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Apache HTTP Server And Haskell
by Kaveh Shahbazian 31 Jul '06

31 Jul '06

Is there a mod_haskell (like mod_perl and mod_python) for Apache HTTP server? Does anyone know about it?

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Fwd: [Haskell-cafe] Apache HTTP Server And Haskell
by Kaveh Shahbazian 31 Jul '06

31 Jul '06

Thanks, But it seems to be dead! (Last news on 18 Apr 2002). Is there a more mature one?

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The possibility of a real OS based on Haskell zippers
by Andy Elvey 30 Jul '06

30 Jul '06

Hi everyone - I'm very much a Haskell newbie, but have followed Haskell (and the various Haskell sites) for quite a few months now, and I absolutely *love* the language. One thing that **fascinated** me a while ago was the site which gave Haskell code for a "Haskell-based fileserver/OS" using zippers (the URI follows) - http://okmij.org/ftp/Computation/Continuations.html#zipper That code looks amazing! So, I was thinking - would it *actually be (theoretically) possible* to create a "real" booting OS (a bit like "House") from the above code? By a "real" OS, I mean something that uses (say) GRUB, and at least has a command-line of some kind. ( A GUI could come later ... ). I think that a "Plan 9-ish" OS (with all its "namespace/URL stuff, and its network-centric approach) would seem to be *particularly well suited* to being done using Haskell. ( I have to say that I love Plan 9's elegance, but I just *can't* manage its controls/GUI, they have me flummoxed ... ) If it were possible, I would *love* to see it - I've actually seriously considered even coughing up say $100 to $200 or so for a contest to create such a thing.... Anyway, "first things first" :-) - is such an OS possible? Many thanks for your time - bye for now - - Andy

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Serializing Functions and Actions for Distributed Programming
by Brian Sniffen 30 Jul '06

30 Jul '06

I'm very excited by the ability to pass functions or IO actions between threads of the same program. But I don't see any language or library support for doing so between programs, or between sessions with the same program. OCaml provides a partial solution: <http://caml.inria.fr/pub/docs/manual-ocaml/libref/Marshal.html> Though all it's really sending is an address and a hash of the binary program. Even SerTH doesn't help with functional types. I seek the knowledge of the Haskell Cafe: is there a reasonable way of addressing this problem? -- Brian T. Sniffen bts(a)alum.mit.edu or brian.sniffen(a)gmail.com http://www.evenmere.org/~bts

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Baffled by Disk IO
by Matthew Bromberg 30 Jul '06

30 Jul '06

I have run into a baffling distinction between the behavior of GHCi and the compiled binary from GHC. I suspect it's something pretty stupid on my part. I have the following test program <test.hs> import Matrix main = let f1 = bRgauss 4 3 f2 = bRgauss 3 2 fu = f1 *. f2 in bsave "fumat" fu ---------------------------------- The type signature of bsave is -- | save a matrix to a file bsave :: String -> m -> IO() bsave str z = do k <- matindx z withCString str (\x -> bmsave x k) and bmsave is a C routine that does a simple write to disk using fprintf. If I compile using GHC and run this as test.exe it does absolutely nothing, no file is actually saved. In fact no file is saved even if I try to print components of fu in the IO monad. Whereas if I load it into GHCi and run main, everything works as expected. Well sort of. Apparently everytime I extract something out of fu and use it in an IO monad it executes all the IO actions in fu, so it's easy to get unwanted behavior with incautious use.

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Cabal in ghc 6.4.2
by allan 30 Jul '06

30 Jul '06

Hello all, (before I start I should say I'm relatively new (a few months) to haskell, so I'm not exactly savvy with the use of the compilers) I have a problem using the cabal with version 6.4.2 of ghc, the same thing occurs with two projects which I wish to compile, I'll use the simplest one since the problem seems to be the same in both cases. The program is a very simple printer thing for haskell the source of which you can download at: http://homepages.inf.ed.ac.uk/s9810217/software/haskell_print/haskellprint.… <http://homepages.inf.ed.ac.uk/s9810217/software/haskell_print/haskell_print…> (but you probably won't need to, in order to diagnose this problem) I wish to compile this with the sequence of commands: ./Setup.hs configure --user --prefix=/home/allan/install ./Setup.hs build this works perfectly on version 6.4 of ghc, however with version 6.4.2 I get the following error: haskellprint$ ./Setup.hs build Preprocessing executables for haskellprint-0.1... Building haskellprint-0.1... Chasing modules from: Main.hs Could not find module `Text.ParserCombinators.Parsec': use -v to see a list of the files searched for (imported from ./Parser.hs) haskellprint$ Now if I run haskellprint$ ./Setup.hs build --verbose then I can see the command line used to compile the program which is: /home/allan/install/bin/ghc -Idist/build -o dist/build/haskell_print/haskell_print --make -Wall -hide-all-packages -i -idist/build/autogen -i. -odir dist/build/haskell_print/haskell_print-tmp -hidir dist/build/haskell_print/haskell_print-tmp -package base-1.0 Main.hs If I copy and paste this into the command line and remove the argument [ -hide-all-packages ] then the program compiles fine. So basically my question is: what am I doing wrong? I assume my problem is in the .cabal file, so I include the contents here: name: haskellprint version: 0.1 license: GPL license-file: LICENSE author: Allan Clark maintainer: a.d.clark(a)ed.ac.uk build-depends: base executable: haskell_print main-is: Main.hs include-dirs: c-sources: other-modules: extra-libraries: extensions: ghc-options: -Wall -- ghc-options: -fwarn-incomplete-patterns -fwarn-missing-fields -- ghc-options: -prof -auto-all -Wall -fglasgow-exts -O -funbox-strict-fields -fasm -optl-Wl,-s thanks in advance for any help. allan

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Is id strict?
by David House 30 Jul '06

30 Jul '06

Hi all. I've seen two definitions of a 'strict function', which I'm trying to unite in my mind: 1) f is strict iff f _|_ = _|_. 2) f is strict iff it forces evaluation of its arguments. There is a large sticking point that in my minds seems to fit (1) but not (2): id. Clearly, id undefined is undefined, but I also don't think id forces evaluation of its argument. There was a mini-discussion concerning this topic last night on #haskell, but if there was a consensus conclusion, it passed me by. Thanks in advance. -- -David House, dmhouse(a)gmail.com

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Sending Ctrl-C under Windows
by Neil Mitchell 29 Jul '06

29 Jul '06

Hi, I want to send Ctrl-C to a process opened by runInteractiveCommand, under Windows. I have found System.Posix.Signals, but on Windows none of those methods exist. For further information, the processes I want to send Ctrl-C to are ghci and hugs, so doing something like terminateProcess isn't what i want, because I don't want the process to be terminated, just the current computation. If there is another way to programmatically terminate the computation started by hugs/ghci (but not the process) via runInteractiveCommand, that would be great too. Thanks Neil

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commandline parsing?
by Marc Weber 29 Jul '06

29 Jul '06

I've been using pesco_cmdline for a while now. But I did notice that it doesn't fit my needs.. And it took me quite a while to get to know why I was getting strange typeable errors when specifying the wrong default value or reading the wrong type.. (these errors occur at runtime thus they don't use haskells strength) So I got another crazy idea. Why not see arguments as tokens and use a parser such as parsec? All you would have to do is creating some kind of syntax tree. Consider an example application : mmewcde (my mega executable which can do everything). mmewcde edit <file> mmewcde mv <file> <file> mmewcde mv --target-dir <dest> <folder> <files>> mmewcde cp <file> <file> mmewcde cp --target-dir <dest> <folder> <files>> mmewcde callsox <infile> <options> <outfile> Why not something like tar? mmewcde tar <x|c|f|v>* ..? Of cause I'll have to write the funtions edit :: String -> IO () mv :: String -> String -> IO () mvDest :: String -> [String] -> IO () ... ... which have to be called Now I'd like to do this: optionParser = many $ oneOf [ edit, mv, mvDest, cp, cpDest, callsox, tar] optionParserWithHelp = optionParser <|> printHelp edit = do string "edit" file <- existingFilename launchEditor file mv = [..] mvDest = do string mv string "--target-dir" td <- existingDirectory files <- many existingFilename map (movefile td) files [...] printHelp = do oneOf $ map string ["-h","--help","--usage"] prettyPrint optionParser main = do args <- getArgs parseAndExecute optionParserWithHelp prettyPrint might look like this (this will be unncommon ;) mmewcde --help: many edit <exitingfile> or mv <exitingfile> <file> .... Would this be nice? Does this already exist somehow? perhaps many can even be made nongreedy so that you can specify more than one command at once mmewcde mv --target-dir d f1 f2 cp --target-dir d2 f3 f4 ... Does this make sense? I think this would lead to self documenting well mantainable code. Marc

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StablePtr's and castStablePtrToPtr
by DeeJay-G615 28 Jul '06

28 Jul '06

I have a query which is asked out of interest's sake... I'm essentially looking for an affirmation of what I think I already understand (or some info if I'm deluded ;)). To put this in context... I have some C code... typedef int func(void *); void from_maybe_int(void *val, func *my_func) { int i; i = my_func(val); printf("value within C: %d\n", i); } And some Haskell code... type FromMaybeInt = StablePtr (Maybe Int) -> IO CInt foreign import ccall "wrapper" wrapFromMaybeInt :: FromMaybeInt -> IO (FunPtr FromMaybeInt) foreign import ccall "from_maybe_int" cFromMaybeInt :: StablePtr (Maybe Int) -> FunPtr GetJust -> IO () functionToPass :: FromMaybeInt functionToPass sPtr = do m <- deRefStablePtr sPtr case m of Nothing -> return (-1) Just i -> return (fromIntegral i) main :: IO () main = do sPtr <- newStablePtr (Just 3) -- for example funPtr <- wrapFromMaybeInt functionToPass cFromMaybeInt sPtr funPtr freeStablePtr sPtr The compiled program works fine. However I wanted to check this was correct usage. As in, is perfectly fine to pass a value of type StablePtr a into C? Am I correct in thinking that StablePtr is defined as a void pointer in C? From my very limited understanding of C, it is also the case that you can implicitly cast a void pointer to any other pointer type and vice-versa. Some appear to deem it bad practice if you explictly give the cast. So therefore I am somewhat hazy on the use for castStablePtrToPtr. I found the ghc docs to be quite cryptic for this function. Google dug up a few examples of it's use in PUGS... which has lead me to think that the function is purely for type 'convience' in Haskell. Is this the case or am I missing a use case here? Disclaimer: My knowledge and experience of C is somewhat limited. Ironically I have started playing with C by way of Haskell. (Sick and twisted I know). Thanks, Daniel James P.S. I must say, I really like the Haskell FFI. Very clean and enjoyable to work with.

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Implementation of the Floyd-Warshall algorithm
by frederic＠ka-ge-ro.org 28 Jul '06

28 Jul '06

Hi, I'm new to Haskell (yet I am very familiar with Lisp and OCaml), and I am trying to implement the Floyd-Warshall algorithm (finding the minimal distance between two nodes in a weighted graph). For an input graph with 101 nodes, the obvious C version takes 0.01 s on my machine. My first totally functional implementation in Haskell took 6s... for a graph with 10 edges. (This version considered that a graph is given as its adjacency matrix, which is represented as a 2-uple in ([k], k -> k -> Double)). [I do not show my code, as I am ashamed of it :-S] My first question is: what would an (efficient?) version of the algorithm using this representation would look like ? Is it possible to do without ressorting to the ST monad ? Now, I have been trying to implement it in a more imperative way, to understand how the ST monad works. It runs in 0.6s for a 101-noded graph, which is much, much faster than the original version but still much slower than the C version. I would be very grateful if someone cared to explain why this is unefficient and how to make it faster (Without using the FFI :-|) Thanks by advance. (BTW, I'm using the ghc-6.42 compiler with -O2 flag). -- Frederic Beal -- Code begins here module FW (bench) where import Control.Monad import Control.Monad.ST import Data.Array.ST update :: STUArray s (Int, Int) Double -> Int -> Int -> Int -> ST s () update arr i j k = do aij <- readArray arr (i, j) ajk <- readArray arr (j, k) aik <- readArray arr (i, k) if aij + ajk < aik then do writeArray arr (i, k) (aij + ajk) else return () updateLine arr i j n = do mapM_ (update arr i j) [0..n] updateRow arr i n = do mapM_ (\x -> updateLine arr i x n) [0..n] updateStep arr n = do mapM_ (\x -> updateRow arr x n) [0..n] -- The actual FW invocation canonicalize = updateStep -- From here on, the "testing" suite count = 100 -- A test array: M[i, j] = 1 + ((x+y) mod count) orgArray :: ST s (STUArray s (Int, Int) Double) orgArray = do v <- newArray ((0, 0), (count, count)) 0.0 mapM_ (\x -> mapM_ (\y -> writeArray v (x, y) ((1+) $ fromIntegral (mod (x+y) count))) [0..count]) [0..count] return v sumDiag :: STUArray s (Int, Int) Double -> Int -> ST s Double sumDiag arr n = do foldM (\y x -> do a <- readArray arr (x, x) return $ a + y) 0.0 [0..n] orgDiag = do arr <- orgArray v <- sumDiag arr count return v cptDiag = do arr <- orgArray canonicalize arr count v <- sumDiag arr count return v bench = do val <- stToIO cptDiag diag <- stToIO orgDiag print val print diag

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if-then-else as rebindable syntax (was Re: Why does Haskell have the if-then-else syntax?)
by Niklas Broberg 28 Jul '06

28 Jul '06

I would be happy to write up a trac-ticket for this - I could even try to implement it in GHC. However, I'm surprised that you agree with it so easily since it breaks some Haskell 98-ish stuff in un-nice ways. :-) First of all, programs that import names from the Prelude explicitly would no longer be able to use if-then-else unless they also added 'cond' to that input list (or redefined it of course). This shouldn't really be a problem, since the rebindable syntax is turned on by adding some flag anyway, and if you add that flag you know you're no longer H98. Still, it's going to break a lot of existing programs. The second problem is that it would require the addition of the cond function to the Prelude. This will probably not break too many existing programs, but still it is a more serious problem since it will have effect even without any flags to GHC. Or is it possible to govern the contents of the Prelude based on flags? I would really like to see this implemented, and I don't think the above is serious enough that we shouldn't. There may be some that don't agree though. Speak up now, or forever hold your peace! Also, is cond the best name for the suggested function? If we don't expect anyone to really use it without the sugar, we could name it whatever weird thing so as to break as few existing programs as possible. It would make explicit import a bit more akward though. But I suspect that if this function did exist in the Prelude, people would start using it a lot. Does anyone have any better suggestions, or is cond the name of the day? /Niklas On 7/27/06, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: > GHC does indeed include the notion of "rebindable syntax". It would be > straightforward to extend it to include if-then-else. In effect, that > would mean that > if e1 then e2 else e3 > would behave exactly like > cond e1 e2 e3 > including from the point of view of typing. (You could choose a > different name than 'cond'.) Then by importing a 'cond' with (say) type > > cond :: MyBool -> b -> b > you could use a different kind of Boolean. You could even overload the > bool: > cond :: Boolean a => a -> b -> b > > This could be done with a few hours work. But not a few minutes. Want > to put a feature request in Trac? > > Simon > > | -----Original Message----- > | From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Niklas > | Broberg > | Sent: 27 July 2006 09:01 > | To: Haskell-cafe > | Subject: Re: [Haskell-cafe] Why does Haskell have the if-then-else > syntax? > | > | I often find myself at odds with this choice. The reason is that I use > | Haskell as a host for embedded languages, and these often come with > | their own control flows. So I find myself wanting to write my own > | definition of the if-then-else construct that works on terms of some > | other type, e.g. tests on values of type Exp Bool instead of Bool, and > | at the same time make sure that the user doesn't use the built-in > | if-then-else. Sure, I can (and do) call my own version if_, ifElse or > | something else along those lines, but it's sure to be a constant > | source of programmer errors, writing if-then-else instead of if_ by > | habit. > | > | A thought that has crossed my mind on several occasions is, why not > | make the syntactic if-then-else construct rebindable, like the do > | notation? I think I know the answer already -- the do notation is > | syntactic sugar for >>= and company so it's easy to translate it into > | non-prelude-qualified versions of functions with those names. This is > | not the case for if-then-else. But it could be, the prelude could > | define a function if_ (or whatever) that the if-then-else construct is > | made to be sugar for, and thus also amenable to rebinding by not > | prelude-qualifying. > | > | /Niklas > | > | On 7/27/06, Paul Hudak <paul.hudak(a)yale.edu> wrote: > | > Mike Gunter wrote: > | > > | > >I had hoped the "History of Haskell" paper would answer a question > | > >I've pondered for some time: why does Haskell have the if-then-else > | > >syntax? The paper doesn't address this. What's the story? > | > > > | > >thanks, > | > >-m > | > > > | > > > | > Thanks for asking about this -- it probably should be in the paper. > Dan > | > Doel's answer is closest to the truth: > | > > | > I imagine the answer is that having the syntax for it looks > nicer/is > | > clearer. "if a b c" could be more cryptic than "if a then b else > c" > | > for some values of a, b and c. > | > > | > except that there was also the simple desire to conform to > convention > | > here (I don't recall fewer parentheses being a reason for the > choice). > | > In considering the alternative, I remember the function "cond" being > | > proposed instead of "if", in deference to Scheme and to avoid > confusion > | > with people's expectations regarding "if". > | > > | > A related issue is why Haskell does not have a "single arm" > conditional > | > -- i.e. an "if-then" form, which would evaluate to bottom (i.e. > error) > | > if the predicate were false. This was actually discussed, but > rejected > | > as a bad idea for a purely functional language. > | > > | > -Paul > | > > | > _______________________________________________ > | > Haskell-Cafe mailing list > | > Haskell-Cafe(a)haskell.org > | > http://www.haskell.org/mailman/listinfo/haskell-cafe > | > > | _______________________________________________ > | Haskell-Cafe mailing list > | Haskell-Cafe(a)haskell.org > | http://www.haskell.org/mailman/listinfo/haskell-cafe >

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heap issues
by Jeff Polakow 27 Jul '06

27 Jul '06

Hello, Why would ghci run out of heap space (and crash) the second time I run a computation? More specifically, I have a ghci session which goes something like this: *Analysis>run [ ... print out of a very long list ...] *Analysis>run [ ... partial print out GHC's heap exhausted: current limit is 268435456 bytes; Use the `-M<size>' option to increase the total heap size. Shouldn't the garbage collector free up everything in between my top-level function executions? Also, there doesn't appear to be a '-M<size>' flag for ghc (I'm using 6.4.2 on windows xp). thanks, Jeff -- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: [Haskell-cafe] Why Haskell?
by Matthew Bromberg 27 Jul '06

27 Jul '06

> I used what I thought, initially was an elegant contruction technique in > Haskell. Something like this > do > ... > sequence $ [ reffill b s | s <- [0..(fi temits)-1], b <- [0..(fi nc)-1]] > ...(push list on to matrix stack) Try the sequence_ (note the underscore) function, it should be a big win here. Cheers, Spencer Janssen Now thats interesting. I can see that this function is more appropriate since I do not need to retrieve data from the IO monad, but what I don't understand is why it's actually faster. I will give it a try and test it on a large set to see if things change. Thanks for the tip.

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[FGL] Works on ghc, not on Hugs?
by Stephane Bortzmeyer 27 Jul '06

27 Jul '06

[Warning: beginner with FGL, the Functional Graph Library.] My first program with Data.Graph.Inductive works fine on ghc but raises a type error with hugs, which strikes me as odd. % hugs -98 smalltest.hs __ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Report bugs to: hugs-bugs(a)haskell.org || || Version: 20050308 _________________________________________ Hugs mode: Restart with command line option +98 for Haskell 98 mode ERROR "smalltest.hs":7 - Type error in application *** Expression : mkGraph mynodes myedges *** Term : myedges *** Type : [(Integer,Integer,[Char])] *** Does not match : [LEdge [Char]] ghc works: % ghc --make smalltest.hs Chasing modules from: smalltest.hs Compiling Main ( smalltest.hs, smalltest.o ) Linking ... % ./a.out 0:"Toto"->[("Couic",1),("Crac",2)] 1:"Tata"->[] 2:"Shadok"->[] 3:"Machin"->[] Here is the code: import Data.Graph.Inductive mynodes = [(0, "Toto"), (1, "Tata"), (2, "Shadok"), (3, "Machin")] myedges = [(0, 1, "Couic"), (0, 2, "Crac")] graph :: Gr String String graph = mkGraph mynodes myedges main = do print graph

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Re: [Haskell-cafe] Why does Haskell have the if-then-else syntax?
by Donn Cave 27 Jul '06

27 Jul '06

Quoth "Sebastian Sylvan" <sylvan(a)student.chalmers.se>: | On 7/27/06, mvanier <mvanier(a)cs.caltech.edu> wrote: | > As opposed to what? | | For example case-of, guards (in combination with let or where), or | just a function: | | if :: Bool -> a -> a -> a | if True t _ = t | if False _ e = e | | -- example usage | myAbs x = if (x < 0) (negate x) x That looks to me like a different way to spell if then else, but maybe that's the answer to the question - conceptually, for every "then" there really is an "else", however you spell it, and only in a procedural language does it make any sense to leave it implicit. The exception that proves the rule is "else return ()" -, e.g., if_ :: Bool -> IO () -> IO () if_ True f = f if_ False _ = return () main = do args <- getArgs if_ (length args > 0) (print args) Strictly speaking that generalizes to any functional context where a generic value can be assigned to the else clause, but there don't tend to be that many other such contexts. Does that answer the question? Donn Cave, donn(a)drizzle.com

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Re: ldap-haskell questions
by Donn Cave 26 Jul '06

26 Jul '06

Quoth Ferenc Wagner <wferi(a)tba.elte.hu>: ... | find -type f | xargs grep -i tls | revealed nothing in my ldap-haskell tree. Isn't your interface | available for the public by chance? It isn't at the moment, and while I don't have anything like a proprietary interest to keep me from posting it somewhere, at the moment I like the idea better of trying to contribute that and maybe one or two other things to this one. I think mine's OK for 300 lines of code, but ldap-haskell has some nice things and is probably a better way forward. The fun contribution will be SASL bind. The TLS option is fairly trivial - I think if you're in a hurry and don't want to wait for it from ldap-haskell, you might be able to hack something up yourself. Now that I look at what I did with ldap_set_option, I recall that s actually a little difficult to assign a type, since the third parameter is sometimes Int, Enum, TimeVal, String, and LDAP_OPT_* values overlap in a way that doesn't really say "enum" to me. So it's reasonably easy to do something, but harder to do it in a way that's generally useful. I'm also unsure of the portability of the things I'd propose to add. I know only the OpenLDAP API, but something about LDAP_OPT_X_TLS hints that it may be non-standard. Donn Cave, donn(a)drizzle.com

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Newbie Q: Deriving MyOrd from Eq problem
by Dmitri O.Kondratiev 26 Jul '06

26 Jul '06

I am trying to derive MyOrd class from Eq (Prelude): class Eq a => MyOrd a where (%<=), (%>), (%>=) :: a -> a -> Bool x %<= y = (x < y || x == y) x %> y = y < x x %>= y = (y < x || x == y) I get these errors: ClassTest.hs:28:21: Could not deduce (Ord a) from the context (MyOrd a) arising from use of `<' at C:/wks/haskell-wks/ClassTest.hs:28:21 Probable fix: add (Ord a) to the class or instance method `%<=' In the first argument of `(||)', namely `x < y' In the definition of `%<=': %<= x y = ((x < y) || (x == y)) In the definition for method `%<=' ClassTest.hs:29:20: Could not deduce (Ord a) from the context (MyOrd a) arising from use of `<' at C:/wks/haskell-wks/ClassTest.hs:29:20 Probable fix: add (Ord a) to the class or instance method `%>' In the definition of `%>': %> x y = y < x In the definition for method `%>' ClassTest.hs:30:21: Could not deduce (Ord a) from the context (MyOrd a) arising from use of `<' at C:/wks/haskell-wks/ClassTest.hs:30:21 Probable fix: add (Ord a) to the class or instance method `%>=' In the first argument of `(||)', namely `y < x' In the definition of `%>=': %>= x y = ((y < x) || (x == y)) In the definition for method `%>=' Failed, modules loaded: none. Q: What's wrong? Why 'Ord' gets into play here? Thanks, Dima

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ghc hackathon start time?
by Greg Fitzgerald 26 Jul '06

26 Jul '06

What time on September 14th will the GHC Hackathon begin? I don't need a precise answer, just a quick one. Plus or minus an hour will do. Thanks, Greg

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Re: [Haskell-cafe] Newbie Q: Deriving MyOrd from Eq problem
by voigt.16734551＠bloglines.com 26 Jul '06

26 Jul '06

Hi Dmitri, I don't have answers for all your questions. But some, at least. --- Dmitri O.Kondratiev" <dokondr(a)gmail.com wrote: > -- Ok, then I can derive MyOrd class directly from Ord: > > class Ord a => MyOrd a where > (%<), (%<=), (%>), (%>=) :: a -> a -> Bool > x %< y = x < y > x %<= y = (x < y || x == y) > x %> y = y < x > x %>= y = (y < x || x == y) > > instance (MyOrd a, MyOrd b) => MyOrd (a,b) where > (x1, y1) %< (x2, y2) = (x1 %< x2) && (y1 %< y2) > (x1, y1) %> (x2, y2) = (x1 %> x2) && (y1 %> y2) > (x1, y1) %<= (x2, y2) = (x1 %<= x2) && (y1 %<= y2) > (x1, y1) %>= (x2, y2) = (x1 %>= x2) && (y1 %>= y2) > > greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool > greaterMyOrd (x,y) (z,t) = (x,y) %> (z,t) > > -- This should work, right? Yet I get this error message: Only if you tell the compiler to allow constraints of the form (C t), where C is a class and t is not a variable. That's exactly what GHC tells you to try, using -fglasgow-exts. > -- Notwithstanding :) when i comment out declaration: > > -- greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool > > -- program gets compiled and checking type of 'greaterMyOrd' gives: > > *ClassTest> :t greaterMyOrd > greaterMyOrd :: (MyOrd (a, b)) => (a, b) -> (a, b) -> Bool > > -- which is the same as I tried to declare in the program source. > What is hapenning here? That is indeed a bit funny. Using your instance declaration, GHC could have reduced the constraint "MyOrd (a,b)" to "MyOrd a, MyOrd b". I think there was a discussion on why it does not do so very recently. Maybe you can find it in the archive. > -- Now, when trying to use function 'greaterMyOrd' I get: > > *ClassTest> greaterMyOrd (2, 3) (1, 2) > > <interactive>:1:0: > Ambiguous type variable `a' in the constraints: > `MyOrd a' arising from use of `greaterMyOrd' at <interactive>:1:0-11 > `Num a' arising from the literal `2' at <interactive>:1:14 > Probable fix: add a type signature that fixes these type variable(s) That is because GHC doesn't know which type your literal 2 comes from. So it doesn't know whether there is a MyOrd instance for it, or which to choose. The Num comment comes from GHC only knowing that literal 2 will be of some type in the Num class. But that doesn't tell it anything about possible MyOrd instances. > -- Then I try to declare argument types explicitely and get this: > > *ClassTest> greaterMyOrd (2::MyOrd, 3::MyOrd) (1::MyOrd, 2::MyOrd) > > <interactive>:1:17: > Class `MyOrd' used as a type > In an expression type signature: MyOrd > In the expression: 2 :: MyOrd > In the first argument of `greaterMyOrd', namely `(2 :: MyOrd, 3 :: MyOrd)' > *ClassTest> Well, just as GHC tells you, you should use a type for the signature, not a typeclass. Try something like greaterMyOrd (2::Int, 3::Int) (1::Int,2::Int). Maybe the annotations on the first tuple suffice. > -- I am lost. Please enlight me, what am I doing wrong trying to > create my own class, its instance and then using it. Hope that helped. BTW, you might get problems with overlapping instances. Because with your instance declaration there are two possible ways, for example, to get a MyOrd instance for (Int,Int). The first is via: Ord Int => MyOrd Int => MyOrd (Int,Int) The second is via: Ord Int => Ord (Int,Int) => MyOrd (Int,Int) Ciao, Janis.

1 0

Frantk
by Alvaro Galan 26 Jul '06

26 Jul '06

Hi, Im trying to use Frantk from winhughs but i dont know how to import the file, where to put the sources to work, how to import other libaries, etc. If i wanto to import Frantk.lh where i have to place (somo folder, inside the winhugs tree, where?) it or import it? Thanks in advance. Alvaro. -- It's only Rock'n'Roll (..but I like it)

2 1

scripting in haskell
by mvanier 26 Jul '06

26 Jul '06

Hi, I was playing around with runhaskell (runghc to be precise), and I discovered the limitation wherein you have to use the file suffix ".hs". Don't get me wrong, runhaskell is great, but if you didn't have that restriction it would make haskell much more attractive to many programmers who write standalone scripts. Of course it's always possible to do e.g. (file: hello) #! /bin/sh runhaskell Hello.hs (file: Hello.hs) module Main where main :: IO () main = putStrLn "hello, world!" but that's pretty inconvenient. What I'd like is: (file: hello) #! /usr/bin/env runhaskell module Main where main :: IO () main = putStrLn "hello, world!" which is _almost_ doable; you have to name the file "hello.hs". What would really be optimal is something like (file: hello) #! /usr/bin/env runhaskell -fglasgow-exts -- extra arguments to runhaskell !# module Main where main :: IO () main = putStrLn "hello, world!" Looking through the mailing list I see something from May 2005 on this topic about adding the equivalent of gcc's "-x" option to ghc. What's the status of that? Cheers, Mike

3 3

Universal machines in Haskell
by dons＠cse.unsw.edu.au 26 Jul '06

26 Jul '06

Now the ICFP contest is now over, several of the entrants have been interested in seeing if we can't write a decent universal machine (the 'hardware' of this year's contest), in Haskell. This is an interesting problem for Haskell, since the spec encourages the use of mutable variables and mutable arrays, as well as a bit of IO. Getting C-like performance from Haskell, for this problem, is a bit of a challenge, and highlights some corners in our libraries. So, we're collecting any UMs in Haskell, with the goal to breed some good solutions, and learn a bit about writing low level machines in Haskell. The website is here, http://www.cse.unsw.edu.au/~dons/um.html If you wrote a Haskell UM, or a faster C UM, free to submit it! Just mail me a darcs repo url, a source module url, or the source itself. -- Don

1 0

Why Haskell?
by Matthew Bromberg 25 Jul '06

25 Jul '06

I am currently in the middle of a reasonably large software simulation of a wireless network that I'm programming in Haskell and in C. Progress has been slower than anticipated and I occasionally (probably because of looming deadlines) ask myself did I take the right approach? Here are the reasons why I chose Haskell initially 1) It's mathematical elegance, I thought would lend itself to the complex mathematical expressions I am implementing. 2) It rates reasonably high in performance comparisons with other languages http://shootout.alioth.debian.org/gp4sandbox/benchmark.php?test=all&lang=all 3) It has a very active community who are courteous and quite helpful 4) It has an interactive shell. 5) It integrates easily with C. The idea would be to implement hooks to some fast matrix libraries and some plotting routines in C and then develop the higher level logic in Haskell. Since marshalling arrays and such, is a bit of a pain I structured my C matrix library in the form of stack based machine, using reference counting to handle memory management and to minimize the amount of actual data I need to pass between C and Haskell. This seems to work fairly well. However I have been somewhat disabused of my original thought about how development would proceed. I had hoped that I could get within a factor of two or so of the fast prototyping, development times of a good scripting language such as Matlab, Python or Lua, but I don't think I'm achieving this unfortunately. Here are some reasons why 1) Lack of debugging support. Yes there are print statements and trace, but I would like to set a breakpoint. It would be nice to do so and launch the GHCi interpreter with all the variable context supported. A google search revealed that there is current work on this, but unfortunately the package is in cabal, which has spotty support in windows it seems. 2) Recompiling binaries (necessary in order to link in foreign object code into GHCi) is slow using GHC. Moreover I have to restart GHCi if I want to reload a changed DLL (unless there is a way to unload a DLL in GHCi). It also requires jumping around between several console windows to get the job done. (I'm not using an IDE does one exist?) 3) Lack of automatic type coercion for closely related types. In particular I have to use CInt and CDouble to go into and out of C. Unfortunately only a small subset of library functions will actually support CInt so I have to explicitly coerce this values. I usually have a host of compile errors initially because of this silly issue and it makes the code ugly. What would help would be an automatic type conversion from CInt to Int and CDouble to Double. Perhaps some warning level could flag these if this were 'too dangerous'. 4) GHCi is really not as useful as I'd hoped. You can not just cut and paste Haskell code from a text file and run it in the interpreter. There is also this context issue concerning what modules are actually in scope. So although in Haskell once I import a module, all of its functions are in scope, in GHCi, if I load this module only the exported functions from that module are in scope. The result, again, is that I can not get an apples to apples idea of what is happening in my code. Thus I have been using GHCi primarily as a syntax checker for Haskell constructs. 5) Learning new programming patterns may have a role to play, but sometimes things like iteration is cleaner than recursion. I worry about performance issues however. I had to create a 50 Mbyte matrix and I used what I thought, initially was an elegant contruction technique in Haskell. Something like this do ... sequence $ [ reffill b s | s <- [0..(fi temits)-1], b <- [0..(fi nc)-1]] ...(push list on to matrix stack) The refill function generates a matrix based on basestation index b and SU index s and pushes it on the Matrix stack in C, inside an IO monad. So it returns IO(). The list comprehension ends up doing a cartesian product of all (b,s) pairs which in this case is a large number. fi is my shorthand for doing an annoying CInt to Int conversion. The problem with this code is that it takes about 500 Mbytes in memory instead of 50 Mb. Apparently it loads up all the potential IO actions into the huge list before executing it, taking the factor of 10 or so hit. There is probably a nice space saving way of doing this, that executes each IO action before generating the next one, but I was in too much of a hurry to figure it out. This also makes me wonder about the efficiency of the C interface. Each IO action calls a C function (using the unsafe specifier in the import declaration). I hope there isn't too much overhead with this. In my case I do almost no marshalling. Most calls have no arguments and no returns, or at most require a handful of CInts or CDoubles. My application has 60-70% of it's statements in an IO monad calling lots of C code. I begin to wonder if all those statements shouldn't in fact just be in C to begin with. Thus I begin to wonder why I'm using Haskell. Maybe I just need some encouragement that my choice will pay off in the end.

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Internships on GHC and Haskell at MSR Cambridge
by Simon Peyton-Jones 25 Jul '06

25 Jul '06

Gentle Haskellers Would you be interested in working for three months at Microsoft Research, Cambridge, on a project related to GHC or Haskell? MSR Cambridge now takes interns *year-round*, not just in the summer months. Simon Marlow and I are keen to attract motivated and well-qualified folk to work with us on our research, and on improving or developing GHC. If you or one of your students is interested, you can find more details here http://hackage.haskell.org/trac/ghc/wiki/Internships Our next empty slot is the Oct-Dec period, but you are welcome to apply for some later period. Simon PJ PS: There are lots of other programming-language folk at MSR (F#, security, probabilistic languages...), and stuff beyond that (machine learning, systems and networks...); see http://research.microsoft.com/aboutmsr/labs/cambridge/

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Re: Re[4]: [Haskell-cafe] REALLY simple STRef examples
by Chad Scherrer 23 Jul '06

23 Jul '06

The IO monad hasn't given me too much trouble, but I want to be sure to structure things the way "they should be". If I get everything running using IO first and then have type-checking problems with ST, it will be tempting to just slap on an unsafePerformIO and call it good. Sure, it's really doing the same thing anyway, but it just comes out looking like a hack. On 7/20/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Hello Chad, > > Friday, July 21, 2006, 12:26:58 AM, you wrote: > > > Ok, I see now why the return is necessary. > > btw, it may be helpful to read "IO inside" material. ST monad is not > very different from IO monad - it only limited to operations on STRef > and STArray, so that it can't have side-effects visible outside of > runST statement used to run ST computation >

5 4

incoherent instance selection when it should be still coherent
by Ralf Lammel 22 Jul '06

22 Jul '06

The following pain is sort of ghc (6.4.2) specific. (The same behavior is achievable with hugs -98 +O which is Ok in so far that +O (as opposed to +o) is not strongly supposed to be coherent.) Note the following GHC options preceding the code. They do not contain -fallow-incoherent-instances. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} -- A single parameter class with two silly instances class Foo x where foo :: x -> (); foo = const () instance Foo () instance Foo Bool -- A two-parameter class with a generic and a non-generic instance class (Foo x, Foo y) => Bar x y where bar :: x -> y -> String instance (Foo x, Foo y) => Bar x y where bar _ _ = "generic instance" instance Foo z => Bar () z where bar _ _ = "non-generic instance" -- An existential wrapper around foos data Wrap = forall x. Foo x => Wrap x -- A wrapper-based variation on the type-class member bar uuh :: Wrap -> Wrap -> String uuh (Wrap x) (Wrap y) = bar x y -- Let's try all unwrapped and wrapped combinations of bar and uuh t1 = () t2 = True w1 = Wrap t1 w2 = Wrap t2 main = do print $ bar t1 t1 print $ uuh w1 w1 -- uuh! print $ bar t1 t2 print $ uuh w1 w2 -- uuh! print $ bar t2 t1 print $ uuh w2 w1 print $ bar t2 t2 print $ uuh w2 w2 We get: {- "non-generic instance" "generic instance" "non-generic instance" "generic instance" "generic instance" "generic instance" "generic instance" "generic instance" -} This means that the generic instance is consistently chosen by uuh. This is clearly incoherent. I would also complain that uuh type-checks in the first place. Opinions? Regards, Ralf

1 0

HXT - problems with runIOSLA or xread
by Niklas 21 Jul '06

21 Jul '06

Hi everybody, and especially Uwe Schmidt et al. I ran into some problems trying to process some xml from a string source, illustrated in the code below. From file, using readDocument, everything works great though. I've probably missed a finer detail or two in the arrow handling, or it could be a bug in HXT (if I run the code in ghci, it will crash hard). Output: -->main.exe NTree (XTag (QN {namePrefix = "", localPart = "small", namespaceUri = ""}) []) [ NTree (XTag (QN {namePrefix = "", localPart = "xml", namespaceUri = ""}) []) [], NTree (XTag (QN {namePrefix = "", localPart = "example", namespaceUri = ""}) []) []] --> So it seems that the string gets read, since it shows up as a result from procString, but I get no output from writeDocument in writeStrDoc. Could anybody enlighten me? I'm using GHC 6.5 and HXT 6.0 on Windows XP. Regards, /Niklas ---8<--- module Main where import Text.XML.HXT.Arrow wa = [(a_indent, "1"), (a_remove_whitespace, "1")] main = procString tststr >>= mapM_ print . snd procString = runIOSLA writeStrDoc (initialState ()) writeStrDoc = xread >>> writeDocument wa "-" tststr = "<small><xml/><example/></small>" --->8---

1 0

Still not dead
by Einar Karttunen 21 Jul '06

21 Jul '06

Hello As many of you may have noticed I have been away for some months. This has been due to health problems which have unfortunately kept me unable to work on Haskell projects. I am not dead and will be working on resolving the backlog of messages (will probably take a week). I will be slowly back to hacking things when I get everything fixed. - Einar Karttunen

1 0

seq - RTFM?
by Dusan Kolar 21 Jul '06

21 Jul '06

Hello all, my question is probably dull. So answers to better investigate manual are welcome. Why is this correct? ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4.1, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Prelude> putStr "Ahoj\n" Ahoj Prelude> putStr "Ahoj\n" `seq` 3+3 6 Prelude> :q Leaving GHCi. And not Prelude> putStr "Ahoj\n" Ahoj Prelude> putStr "Ahoj\n" `seq` 3+3 Ahoj 6 ??? Does it have something common with monads or is it a behavior of seq? Thanks, Dusan

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Re: [Haskell-cafe] REALLY simple STRef examples
by Chad Scherrer 21 Jul '06

21 Jul '06

Hi, > The short answer: use > runST (long expression) > rather than > runST $ long expression > > when it comes to higher-ranked functions such as runST. I suppose the same holds for runSTUArray, right? But this still gives me that same error, about being less polymorphic than expected. (+=) x y = let updateX i = do xi <- readArray x i writeArray x i (xi + y!i) in sequence_ . map updateX $ indices x sumArrays [] = error "Can't apply sumArrays to an empty list" sumArrays (x:xs) = runSTUArray result where result = do x0 <- thaw x mapM_ (x0 +=) xs x0 -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: Why is there no splitBy in the list module?
by dons＠cse.unsw.edu.au 21 Jul '06

21 Jul '06

marco-oweber: > There is already lines. > Why not generalise it to take an additional parameter '\n' and call it > split or splitBy? > There are some cases where you want to split a list not on '\n'. This comes up a lot. I thought at some time last year there'd been a long discussion about all the various flavours of 'split' that we could come up with. But then, what happened to the code? Did we not agree on a version to go into Data.List? Hacking up your own custom split (or a tokens/splitOnGlue) must be one of the most common questions from beginners on the irc channel. Even fps has a split/splitBy: http://www.cse.unsw.edu.au/~dons/fps/Data-ByteString.html#v%3Asplit http://www.cse.unsw.edu.au/~dons/fps/Data-ByteString.html#v%3AsplitWith http://www.cse.unsw.edu.au/~dons/fps/Data-ByteString.html#v%3Ajoin Anyone rememeber what the result of the "let's get split into the base library" movement's work was? -- Don (Moved to haskell-cafe@)

10 16

Re: rigid variables
by Rodney D Price 20 Jul '06

20 Jul '06

Thanks for the responses. The semantics of check as described the "Data Invariants" paper on STM are really close to what I am looking for. Unfortunately, that work is not in GHC yet, so I'm attempting my own poor substitute (below). I have a store that is to be accessed by other threads, and I want to throw an error under certain conditions. (The names below are borrowed from the "Scheme in 48 Hours" tutorial.) In reality I'm looking for an inconsistent store, but for simplicity's sake I've just put in a standard read operation below. Is there an easier way? Thanks, -Rod -- module Store where import Control.Monad.Error import Control.Concurrent.STM data StoreError = DoesNotExist String | Default String instance Error StoreError where noMsg = Default "Store error" strMsg = Default type ThrowsError = Either StoreError type IOThrowsError = ErrorT StoreError IO type Store a = TVar [ (String, TVar a) ] -- read from the store getSTM :: String -> Store a -> STM (ThrowsError a) getSTM name store = do st <- readTVar store case lookup name st of Nothing -> return (Left $ DoesNotExist name) Just rd -> readTVar rd >>= return . Right get :: String -> Store a -> IOThrowsError a get name store = do d <- liftIO $ atomically $ getSTM name store case d of Left err -> throwError err Right d' -> return d'

1 0

doubt on gui
by Alvaro Galan 20 Jul '06

20 Jul '06

Hi, im almost new in haskell world, but im trying to do a simple graphical interface for a small program, i developed the program under winhugs, and now i want to develop the gui also with it, but all the libraries and kits that i download from the haskell page is either for ghc or need to be make or stuff, my question, is or isnt a library or something that i can download, then import and use it on winhugs without compile, download a ton of things, etc etc? is there some way of developing a very easy gui (based on radio buttons) with normal librarys on winhugs?, or its compulsory get ghc? Kind Regards. Alvaro. -- It's only Rock'n'Roll (..but I like it)

2 1

Re: [Haskell] rigid variables
by Robert Dockins 20 Jul '06

20 Jul '06

[moved to cafe] On Jul 20, 2006, at 12:48 PM, Rodney D Price wrote: > I've gotten this sort of error several times, which mysteriously > disappears > when I add more functions to the code: > > storeError.hs:13:38: > Couldn't match expected type `a' (a rigid variable) > against inferred type `String' > `a' is bound by the type signature for `throwError' > at <no location info> > Expected type: a > Inferred type: String > In the first argument of `return', namely `msg' > In the call (return msg) > > (This is GHCi.) The code is below. The type variable a can't be > bound to > String, obviously, but a relative novice like myself has no idea > why. Can > someone tell me? > > Thanks, > > -Rod > > -- > module Store where > > import Control.Monad.Error > import Control.Concurrent.STM > > data StoreError = Default String > > instance Error StoreError where > noMsg = Default "Store error" > strMsg = Default > > instance MonadError StoreError STM where > throwError (Default msg) = return msg Lets take a look here at the definition of MonadError from Control.Monad.Error: class Monad m => MonadError e m | m -> e where throwError :: e -> m a catchError :: m a -> (e -> m a) -> m a In the signature for 'throwError' there are three type variables: e, m and a. e and m are bound by the instance declaration, but a is free. In Haskell the rule is that free variables are implicitly bound with a universal quantifier. So, the type for throwError can be regarded as, throwError :: forall a. e -> m a, for some concrete choices of e and m which are determined by the instance. The forall means that a user of this function can put any type there that he likes. In other words, the monadic action created by 'throwError', when executed, evaluates to _a value of any type at all_ (yes, I know the terminology is a little loose here). That means you can't just 'return msg', which has type 'm String' because a user might have used throwError to create an action of type 'm Int', for example. In fact, you won't really be able to return anything at all, because there isn't any way to write a program that can generate a value of any unknown type. This should hopefully correspond to your intuition about what throwing an exception does. The error generated by the typechecker basically tells you that the function you have written is not polymorphic enough. It has type 'StoreError -> STM String' rather than 'forall a. StoreError -> STM a' as it ought. I think perhaps you have misunderstood how MonadError is used. The idea is to expose to users a particular non-local control flow construct (throw/catch style exceptions) by hiding all the stuff necessary for that inside the monad plumbing. Usually, whoever writes the monad itself will provide the necessary instances. It's often not possible to write instances like this by using the external API of the monad. This is particularly the case for the abstract monads available in GHC (IO, ST, and STM). In short, I don't think you'll be successful in writing a 'MonadError' instance for STM that has the customary semantics. What you may be looking for is the ErrorT monad transformer, which will let you layer error handling over STM. It's hard to know with what info you've provided here. If you give a few more details on what you're trying to accomplish, someone may be able to give you a push in the correct direction. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

1 0

Re: [Haskell-cafe] REALLY simple STRef examples
by Chad Scherrer 20 Jul '06

20 Jul '06

Whoa. That changes everything I thought I knew about ($). Come to think of it, one of the examples that does work it written main = print $ runST f where f is defined separtely. So that's consistent. I'll take a look at the references. Thanks! > > Indeed. The short answer: use > runST (long expression) > rather than > runST $ long expression > > when it comes to higher-ranked functions such as runST. > A longer answer: > http://www.haskell.org/pipermail/haskell-cafe/2004-December/008062.html > > > let me know what i need to read to fix it myself > MLF (see Daan Leijen, A. Loeh, `Qualified types for MLF', ICFP05) > >

1 0

trace function
by Alexander Vodomerov 20 Jul '06

20 Jul '06

Hello! The function trace is supposed to write debug messages to console. However, when I trying to run the following program import Debug.Trace main = do putStrLn "xxx" return (trace "yyy" ()) putStrLn "zzz" only xxx and zzz is displayed. yyy is missing. Why trace is not working? PS. GHC-6.4.2, Debian Linux on x86. With best regards, Alexander.

3 2

Opening a file that another process is writing
by Maurício 20 Jul '06

20 Jul '06

Hi, I want to open for reading a log file that another process is locking for write. I know it's possible, since 'cat' and 'vim' can read that file (but not edit it, of course). How can I do that in Haskell? 'openFile' says "permission denied". Thanks, Maurício

2 1

problems with receiving mail lists
by Bulat Ziganshin 20 Jul '06

20 Jul '06

Hello Simon, Simon, you are administrator of many haskell mail lists, so i wrote to you. last day i don't receive messages in these mail lists. investigating the problem, i found on haskell-cafe subscription page that there are some problems in mail delivery to my address. i'm not 100% sure but i think that it is because gmail server last days work too slow and in many cases login to aborts because of timeouts. moreover, i think that the same problem with receiving messages from haskell mail lists may expose other gmail users i propose to temporarily increase timeouts of message delivery system which serves these mail lists. may be there is other solutions please duplicate your message to cafe too, because i'm not sure that i will receive it directly ps: and question to other gmail users - are you had problems with receiving email? may be it's some local problem? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

2 1

test message
by Simon Marlow 20 Jul '06

20 Jul '06

Trying to diagnose problems with mailing lists...

1 0

The History of Haskell
by Simon Peyton-Jones 20 Jul '06

20 Jul '06

Friends, Phil Wadler, John Hughes, Paul Hudak and I have been writing a paper about the The History of Haskell We've submitted an earlier draft to the History Of Programming Languages conference (HOPL'07), and it's been accepted. We have to submit a more-or-less final draft by 1 September. This message is to invite you to read it, tell us what you think, and help us improve it. Here it is, along with a Wiki page for you to write comments: http://haskell.org/haskellwiki/History_of_Haskell Enjoy! Simon PS: If you followed a link earlier today, and have a version dated Feb, discard it. The one you want is dated July 14th.

4 3

REALLY simple STRef examples
by tpledger＠ihug.co.nz 19 Jul '06

19 Jul '06

Chad Scherrer wrote: > x = runST $ return 1 > > y = runST $ do {r <- newSTRef 1; readSTRef r} > > Neither of these works in ghci x = runST (return 1) y = runST (do {r <- newSTRef 1; readSTRef r}) The escaping s is something to do with rank 2 polymorphism. (Search for "rank" in the ghc user guide, for example.) The hassle is that runST must always be applied to an argument (e.g. not passed to ($)), and a benefit is that you're protected from using an STRef you created in the context of one runST, in the context of another runST. Regards, Tom

1 0

trace function
by tpledger＠ihug.co.nz 19 Jul '06

19 Jul '06

Alexander Vodomerov wrote: > import Debug.Trace > > main = do > putStrLn "xxx" > return (trace "yyy" ()) > putStrLn "zzz" > > only xxx and zzz is displayed. yyy is missing. > Why trace is not working? Nothing uses the value of (trace "yyy" ()), so it is never evaluated. Try this instead, which uses the value for a pattern match: () <- return (trace "yyy" ()) Or this, which makes the trace part of the sequence of IO actions: trace "yyy" (return ()) HTH, Tom

1 0

Compiling ghc for using STM
by Asfand Yar Qazi 19 Jul '06

19 Jul '06

Hi, I finally got my spiffy dual-core processor (an Opteron 165 no-less) and want to learn STM, since I think it and haskell are the future of concurrent programming. How do I compile Haskell to be able learn STM on it, using "proper" threading? I know there's a parallel haskell flag, but I read somewhere about it running on top of some special server that lets it work in parallel threads or something. Please excuse me if I'm waffing, can anybody help me? Thanks

4 4

irc channel stats
by dons＠cse.unsw.edu.au 19 Jul '06

19 Jul '06

While we're in a period of reflection, pondering the history of haskell, I've prepared some graphs of activity on the IRC channel. Summary: its growing much as the mailing lists are, with more than 5000 users over the past 5 years. Full details here, http://www.cse.unsw.edu.au/~dons/irc/ Another interesting note is that lambdabot's get more and more verbose over time, as it provides further features. It's turning into an ide of sorts. -- Don

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Re: Type-Level Naturals Like Prolog?
by Jared Warren 19 Jul '06

19 Jul '06

Thank you to everyone for the responses. I guess what I should have clarified is that I know how Peano numbers are *normally* encoded in the type language (I am very familiar with the HList library), but I would like to know why the type language appears to require data structures to do so while [Idealised] Prolog has none. Niklas Broberg helpfully corrected my Prolog: > That is not a valid encoding of peano numbers in prolog, so I think > that's where your problems stem from. :-) > > % defining natural numbers > natural(zero). > natural(s(X)) :- natural(X). > > % translate to integers > toInt(zero, 0). > toInt(s(X), N) :- toInt(X, Y), N is Y + 1. Thank you. I can now more precisely state that what I'm trying to figure out is: what is 's', a predicate or a data structure? If it's a predicate, where are its instances? If not, what is the difference between the type language and Prolog such that the type language requires data structures?

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Type level logic programming terminology
by oleg＠pobox.com 19 Jul '06

19 Jul '06

Most systems of (first-order) logic differentiate between function letters (aka, symbols) and predicate letters (symbols). The former are used to build terms; the latter build atomic formulas (which can later be combined in more complex formulas using negation, conjunction, disjunction, and quantification): http://plato.stanford.edu/entries/logic-classical/ Formulas like Even (succ zero) can be interpreted to be `true' or `false' (alternatively: succeed or fail). In the atomic formula above, Even is a predicate symbol. OTH, terms like `succ zero' (an application of a function symbol succ to a constant zero) is not interpreted as true or false: it just is. In Prolog, there is likewise distinction between terms (which can be compared and unified, but cannot 'fail') and goals. The following gives a good introduction in the typed setting: http://www.cs.mu.oz.au/research/mercury/information/doc-release/reference_m… Incidentally, many Prolog systems also permit declarations of goals and of terms (because it helps produce better code). Lambda-Prolog, like Mercury, insists on declarations. The distinction between terms and goals is a bit blurred in Prolog, due to its reflection facilities: one can add a new term as a rule or a fact (e.g., using assert), one can examine the rules or facts, etc. In Haskell type-level programming: type -> term type constructor -> function symbol class name -> predicate symbol class constraint -> atomic formula (atomic goal) There is a subset of Prolog called Datalog, which is commonly defined as Prolog without function symbols. Logic without function symbols is equivalent to logic with function symbols, _provided_ we have the infinite supply of constants. For example, Foo(f(x),g(x)) can be represented as exists y y'. F(x,y) & G(x,y') & Foo(y,y') So, each function symbol f of arity n is represented as a predicate F of arity n+1. That predicate is a functional predicate: for each combination of the first n arguments, there exists exactly one object (for the last argument) that makes the predicate hold. Since terms in logic are finite, the infinitely countable supply of constants is enough. In Datalog however, the extensional database is finite. There is a finite number of facts and hence the finite number of constants. That breaks the above correspondence, and so terms like f(f(f(... x))) can no longer be emulated as the corresponding F predicate is finite (its domain is finite). That's why in pure Datalog (without negation or with stratified negation), termination is assured. This is not the case in pure Prolog because of the possibility to construct longer and longer terms like succ(succ(x)). Regarding termination, minimal models, and the correspondence between logic formulas (and pure, positive, stratified, etc. Datalogs) and P-complete etc. programs, please see http://www.csupomona.edu/~jrfisher/www/logic_topics/positive/positive.html

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deepSeq vs. rnf?
by Chad Scherrer 18 Jul '06

18 Jul '06

Now and again I see references to "deepSeq", and I've never understood how this could be any different than using rnf from Control.Parallel.Strategies. Is there really any difference? When is it better to use one or the other? Thanks, -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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XML transformation difficulties
by Greg Fitzgerald 18 Jul '06

18 Jul '06

I'm trying to think of a way to translate this input, to the output below: Input: <test> <foo name="a"> <wahoo>A</wahoo> </foo> <foo name="b"> <wahoo>B</wahoo> </foo> <foo name="c"> <wahoo>C</wahoo> </foo> <group> <bar ref="b"/> <bar ref="a"/> </group> </test> Output: <test> <group> <wahoo>B</wahoo> <wahoo>A</wahoo> </group> </test> That is, anywhere there is a 'bar', replace it with the contents of the 'foo' it references. I'm having a difficult time representing this with HXT's Arrow API because the value of the 'ref' attribute is the output of an arrow, but I need it to be just a plain string so that I could use it as an input parameter to the 'hasAttrValue' function. A similar problem, using 'processTopDown', once I traverse to a 'bar' node, I need to then traverse the root again to find the 'foo', but I'm in the context of the 'bar' node, not the root. My ears are open to solutions with HaXML or Scrap Your XML-plate, or anything else. Thanks, Greg

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announce: (preliminary) dbus haskell bindings
by Evan Martin 18 Jul '06

18 Jul '06

"D-Bus is a message bus system, a simple way for applications to talk to one another." [1] It's particularly popular on free software desktops (Gnome, KDE). HDBus wraps the DBus APIs so your Haskell code can broadcast messages and make calls to services. For example, on my recent Ubuntu system, these bindings are sufficient to pop up balloon-tip-style notifications via a daemon I wasn't previously aware of. Very preliminary code, so please let me know if you find it useful. (Also, I don't know where it's appropriate to announce this, so please direct me to other mailing lists.) Home page: http://neugierig.org/software/hdbus/ Haddock: http://neugierig.org/software/hdbus/doc/ Repository browser: http://neugierig.org/software/darcs/browse/?r=hdbus;a=summary [1] http://www.freedesktop.org/wiki/Software/dbus

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