seemingly equivalent actions above. So I think the above behaviour is more to do with how shallow seq is, as others have probably already shown (but much of the rest of the thread is beyond me). Prelude> undefined `seq` 5 *** Exception: Prelude.undefined Prelude> (undefined:undefined) `seq` 5 5 Aaron (relative newbie) > On 1/23/07, Brian Hulley <brianh(a)metamilk.com> wrote: > > Brian Hulley wrote: > > > Brian Hulley wrote: > > >> Yitzchak Gale wrote: > > >>> I wrote: > > >>>>> Prelude> let f = undefined :: Int -> IO Int > > >>>>> Prelude> f `seq` 42 > > >>>>> *** Exception: Prelude.undefined > > >>>>> Prelude> ((>>= f) . return) `seq` 42 > > >>>>> 42 > > >>> The monad laws say that (>>= f) . return must be > > >>> identical to f. > > >> > > >> I thought it was: > > >> > > >> return x >>= f = f x > > >> > > >> so here the lhs is saturated, so will hit _|_ when the action is > > >> executed just as the rhs will. > > > > Ooops! But that does not mean the equation holds because for example > > > > Prelude> (return 3 >>= f) `seq` 42 > > 42 > > Prelude> (f 3) `seq` 42 > > *** Exception: Prelude.undefined > > > > In the lhs you only hit _|_ when the composite (>>=) action is actually > > being executed whereas in the rhs you hit _|_ when computing the function > > which will return the action to execute so there is difference. > > > > Brian. > > -- > > http://www.metamilk.com > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

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External Core
by Ricky Barefield 07 Feb '07

07 Feb '07

Dear All, For the last year I've been working on a program to translate between Haskell and Java. Originally this was for my third year dissertation to contribute towards my Masters in computer science at Sheffield University, UK. I have been given the opportunity to continue this project and have become interested in using the Core representation as an intermediary language. I understand that GHC can produce 'hcr' files which are specified in "An external representation for the ghc core language" 2001 and to this point I have been successful. I have also downloaded the tools for manipulating Core available from http://darcs.haskell.org/ghc/utils/ext-core/ I've tried running Happy on these files but get the error `Not enough type arguments for the type synonym "P"' when I try to run the resultant Parser.hs in Hugs and similar errors when run in GHC. What I'm trying to achieve is to read the hcr files into a Haskell data type which I could work with, if anyone could give me any help with using the files for manipulating Core I would be very grateful. Many Thanks, Ricky Barefield 4th Yr Undergraduate University of Sheffield

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Trouble representing a set of flags
by Stefan O'Rear 07 Feb '07

07 Feb '07

I have a structure: data Attr = Attr { fg :: !Color, bg :: !Color, bold :: !Bool, blink :: !Bool, rv :: !Bool, halfBright :: !Bool, underline :: !Bool } deriving(Eq,Show,Ord) newtype Color = Color Int deriving(Eq,Show,Ord) but ghc is doing a phenominally bad job of unboxing it; so I'm trying to do it manually using a bitfield: -- |Data type representing character attributes. newtype Attr = Attr Int deriving (Eq) -- | Set the foreground color of an `Attr'. setFG :: Color -> Attr -> Attr setFG (Color c) (Attr a) = Attr ((a .&. 0xFFFFFF00) .|. c) -- | Get the foreground color of an `Attr'. getFG :: Attr -> Color getFG (Attr a) = Color (a .&. 0xFF) -- | Set the background color of an `Attr'. setBG :: Color -> Attr -> Attr setBG (Color c) (Attr a) = Attr ((a .&. 0xFFFF00FF) .|. (c `uncheckedShiftL` 8)) -- | Get the background color of an `Attr'. getBG :: Attr -> Color getBG (Attr a) = Color ((a .&. 0xFF00) `uncheckedShiftR` 8) -- | Set bold attribute of an `Attr'. setBold :: Attr -> Attr setBold (Attr a) = Attr (a .|. 0x10000) -- | Clear bold attribute of an `Attr'. clearBold :: Attr -> Attr clearBold (Attr a) = Attr (a .&. 0xFFFEFFFF) -- | Examine bold attribute of an `Attr'. isBold :: Attr -> Bool isBold (Attr a) = (a .&. 0x10000) /= 0 Urk. this is NO FUN. How do I stop the copy&paste and still generate fast code? Is there a better way? (Also, GHC seems unable to unbox strict fields that are newtypes. Can this be done, or should I abandon type safety and use type synonyms?) (Yes, this is performance critical code, and I have profiler output to back up this claim.) (Should this be directed to glasgow-haskell-users@ ?)

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Newbie: generating a truth table
by phiroc＠free.fr 07 Feb '07

07 Feb '07

Hello, I would like to create a Haskell function that generates a truth table, for all Boolean values, say, using the following "and" function : and :: Bool -> Bool -> Bool and a b = a && b I have tried creating a second function called "loop", which repeatedly calls "and", but it did not work, because, for some reason unknown to me, "do" does not like repeated function calls loop = do and True True and True False and False True and False False Is there a better way to repeatedly call "and"? Furthermore, is there a way in Haskell to loop through the Boolean values (True and False) and call "and" each time? Last but not least, in the "loop" function above, assuming that there is a way to repeatedly call the "and" function, how could you intersperse "printStr"s between the "and" calls? loop = do printStr("Calling and True True returns ") and True True printStr("Calling and True False returns ") and True False printStr("Calling and False True returns ") and False True printStr("Calling and False False returns ") and False False Many thanks. Best regards, phiroc

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blog: Is C# becoming a functional language?
by Bulat Ziganshin 07 Feb '07

07 Feb '07

Hello haskell-cafe, this seems like a comprehensive overview of FP facilities in C# 3.0: http://blogs.msdn.com/madst/archive/2007/01/23/is-c-becoming-a-functional-l… -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Generalizing three programs
by Andrew Wagner 06 Feb '07

06 Feb '07

Hi everyone, I've got an interesting problem here I'm trying to solve. Actually, I've got several problems which seem to have a very similar structure. I want to find a way to abstract them to solve other problems which can be thought about in the same way. Here they are: http://hpaste.org/307 http://hpaste.org/308 http://hpaste.org/309 Note that these are just sketches, the programs aren't done yet. The general structure of the problem is that an object enters a system, interacts with different parts of the system, and eventually leaves, and we want to monitor some statistics about the interaction, so that we can then make some changes, and run it again, and hopefully improve it. Thanks in advance for any suggestions!

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Final issues to fix for the shootout entries (Was: no subject)
by dons＠cse.unsw.edu.au 06 Feb '07

06 Feb '07

Yeah, have a look on the shootout: http://shootout.alioth.debian.org/gp4/benchmark.php?test=recursive&lang=all Has an ackerman. More detailed comparisions, across the 19 benchmarks on the shootout, with notes on which of the 19 GHC Haskell is significantly slower (>4x) at: Haskell verus Python http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: regex-dna Haskell verus Ruby http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: regex-dna Haskell verus Perl http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: regex-dna Haskell verus Lisp http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: mandelbrot (Double math) spectral-norm (Double math) Haskell verus Clean http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: k-nucleotide ( HashTable benchmark) mandelbrot spectral-norm Haskell verus SML http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: k-nucleotide mandelbrot spectral-norm Haskell versus OCaml http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=ghc&lang2… Loses on: k-nucleotide mandelbrot regex-dna spectral-norm Haskell versus C Loses on: k-nucleotide mandelbrot regex-dna spectral-norm Fixing these last few programs: k-nucleotide: Needs a better hashtable library implementation. For example a Data.HashByteString based on a Trie of ByteString) regex-dna: Should use the regex-tre library instead of regex-posix (algorithm issue only) mandelbort/spectral-norm: Double math is slow. I don't know why. And note that strings aren't a bottleneck anymore! http://shootout.alioth.debian.org/gp4/benchmark.php?test=sumcol&lang=all -- Don seth: > GHC has profiling support. > > (By the way, many mail servers these days discard mail with no subject.) > > I've seen a number of papers comparing the speed of Haskell code to code of other functional languages; there is a periodic "shoot out" with ocaml. > > Some probably have comparisons with imperative languages, and, even if they do not, the methodology should help you. > > Seth Kurtzberg > > On Mon, 5 Feb 2007 11:28:03 -0800 (PST) > Tays Soares <tayscristina(a)yahoo.com.br> wrote: > > > Hello everyone, > > > > I did at my master thesis a compiler that generates Haskell code. Now I need to measure the execution time of my generated code and I've been searched and I don't know if I'm looking with the wrong keywords but I could not find anything. I just need to measure the time of simple functions, like Ackermann and Fibonacci. Does anyone know how to measure the execution time of a Haskell program or function? > > > > Thank you, > > Tays > > > > > > > > __________________________________________________ > > Fale com seus amigos de gra?a com o novo Yahoo! Messenger > > http://br.messenger.yahoo.com/ > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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Function Composition
by Chris Witte 06 Feb '07

06 Feb '07

Can anyone explain why Shape.Polygon would have a different type to (Shape).Polygon, I thought the brackets would be redundant. Here is the output from a Hugs session Animation> :v -- Hugs Version 20050113 Animation> :t Shape Shape :: Shape -> Region Animation> :t Polygon Polygon :: [Vertex] -> Shape Animation> :t (Shape.Polygon) Polygon :: [Vertex] -> Shape Animation> :t ((Shape).Polygon) Shape . Polygon :: [Vertex] -> Region Animation> :t Shape.(Polygon) Shape . Polygon :: [Vertex] -> Region Cheers Chris Witte

2 1

strict bytestring fun
by dons＠cse.unsw.edu.au 06 Feb '07

06 Feb '07

High performance strings on the shootout: http://shootout.alioth.debian.org/gp4/benchmark.php?test=sumcol&lang=all interesting alternative programs 0.5 Haskell GHC #5 1.29 90,880 270 1.0 Clean 2.77 600 136 2.0 C gcc 5.64 444 159 2.1 D Digital Mars #2 5.85 700 153 2.3 C++ g++ #2 6.46 848 244 Ah well, its illegally strict, but its good to know you can do it, eh? (A lazy bytestring version has been submitted, we'll see how that runs). -- Don

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How is laziness defined?
by TJ 06 Feb '07

06 Feb '07

I would think that with 100% laziness, nothing would happen until the Haskell program needed to output data to, e.g. the console. Quite obviously that's not it. So how is laziness defined in Haskell? I remember vaguely someone saying that pattern matching on a value forces it to be evaluated. Is that right? What else? This is one of the things that just boggles my mind everytime I try to wrap it around this thing called Haskell ;) Cheers, TJ

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List operation question
by Eric Olander 06 Feb '07

06 Feb '07

Hi, I'm still somewhat new to Haskell, so I'm wondering if there are better ways I could implement the following functions, especially shiftl: >> moves the first element to the end of the list shiftr :: [a] -> [a] shiftr [] = [] shiftr (x:y) = y ++ [x] >> moves the last element to the head of the list shiftl :: [a] -> [a] shiftl [] = [] shiftl x = [last x] ++ init x -Eric

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A function callable from C
by John Ky 06 Feb '07

06 Feb '07

Hi, The following code works: > type ServiceMainClosure = DWORD -> IO () > > foreign import ccall "wrapper" > mkServiceMainClosure :: ServiceMainClosure -> IO (FunPtr ServiceMainClosure) But the following doesn't: > type ServiceMainClosure = DWORD -> [String] -> IO () > > foreign import ccall "wrapper" > mkServiceMainClosure :: ServiceMainClosure -> IO (FunPtr ServiceMainClosure) System\Win32\Service.hsc:108:8: parse error on input `import' What can I do to get this to work? Thanks -John

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OT: any haskell-friendly / functional programming friendly comp sci programs? (for a 30s guy who did his undergrad in liberal arts)
by Thomas Hartman 06 Feb '07

06 Feb '07

haskellers, I'm contemplating returning to school after a decade as a worker bee, and almost that long as a worker bee doing computer consulting / miscelaneous tech stuff. Ideally I'd like to get a masters, but I don't know if that's feasible this late in the game. If it's not, I might settle for a lesser degree and worry about the masters later. Part of my desire to return to school is that after encountering functional programming over the past year or so (first through lisp, then haskell), I've found something that I'm really interested in, but that it feels the amount of learning I can do around a job just won't cut it to get to the level that I want. That's the personal reason; the practical reason is that I think it would be a good networking opportunity (hankering to start my own company, and not meeting the right kinds of people), and obviously increase my chance of getting better paying, but especially more *interesting* jobs. I have some savings, figure loans would cover the rest, and no wife or kids. So I'll make a rather open ended request for advice. Are there decent comp sci master's programs out there that will take someone who didn't do a hard science in undergrad, but has lots of work-related experience with programming? If not, what's the next best thing? Get a quick bachelor's? Spend six months cramming for the GREs and then try for a master's? Whether master's, bachelor's, or other, I am specifically interested in programs that are "functional" friendly. In other words, I don't want to just go and study algorithms in java for two years. Ideally, I'd like to go somewhere where I could really explore and get good at the functional languages, with haskell my current favorite but also open to others. (For what it's worth, most of my experience is in perl, but I take it seriously as a language and try not to write the kind of throwaway crap that mean people make fun of.) My background is that I have a liberal arts bachelors from an american ivy. Though I enjoyed the program very much, it probably wasn't the wisest financial decision, given the type of career I subsequently gravitated to. But hey, maybe that gives me a different kind of perspective that has its own kind of value. Since, then, there;s mainly work work work for me, with some time off bumming around europe. And, on my own time: learn learn learn. I'm now 31. I've been living in germany for a few years, mainly freelancing as a computer consultant, and am open to programs in europe -- probably either england or germany. Largely because programs out here seem to be significantly cheaper than programs in the states, and being a somewhat older student seems to be less unusual out here. However, truth be told, I have a hankering for my homeland, the good old USA. And if possible somewhere in california, where my extended family is based, or it not that new york, where I have a good network. (But open to other locations as well; cali and new york would just be my top choices.) Distance learning is okay if the program is really good, but doing the campus thing again, perhaps while working part or semi-full time, would be the ideal (though perhaps I'm pushing the age limit there?). This has gotten rather long, so I'll leave it at that. Sorry about going off topic, and thanks in advance for any advice. thomas

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Timezone Database Library
by Martin Percossi 06 Feb '07

06 Feb '07

Hello, is there a haskell library that provides facilities to read and use the tzfile format [1], or equivalent in Windows? TIA Martin [1] http://www.twinsun.com/tz/tz-link.htm

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How to use infinite lists to define the list of all negative integers
by Bahtijar Vogel 06 Feb '07

06 Feb '07

Hi, How am I supposed to use infinite lists to define the list of all negative integers?

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How to remove all numbers from a list that are divisible by an integer
by Miranda Kajtazi 06 Feb '07

06 Feb '07

Hi, Thanx to all for the previous hints how to solve and write the function to calculate the sum of a list of lists. Now I need to know how to remove all numbers that are divisable by an integer. Lets say I have a list [2,4,5,6,8] and when I divide them with number 2, my list should look like [5]...because all other numbers in this list are divisible with 2 therefore I remove or filter all of them. I tried to use some zipWith...filter and other predefined functions..but I can't really find the right solution :(

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Space leaks in large mutable data structures
by C Rodrigues 06 Feb '07

06 Feb '07

I'd like to hear what tips and techniques you guys have for avoiding space leaks. I understand the basic techniques to force evaluation of closures. What I'd like to know is how you avoid space leaks in large, long-lived, mutable data structures. These kind of data are particularly sensitive to laziness in my experience. (I'm using functional data structures and doing a lot of update operations.) If you use monads to enforce strictness, how do you maintain composability throughout the program? _________________________________________________________________ Invite your Hotmail contacts to join your friends list with Windows Live Spaces http://clk.atdmt.com/MSN/go/msnnkwsp0070000001msn/direct/01/?href=http://sp…

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how to calculate the sum of list of lists?
by Miranda Kajtazi 06 Feb '07

06 Feb '07

Help, How to calculate the sum of list of lists in Haskell? Please help me, Miranda

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mixing wxhaskell state and file io
by Martin DeMello 05 Feb '07

05 Feb '07

I'm having a lot of trouble mixing file io and wxhaskell's varCreate/Get/Set functions. I have functions readWords :: String -> IO WordMap wordGrid :: WordMap -> Layout And within my GUI code, the following compiles (ignores the variable, basically): words <- varCreate (do {w <- readWords "words"; return w}) wGrid <- do w <- readWords "words" return $ wordGrid w but I can't get the following (noncompiling code, but it shows what I'm trying to do) working: wGrid <- do w <- varGet words return $ wordGrid w Could someone give me a minimal example of reading in a list of words from a file, binding a wxHaskell variable to the list, and then mapping some GUI code over it? (Also, I'm making the base assumption that varSet and varGet are wxHaskell's analogue of the State monad - should I be looking at using StateT instead?) martin

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Connected!
by Bulat Ziganshin 05 Feb '07

05 Feb '07

Hello haskell-cafe, i've just got ADSL connection here! it's slow (64k) and not cheap, but at least it is completely different from dial-up i've used before -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Win32 help please
by John Ky 05 Feb '07

05 Feb '07

Hi, I'm interested in writing accessing some Win32 functions not available from GHC. I wrote this short little file and tried to compile: mywin32.hsc > module MyWin32 where > > #include <windows.h> Then tried to compile it like this: # hsc2hs mywin32.hsc # ghc -fffi mywin32.hs C:/system/ghc/ghc-6.6/libHSrts.a(Main.o):Main.c:(.text+0x1b): undefined reference to `__stginit_ZCMain' C:/system/ghc/ghc-6.6/libHSrts.a(Main.o):Main.c:(.text+0x3f): undefined reference to `ZCMain_main_closure' collect2: ld returned 1 exit status What am I missing? Thanks -John

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Strange behaviour with writeFile
by C.M.Brown 05 Feb '07

05 Feb '07

Hi, I am observing some rather strange behaviour with writeFile. Say I have the following code: answer <- AbstractIO.readFile "filename" let (answer2, remainder) = parseAnswer answer if remainder == "" && answer2 == "" then do AbstractIO.putStrLn $ "completed" else do AbstractIO.putStrLn answer2 AbstractIO.writeFile "filename" remainder With the above I get an error saying the resources to "filename" are locked. If I add the line "AbstractIO.putStrLn $ show (answer2, remainder) before I call writeFile it suddenly magically works! Has anyone seen strange behaviour like this before? Regards, Chris.

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(no subject)
by C Rodrigues 05 Feb '07

05 Feb '07

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Another Space Leak
by Dominic Steinitz 05 Feb '07

05 Feb '07

Many thanks for the help on the original space leak which is now fixed -see the function pad below and test runs in small constant space. However, that has merely revealed the next space leak. The problem appears to be blockWord8sIn512 :: [Word8] -> [[Word8]] blockWord8sIn512 = unfoldr g where g [] = Nothing g xs = Just (splitAt 64 xs) or its alternative bws :: [Word8] -> [[Word8]] bws [] = [] bws xs = ys:(bws zs) where (ys,zs) = splitAt 64 xs But I can't see why a big expression is being built up. Shouldn't these produce elements of the list when they are required and they are then consumed by foldl'? Dominic. module Main(main) where import Data.Word import Data.Bits import Data.List ss = [0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0] pad = pad' 0 where pad' l [] = [0x80] ++ ps where pl = (64-(l+9)) `mod` 64 ps = replicate pl 0x00 pad' l (x:xs) = x : (pad' $! l+1) xs -- otherwise (l+1) it will be deferred until replicate blockWord8sIn512 :: [Word8] -> [[Word8]] blockWord8sIn512 = unfoldr g where g [] = Nothing g xs = Just (splitAt 64 xs) bws :: [Word8] -> [[Word8]] bws [] = [] bws xs = ys:(bws zs) where (ys,zs) = splitAt 64 xs test :: Int -> Word8 test n = foldl' xor 0x55 (pad (replicate n 0x55)) test1 :: Int -> [Word8] test1 n = foldl' (zipWith xor) [0x01..0x40] (blockWord8sIn512 (pad (replicate n 0x55))) test2 :: Int -> [Word8] test2 n = foldl' (zipWith xor) [0x01..0x40] (bws (pad (replicate n 0x55))) main = do putStrLn (show (test 1000000)) putStrLn (show (test1 1000000)) putStrLn (show (test2 1000000))

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Suggestions for a hReadUntilStr implementation
by Matt Revelle 05 Feb '07

05 Feb '07

Hey there, A few weeks back I was thinking of writing a Haskell program that automated a telnet session. One function that could be useful is a hReadUntilStr - that is, a function that takes a Handle as an input source, a String to match, and a "Num a" as the number of seconds to wait before returning a (String, Bool) where the String is all the text read from the Handle until either matching or timing out and the Bool is true if the input String was matched. Something like: hReadUntilStr :: (Num a) => Handle -> String -> a -> IO (String, Bool) Is this the wrong way to think about the problem? If so, how should it be handled? If not, any ideas on the implementation? Any advice is appreciated. Cheers, Matt

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Space Leak Help
by Dominic Steinitz 05 Feb '07

05 Feb '07

I have re-written SHA1 so that is more idiomatically haskell and it is easy to see how it implements the specification. The only problem is I now have a space leak. I can see where the leak is but I'm less sure what to do about getting rid of it. Here's the offending function: pad :: [Word8] -> [Word8] pad xs = xs ++ [0x80] ++ ps ++ lb where l = length xs pl = (64-(l+9)) `mod` 64 ps = replicate pl 0x00 lb = i2osp 8 (8*l) I've thought about zipping the xs with [1..] which will give me a length as I go. Is this the right way to go are there better techniques for dealing with this? I've attached the full source below. Dominic. module Main(main) where import Data.Char import Data.Bits import Data.List import Data.Word import System import Codec.Utils type Rotation = Int rotL :: Rotation -> Word32 -> Word32 rotL s a = shiftL a s .|. shiftL a (s-32) instance Num [Word32] where a + b = zipWith (+) a b f n x y z | (0 <= n) && (n <= 19) = (x .&. y) .|. ((complement x) .&. z) | (20 <= n) && (n <= 39) = x `xor` y `xor` z | (40 <= n) && (n <= 59) = (x .&. y) .|. (x .&. z) .|. (y .&. z) | (60 <= n) && (n <= 79) = x `xor` y `xor` z | otherwise = error "invalid index for f" k n | (0 <= n) && (n <= 19) = 0x5a827999 | (20 <= n) && (n <= 39) = 0x6ed9eba1 | (40 <= n) && (n <= 59) = 0x8f1bbcdc | (60 <= n) && (n <= 79) = 0xca62c1d6 | otherwise = error "invalid index for k" -- Word120 -> Word512 -> Word120 oneBlock ss xs = (as!!80):(bs!!80):(cs!!80):(ds!!80):(es!!80):[] where ws = xs ++ map (rotL 1) (zipWith4 xxxor wm3s wm8s wm14s wm16s) where xxxor a b c d = a `xor` b `xor` c `xor` d wm3s = drop (16-3) ws wm8s = drop (16-8) ws wm14s = drop (16-14) ws wm16s = drop (16-16) ws as = (ss!!0):ts bs = (ss!!1):as cs = (ss!!2):(map (rotL 30) bs) ds = (ss!!3):cs es = (ss!!4):ds ts = (map (rotL 5) as) + (zipWith4 f [0..] bs cs ds) + es + (map k [0..]) + ws ss :: [Word32] ss = [0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0] pad :: [Word8] -> [Word8] pad xs = xs ++ [0x80] ++ ps ++ lb where l = length xs pl = (64-(l+9)) `mod` 64 ps = replicate pl 0x00 lb = i2osp 8 (8*l) blockWord8sIn512 :: [Word8] -> [[Word8]] blockWord8sIn512 = unfoldr g where g [] = Nothing g xs = Just (splitAt 64 xs) getWord32s :: [Word8] -> [Word32] getWord32s s = map f [0..15] where f i = foldl (+) 0 $ map (\n -> toEnum (fromEnum (s!!(i*4+n))) `shiftL` (fromIntegral (8 * (3-n)))) [0..3] blockWord32sIn512 :: [Word8] -> [[Word32]] blockWord32sIn512 = (map getWord32s) . blockWord8sIn512 . pad -- Word120 -> Word512 -> Word120 hashOnce ss a = ss + oneBlock ss a hash = foldl' hashOnce ss . blockWord32sIn512 convert :: String -> [Word8] convert = map (fromIntegral . ord) short :: [Word8] short = convert "abc" message :: [Word8] message = convert "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq" performance n = (convert . take n . repeat) 'a' test n = mapM_ (putStrLn . show . hash) [short, message, performance n] main = do progName <- getProgName args <- getArgs if length args /= 1 then putStrLn ("Usage: " ++ progName ++ " <testSize>") else test (read (args!!0))

6 12

Alternate instance Show (Maybe a)?
by Sergey Zaharchenko 04 Feb '07

04 Feb '07

Hello list, Suppose I want show Nothing to return "", and show (Just foo) return show foo. I don't seem to be able to. Looks like I either have to use some other function name, like `mShow', or have to import Prelude hiding Maybe, reimplement Maybe, write all the other useful instances (Functor, Monad) for it, etc. Not particularly hard, but looks ugly. Isn't there a better solution? I recall some discussion about this, but can't find it in the archives... TIA, -- DoubleF No virus detected in this message. Ehrm, wait a minute... /kernel: pid 56921 (antivirus), uid 32000: exited on signal 9 Oh yes, no virus:)

4 4

The usage of runghc ?
by keepbal 04 Feb '07

04 Feb '07

I tried to install haskell-fastcgi ( http://www.cs.chalmers.se/~bringert/darcs/haskell-fastcgi/ ). >runghc Setup.hs configure --prefix=$HOME/extra >runghc -I$HOME/extra/include Setup.hs build Preprocessing library fastcgi-2006.10.9... FastCGI.hsc:50:21: error: fcgiapp.h: No such file or directory compiling Network/FastCGI_hsc_make.c failed command was: ghc -c Network/FastCGI_hsc_make.c -o Network/FastCGI_hsc_make.o Setup.hs: got error code while preprocessing: Network.FastCGI >runghc --help runghc: syntax: runghc [-f GHCPATH] [GHC-ARGS] FILE ARG... ?

2 1

Proposal for allowing more use of layout to avoid parentheses and commas
by Brian Hulley 04 Feb '07

04 Feb '07

Hi, Following a recent thread on the Haskell' mailing list about the nusiance of having to deal with commas in tuples (when cutting and pasting things to re-order the elements there's always a pesky comma left over in the wrong place), I've written up a proposal for a very simple syntax tweak that would allow you to use the layout rule for function arguments, tuple/list/record contents etc for some future version of Haskell at http://www.haskell.org/haskellwiki/Accessible_layout_proposal This post is not intended to spark a lot of discussion just to point out the existence of the page for anyone interested. I've also added a link to all the new proposals from http://www.haskell.org/haskellwiki/Future . (If I don't reply to any follow up posts to this thread it's not because I'm ignoring them. It will just be because I have to try to get down to actually writing more of my program in the next few weeks, and I find thinking up ideas and reading posts, books, cooking, shopping, watching dvds etc, is infinitely more diverting than the painstakingly hard work of actual coding (though of course it's great in the rare occasions when things fall into place) ... ;-)) Brian. -- http://www.metamilk.com

1 0

(a -> [b]) vs. [a -> b]
by Chad Scherrer 03 Feb '07

03 Feb '07

Are (a -> [b]) and [a -> b] isomorphic? I'm trying to construct a function f :: (a -> [b]) -> [a -> b] that is the (at least one-sided) inverse of f' :: [a -> b] -> a -> [b] f' gs x = map ($ x) gs It seems like it should be obvious, but I haven't had any luck with it yet. Any help is greatly appreciated. Thanks, Chad

6 7

It matters how Type Synonyms are defined?
by Bryan Burgers 03 Feb '07

03 Feb '07

Today, I was plugging away on a program and I ran into a problem. It seems that ErrorT can or can not take a type synonym as its monad, depending on how the type synonym was defined. For example, consider this GHCi interactive run: > :k Maybe Maybe :: * -> * > let { a :: ErrorT String Maybe Bool; a = undefined } > :t a a :: ErrorT String Maybe Bool > :k State (Scope VVar) State (Scope VVar) :: * -> * > let { a :: ErrorT String (State (Scope VVar)) Bool; a = undefined } > :t a a :: ErrorT String (State (Scope VVar)) Bool ScopeState is defined in a file as: > type ScopeState a = State (Scope VVar) a > :k ScopeState ScopeState :: * -> * > let { a :: ErrorT String ScopeState Bool; a = undefined } <interactive>:1:6: Type synonym `ScopeState' should have 1 argument, but has been given 0 In the type signature: a :: ErrorT String ScopeState Bool Now, I was going to ask something like, "How can I define my type synonym so I can do this," but I figured out while writing this email that if I define ScopeState a different way: > type ScopeState = State (Scope VVar) > :k ScopeState ScopeState :: * -> * > let { a :: ErrorT String ScopeState Bool; a = undefined } > :t a a :: ErrorT String ScopeState Bool So, my new question is: Why does it matter how ScopeState is defined? Bryan Burgers

4 3

Levels of recursion
by Andrew Wagner 03 Feb '07

03 Feb '07

I won't speak for anyone else, but I remember when recursion first "clicked". It was in a 400-level data structures and algorithms class. I had already done a fair amount of chess programming (which of course does a massive amount of recursion), but it still seemed a bit magical to me. When the professor pointed out that you simply have to assume that the recursive call will work, and then the whole recursive definition will, a light bulb went on. Of course, once I started learning haskell, such "aha!" moments quickly became a way of life. Things like map, foldr, and the lazy definition of fibs all made me stop and think for a while, and when I understood them, I was better for it. I had a sort of interesting moment like this last night. In hacking on some code, I wrote the following function: combine :: [Int] -> [Int] -> [[Int]] combine [] _ = [] combine (x:xs) ys = (take x ys) : (combine xs (drop x ys)) To me, this is recursion. Define your base case, take care of one part of the problem, and then call yourself recursively to take care of the rest. This is how I was taught recursion, and this is how I think about it. Now for the fun part. A much more experienced haskeller told me he preferred to write it like this: combine' :: [Int] -> [Int] -> [[Int]] combine' xs ys = snd $ mapAccumL aux ys xs where aux ys n = (b,a) where (a,b) = splitAt n ys Ack! Where'd the nice "base-case, take a chunk, recursively do the rest" pattern go? Suddenly, recursion isn't so simple when it's done using composition and higher-order functions which abstract recursive patterns. Like I said, I'm familiar with map, foldr, etc. But this is the first time it's dawned on me that I need to think in more general recursive patterns like this instead of simple, raw recursion. That map and foldr aren't JUST a nice way of doing things to a little list, but they are recursive operators, building blocks that I need to use as fluently as anything else. I suspect I'm not alone, though of course I could be wrong. But I would bet that a significant difference between newbies and more experienced Haskell programmers is the degree to which they can think in this composition/HOF way. Where the beginner wants to program using raw, simple recursion, the more experienced sees an opportunity to apply an abstraction. So, a couple of questions to ponder about this: Is this unique to Haskell, or could the same be said about any functional language? How can we teach this better to newbies? Most of what I see in the tutorials is "Higher order functions accept as parameters and/or return other functions. Here's some examples: <explanation of map>, <explanation of foldr>. Moving on, ..." But clearly, this is something important, and I think we can do a better job of teaching it. Suggestions?

4 3

"Write Yourself a Scheme in 48 Hours"
by Shannon -jj Behrens 03 Feb '07

03 Feb '07

I'm going through the "Write Yourself a Scheme in 48 Hours" <http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/overview.html> tutorial. I like it a lot, but I have some concerns. Are the exercises in the tutorial known to be solvable by mere mortals? For instance: "Rewrite parseNumber using...explicit sequencing with the >>= operator" http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/parser.html#s… There aren't any examples of using >>= previous to this question. Furthermore, the link to the Standard Prelude is not helpful because there aren't any examples of how to use >>=. Furthermore, consider the exercise: "Change parseNumber to support the Scheme standard for different bases. You may find the readOct and readHex functions useful." http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/parser.html#s… I struggled against this for a couple hours last night. How is the reader supposed to figure out readOct, which is part of ReadS, without understanding the whole ReadS business? If the reader does understand the ReadS business, he probably already understands Haskell far better than the tutorial seems to suggest. I eventually figured out how to write: parseHexNumber = do char '#' char 'x' s <- many1 (oneOf "0123456789abcdefABCDEF") case readHex s of [(n,"")] -> return n but it was no small feat. Furthermore, it was only possible because I had already spent so much time trying to understand "A Gentle Introduction to Haskell". Worst of all, once I had it all implemented, the parser *from* the tutorial: parseExpr :: Parser LispVal parseExpr = parseAtom <|> parseString <|> parseNumber led to some surprising results. It turns out that "#o9", which should be an invalid attempt at an octal number, is getting parsed as an atom. There's a whole layer of difficulty that seems insurmountable by mere mortals like me using just this tutorial and minimal reference usage. What am I missing? Is it really solvable by mere mortals who don't already know Haskell, the Parsec module, etc.? Thanks, -jj -- http://jjinux.blogspot.com/

3 3

Become a GHC build slave!
by Simon Marlow 03 Feb '07

03 Feb '07

Thanks largely to Ian Lynagh, GHC now has a BuildBot infrastructure to automate nightly builds on multiple platforms. This replaces the old set of shell scripts that we used to run nightly builds; now adding new clients to the setup is relatively easy, instructions are here: http://hackage.haskell.org/trac/ghc/wiki/BuildBot So far we have various Windows builds running, and I'll be moving over the existing Linux nightly builds in due course. If you have a spare machine or a machine that is idle overnight, and you'd like to use it to run automated GHC builds, then please take a look at the above page for how to get started. We especially need platforms that we don't use regularly (i.e. not Windows or Linux/x86/x86_64). Cheers, Simon

2 1

A distributed and replicating native Haskell database
by Joel Reymont 03 Feb '07

03 Feb '07

Folks, Allegro Common Lisp has AllegroCache [1], a database built on B-Trees that lets one store Lisp objects of any type. You can designate certain slots (object fields) as key and use them for lookup. ACL used to come bundled with the ObjectStore OODBMS for the same purpose but then adopted a native solution. AllegroCache is not distributed or replicating but supports automatic versioning. You can redefine a class and new code will store more (or less) data in the database while code that uses the old schema will merrily chug along. Erlang [2] has Mnesia [3] which lets you store any Erlang term ("object"). It stores records (tuples, actually) and you can also designate key fields and use them for lookup. I haven't looked into this deeply but Mnesia is built on top of DETS (Disk-based Term Storage) which most likely also uses a form of B-Trees. Mnesia is distributed and replicated in real-time. There's no automatic versioning with Mnesia but user code can be run to read old records and write new ones. Would it make sense to build a similar type of a database for Haskell? I can immediately see how versioning would be much harder as Haskell is statically typed. I would love to extend recent gains in binary serialization, though, to add indexing of records based on a designated key, distribution and real-time replication. What do you think? To stimulate discussion I would like to ask a couple of pointed questions: - How would you "designate" a key for a Haskell data structure? - Is the concept of a schema applicable to Haskell? Thanks, Joel [1] http://franz.com/products/allegrocache/index.lhtml [2] http://erlang.org/faq/t1.html [3] http://erlang.org/faq/x1409.html -- http://wagerlabs.com/

3 3

Boost equivalent
by John Ky 02 Feb '07

02 Feb '07

Hi, Does the Haskell community have an equivalent to C++ community's Boost project with the aim of writing libraries for the eventual inclusion into Haskell? Thanks -John

9 8

Array copy performance
by Chris Kuklewicz 02 Feb '07

02 Feb '07

Hi, I am trying to improve the memory usage of regex-tdfa and I wanted to ask an array question. If I have two identical STUArrays (same type and bounds) then what is the most efficient way to overwrite the data in the destination with the data in the source? Does this work for STArrays? Is there a GHC only solution? Is there a way to avoid the long loop? > forM_ (range b) $ \index -> > readArray source index >>= writeArray destination index Is Data.Array.Storable the only route? I do not think Data.Array.Diff will work well since I have one source going to multiple destinations, each with a few different changes. -- Chris

3 2

snd and tuples of various sizes...
by Tim Newsham 02 Feb '07

02 Feb '07

This seems to make using tuples of various sizes easier (and can also be applied to non-tuples). I think it more closely matches how I describe something in spoken language (when I say "second" its obvious what that means for any tuple size): {-# OPTIONS_GHC -fglasgow-exts #-} module Main where class Second a b | a -> b where snd :: a -> b instance Second (a,b) b where snd (a,b) = b instance Second (a,b,c) b where snd (a,b,c) = b instance Second (a,b,c,d) b where snd (a,b,c,d) = b instance Second (a,b,c,d,e) b where snd (a,b,c,d,e) = b instance Second (a,b,c,d,e,f) b where snd (a,b,c,d,e,f) = b instance Second [a] a where snd [] = error "don't got none" snd (x:y:xs) = y main = do print $ snd (1,2) print $ snd (8,9,3) print $ snd (9,8,9,3) print $ snd (4,9,8,9,3) print $ snd [3,4] Tim Newsham http://www.thenewsh.com/~newsham/

4 3

Let's welcome the Ruby hackers!
by dons＠cse.unsw.edu.au 02 Feb '07

02 Feb '07

As seen here, reports from 'Rails Edge': http://notes-on-haskell.blogspot.com/2007/01/haskell-open-secret.html > It seems like everyone is turning onto Haskell these days. > At Rails Edge last week, I saw a few telltale signs that some of the > speakers (including a few members of the Rails core team) were playing with > Haskell. In one case, a speaker was flipping through TextMate projects, and > briefly displayed one project named "Haskell". In another case, the > presenter's browser had a link to All About Monads centered in the middle > of the bookmarks bar. > Of course, I had to take the opportunity to see why these speakers were > interested in Haskell. One speaker was looking into Parsec for some > insights into language design (for DSLs, probably), while another was > revisiting the language after he tried to learn it a few years ago. > It turns out that a few members of the Rails team have informally chosen > Haskell as their language of the year this year. Nothing formal, just a > bunch of folks who hang out on irc periodically trading bits and pieces of > Haskell. > Somehow, I think this bodes well for both Rails and Haskell. More from the same guy: http://notes-on-haskell.blogspot.com/2007/01/ruby-vs-haskell-choose-what-wo… (On not writing Rails support code in C, instead in Haskell). So a big hello to any Ruby/Rails hackers lurking out there! Free lambdas for all if you drop by #haskell... Cheers, Don

5 4

ANN: Takusen 0.6
by Alistair Bayley 02 Feb '07

02 Feb '07

We are pleased to announce a new release of Takusen. There are a large number of changes and bug-fixes: - Oracle support for processing cursors returned from procedure calls - withContinuedSession supports connection reuse (e.g. for persistent connections and connection pooling) - new README file with useful information about getting started with Takusen - new UTF8 en/de-coder (the old one was buggy) - PostgreSQL support for CalendarTime - improved Cabal Setup.hs script, which does a better job of modifying the installation to reflect installed DBMS's. This gives good ghci support (PostgreSQL users on Windows have a small extra step) - prepared statement API requires that users distinguish between queries and DML Our long-term plans are the same as ever: - large objects - ODBC and MS Sql Server backends Release: http://darcs.haskell.org/takusen/takusen-0.6.tar.gz Source: darcs get --partial http://darcs.haskell.org/takusen Oleg and Alistair

1 0

Channel9 Interview: Software Composability and the Future of Languages
by Neil Bartlett 02 Feb '07

02 Feb '07

No doubt many of you will have seen the interview[1] on Channel9 with Anders Hejlsberg, Herb Sutter, Erik Meijer and Brian Beckman. These are some of Microsoft's top language gurus, and they discuss the future evolution of programming languages. In particular they identify composability, concurrency and FP as being important trends. However their focus is on borrowing features of FP and bringing them into mainstream imperative languages; principally C#. Naturally the subject of Haskell comes up repeatedly throughout the interview. Disappointingly they characterize Haskell as being an impractical language, only useful for research. Erik Meijer at one point states that programming in Haskell is too hard and compares it to assembly programming! Yet the interviewees continually opine on the difficulty of creating higher level abstractions when you can never be sure that a particular block of imperative code is free of side effects. If there were ever a case of the answer staring somebody in the face... I found this interview fascinating but also exasperating. It's a real shame that no reference was made to STM in Haskell. I don't know why the interviewer doesn't even refer to the earlier Channel9 interview with Simon Peyton Jones and Tim Harris - it appears to be the same interviewer. Still, it's nice to see that ideas from Haskell specifically and FP generally are gaining more and more ground in the mainstream programming world. It also highlights some of the misconceptions that still exist and need to be challenged, e.g. the idea that Haskell is too hard or is impractical for real work. [1] http://channel9.msdn.com/Showpost.aspx?postid=273697

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Re: [Haskell-cafe] Channel9 Interview: Software Composability and the Future of Languages
by Steve Downey 02 Feb '07

02 Feb '07

The 70's and early 80's were very different in terms of information propagation. I really miss some the journals available back then, because the editors really did their jobs, both in selecting and helping to convey, information. OO did get oversold. The same way that putting it on the internet did at the beginning of this century (I love saying that, now, where's my flying car) but just like many of the good principles of structured programming inform OO, it should be possible to take good OO and apply it functionally. On 1/30/07, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Hello Steve, > > Friday, January 26, 2007, 10:03:09 PM, you wrote: > > > Haskell _is_ hard, although I don't think it's _too_ hard, or I wouldn't > ... > > > The audience for programming languages like Haskell is always going to > > be small, because it appeals to those who want to understand how the TV > > works, > > i don't think so :) imho, we just don't have good _teachers_. in > 70's, OOP audience was also small, but it was popularized later and > now every student know about polymorphism via inheritance. but most of > OOP programmers don't reinvent the wheels, they just use "patterns" > described in OOP bestselling books > > i have a positive experience of making "complex" concepts easy and > available for wide audience ([1]-[5]), [1] was even used to teach > students in some college. and i guess that good Haskell books, such as > yaht and printed ones, also make it easy to learn Haskell. but we need > to gather much more attention to Haskell to make it as "patternized" > as structured-programming and OOP. _nowadays_ there is no even one > "advanced Haskell" or "Haskell in Real World" book and this means that > anyone who want to learn Haskell in deep should study those terrible papers > > (well, it's very like higher education in Russia - no one really > teaches you at our colleges so you should either learn yourself or die :) > but this means that at least whose who still alive, are Real Machos :) > > the same apply to Haskell - now the only way to learn it is to learn > yourself, so we all definitely are cool mans. once i even got C# job > offer only because i know Haskell :) > > > [1] http://haskell.org/haskellwiki/IO_inside > http://haskell.org/haskellwiki/OOP_vs_type_classes > http://haskell.org/haskellwiki/Modern_array_libraries > http://haskell.org/bz/th3.htm > http://haskell.org/bz/thdoc.htm > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

ANNOUNCE: The Monad.Reader - Issue 6
by Wouter Swierstra 02 Feb '07

02 Feb '07

Dear all, I pleased to announce that the latest issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 6 consists of the following three articles: * Bernie Pope - Getting a Fix from the Right Fold * Dan Piponi - Adventures in Classical-Land * Russell O'Connor - Assembly: Circular Programming with Recursive do The Monad.Reader is a quarterly magazine about all things Haskell. It is less-formal than journal, but somehow more enduring than a wiki- page or blog post. If you'd like to submit something to the next issue of The Monad.Reader, please get in touch. The preliminary deadline for Issue 7 is March 30, 2007. I will send out an official call for copy in the coming weeks. For now, enjoy Issue 6! Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

8 9

"Circular" programming (aka time-travel) resources?
by Justin Bailey 02 Feb '07

02 Feb '07

In "The Monad.Reader - Issue 6", that just came out, there is a really interesting article that uses a "circular" technique to implement an assembly language in Haskell. The technique demonstrated seems fascinating. Can someone point me to more resources on the topic? A quick google search turned up a couple of blogs and some papers - but is there more out there? Thanks for any help! Justin

3 2

Takusen - error handling and DBM monad
by Paul Moore 02 Feb '07

02 Feb '07

I'm continuing (in my occasional spare time) to try to write a "simple database query" in takusen. The query is OK now, but I'm struggling with error handling. I have a simple error handler catcher :: DBException -> DBM mark Session () catcher x = do liftIO $ putStrLn $ show x All it does is print the exception. The DBM mark Session () type, I deduced by fiddling until something works. Now, I can protect my query with something like main = do withSession (connect "..." "..." "...") ( do catchDB (do -- simple query, returning reversed list of rows. r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ putStrLn $ show r ) catcher ) So far, so good. The syntax is messy, but a little refactoring should fix that. But this doesn't catch errors in the connect call. Wrapping the withSession in catchDB doesn't work, as withSession is in the IO monad [1], not the DBM one. But using catch with catcher doesn't work, as that expects an error handler in the IO monad, but catcher is in the DBM monad. I'm getting very confused about the relationship between the DBM stuff and the IO monad. I'm sure there are good reasons for the DBM monad, but at the moment I'm missing them, and all I'm doing is trying random things, to little avail. If someone could give me a better understanding of the reasoning behind the DBM stuff, plus some pointers on how I should be interleaving IO and DBM stuff, I'd be hugely grateful. Apologies if this post is vague and/or badly expressed, but I'm currently a bit short of time, and fairly frustrated over what I feel should be a simple task (that imperative vs functional mismatch, again :-)) Thanks, Paul. [1] Excuse sloppy terminology here!

3 4

Think of a monad...
by Eric Y. Kow 01 Feb '07

01 Feb '07

Dear Haskellers, In the recent HWN, I noticed a new monad metaphor by Don Stewart: Think of a monad as a spacesuite full of nuclear waste in the ocean next to a container of apples. now, you can't put oranges in the space suite or the nucelar waste falls in the ocean, *but* the apples are carried around anyway, and you just take what you need This metaphor very clearly captures the essence of monads. Perhaps it will be even more helpful if accompanied by a small illustration: http://koweycode.blogspot.com/2007/01/think-of-monad.html I hope this turns out to be useful to somebody, -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

4 3

data package
by Pavel Rozenblioum 01 Feb '07

01 Feb '07

Hi, I am trying to compile the GLR examples from Happy 1.16, but I get the message that I am missing the "data" package. Where can I download it from? I am using GHC 6.6 /pavel

1 0