- Haskell-Cafe - Haskell.org

Re: [Haskell] Why functional programming matters
by Don Stewart 31 Jan '08

31 Jan '08

stephan.friedrichs: > Simon Peyton-Jones wrote: > >[...] > > > >2. War stories from real life. eg "In company X in 2004 they rewrote > >their application in Haskell/Caml with result Y". Again, for my purpose > >I can't tell very long stories; but your message can give a bit more > >detail than one might actually give in a presentation. The more concrete > >and specific, the better. E.g. what, exactly, about using a functional > >language made it a win for you? > > > > We [1] implemented an ad-hoc chat system in Haskell in the SEP [2] at > the TU-Braunschweig. Wow! Looks like lots of great code in there, http://sep07.mroot.net/documentation/barracuda/ Text.ParserCombinators.Parsec.XML Codec.Encryption.PKCS1 Network.GnuTLS Data.Heap Will we see this packaged for hackage.haskell.org soon? Some of those modules look very reusable. -- Don

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trying to use unix's openFD ..
by Galchin Vasili 31 Jan '08

31 Jan '08

Hello, The signature for openFD is openFd: FilePath -> OpenMode -> Maybe FileMode -> OpenFileFlags -> IO Fd I am currently reading http://www.haskell.org/ghc/docs/latest/html/libraries/unix/System-Posix-Fil… and diving down into links but can't find the definition of data type FileMode. ??? Basically I want to open in O_RDONLY filemode. Kind regards, Vasili

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Improving 'project'
by Justin Bailey 31 Jan '08

31 Jan '08

All, project doesn't easily let me add a few columns to an existing query (or take a few columns away). Instead, each use of project requires me to build the entire list of columns I'd like to pass on by hand. Before I go further, if there is a way to do that, please let me know. An example of what I'd like to do is below. Imagine I want to add a "selected" boolean column to my query. select_customers = do cust <- table customers project ( (selectedField <<- False) # cust) So I get all the columns in my customers table, plus the "selected" column. Unfortunately the code above doesn't work. cust has type "Rel r", and project wants a "Record r", so I originally tried something like: select_customers = do let toRec :: Rel r -> Record r toRec _ = undefined ... project (unRel cust) But project needs to recover the "Expr" values associated with 'r' and this fails. I then came up with this solution, which lets me add columns at the head of a query: class RecoverExpr a b where toExpr :: a -> b instance RecoverExpr RecNil RecNil where toExpr rec = RecNil instance (FieldTag f, RecoverExpr r r) => RecoverExpr (RecCons f (Expr b) r) (RecCons f (Expr b) r) where toExpr ~rec@(RecCons _ r) = RecCons (attribute . fieldName $ recField $ rec) (toExpr r) recField :: RecCons f a b -> f recField _ = undefined rest :: (RecoverExpr r r) => Rel r -> r rest qry = let unRel :: Rel r -> r unRel _ = undefined in toExpr . unRel $ qry Then I can write: select_customers = do cust <- table customers_tbl ... project (selectField <<- False # (rest cust)) And it actually works. However, it seems over-engineered. My questions are: 1) Is there a way to make project behave like I want without these tricks? 2) If not 1, would it make more sense for the Query monad to pack around the original Expr values? 3) If not 2, how can the solution above be better? I'd like to extend it to adding/removing arbitrary columns and more control over column order. Thanks for any and all thoughts! Justin

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Re: [Haskell-cafe] Simple network client
by Gary Bickford 31 Jan '08

31 Jan '08

One rather funky but effective solution might be to use the tftp protocol. No security, but simple, flexible and efficient. I think there are C libraries that implement it. This would take care of handshaking binary data. I have no idea if anyone has ever used it in Haskell. GB On Wed, 2008-01-30 at 12:50 -0500, haskell-cafe-request(a)haskell.org wrote: > robably not all of them, but some of them, definitely. > > If you want to transmit an arbitrary bytestring then I'm pretty sure > that transmitting a length word first is the way to go. An arbitrary > bytestring can have any value in it, so there are no values left to > act > as delimiters :) You'd have to have some kind of escaping mechanism, > like show, which is expensive. -- "Isn't it funny how the Global Village includes everybody but the villagers?" (http://wiki.laptop.org/go/OLPC_Publications)

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ffi and pass by value structs
by Federico Squartini 31 Jan '08

31 Jan '08

Is there a way in the foreign function interface to pass to a function a struct by value? I explain it better. There are two ways in c to pass structs, as in the following example: /* begin c code */ struct couple{ int a; int b; }; //by value int print_couple (struct couple cpl) { printf("%d\n", cpl.a); printf("%d\n", cpl.b); return(0); } //by address int print_couple_ptr (struct couple *cpl) { printf("%d\n", cpl->a); printf("%d\n", cpl->b); return(0); } /* end c code */ Apparently in the ffi it is possible to import the second version, as: foreign import ccall "print_couple_ptr" printCouple::Ptr Couple->IO Int But not the first, as a struct it's not an atomic type. So how does one solve this issue? Cheers, Federico

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NDP
by Stephan Friedrichs 30 Jan '08

30 Jan '08

Hi all, is someone familiar with compiling ndp (nested data parallel Haskell), "Speed with less convenience"-version? I followed the guide at http://www.haskell.org/haskellwiki/Data_Parallel_Haskell/PackageNDP but executing "make" in the examples directory issues the following error: (.text+0x8d5): undefined reference to `__stginit_parallelzm1zi0zi0zi0_ControlziParallelziStrategies_' manually compiling the dotp example on the link mentioned above doesn't work either: [...] (.data+0x5c): undefined reference to `ndp_DataziArrayziParallelziUnliftedziFlatziUArr_zdf14_closure' DotP.o: In function `sz3_srt': (.data+0x88): undefined reference to `ndp_DataziArrayziParallelziUnliftedziFlatziBasics_toU_closure' DotP.o: In function `sz3_srt': (.data+0x8c): undefined reference to `ndp_DataziArrayziParallelziUnliftedziFlatziUArr_zdf14_closure' /home/sfriedrichs/ghc/rts/libHSrts.a(Main.o): In function `real_main': Main.c:(.text+0x12): undefined reference to `__stginit_ZCMain' Main.c:(.text+0x2c): undefined reference to `ZCMain_main_closure' Does someone know, what's wrong? I'm using ghc-6.9.20080124 as source base (which compiled and works just fine). //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Stacking monads - beginner design question
by Adam Smyczek 30 Jan '08

30 Jan '08

Hi, My application has to manage a data set. I assume the state monad is designed for this. The state changes in functions that: a. perform IO actions and b. return execution status and execution trace (right now I'm using WriteT for this). Is the best solution: 1. to build a monad stack (for example State -> Writer -> IO) or 2. to use IORef for the data set or 3. something else? Are monad stacks with 3 and more monads common? How could an example implementation look like? What I have for now is: -- Status data Status = OK | FAILED deriving (Show, Read, Enum) -- Example data set manages by state type Config = [String] -- WriterT transformer type OutputWriter = WriterT [String] IO Status -- example execute function execute :: [String] -> OutputWriter execute fs = do rs <- liftIO loadData fs tell $ map show rs return OK -- run it inside e.g. main (s, os) <- runWriterT $ execute files How do I bring a state into this, for example for: execute fs = do ?? conf <- get ?? -- get Config from state rs <- liftIO loadData conf fs ?? set conf ?? -- change Config and set to state tell "new state:" tell $ show conf return OK Do I have to use and how do I use StateT in this context: data DataState = StateT Config OutputWriter ?? and how do I run it runStateT . runWriterT? Thanks for help, Adam

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Deleting list of elements from Data.Set
by Gracjan Polak 30 Jan '08

30 Jan '08

My strictness analyser in my brain hurts. Which one (foldl,foldl',foldr) is the best way? Prelude Data.Set Data.List> let s = fromList [1,2,3,4,5] Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.0 ... linking ... done. Prelude Data.Set Data.List> foldl (.) id (Data.List.map Data.Set.delete [1,3,5]) s fromList [2,4] Prelude Data.Set Data.List> foldl' (.) id (Data.List.map Data.Set.delete [1,3,5]) s fromList [2,4] Prelude Data.Set Data.List> foldr (.) id (Data.List.map Data.Set.delete [1,3,5]) s fromList [2,4] Which one is best? -- Gracjan

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Security Notice: Buffer overflow fixed in PCRE library
by ChrisK 30 Jan '08

30 Jan '08

The PCRE library has just fixed a buffer overflow (related to UTF-8 mode). There are several haskell wrappers for the pcre library. If you use a wrapper for the PCRE library (libpcre) then you may want to upgrade the underlying library. http://pcre.org/news.txt states: > News about PCRE releases > ------------------------ > > Release 7.6 28-Jan-08 > --------------------- > > The main reason for having this release so soon after 7.5 is because it fixes a > potential buffer overflow problem in pcre_compile() when run in UTF-8 mode. In > addition, the CMake configuration files have been brought up to date. Cheers, Chris

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HDBC-odbc with GHC-6.8.2 and Cygwin
by Joseph Hallett 30 Jan '08

30 Jan '08

Version 1.1.4.0 of the HDBC-odbc package crashes GHCi when I issue: Prelude> :m +Database.HDBC.ODBC Prelude Database.HDBC.ODBC> connectODBC "" I'm using Cygwin and GHC-6.8.2 on a Windows XP machine. Furthermore, using connectODBC with a valid connection string also results in GHCi crashing (using GHCi-6.8.2 and HDBC-odbc-1.1.4.0) -- actually it causes GHCi to exit abruptly without an error message. On GHC-6.6.1, using HDBC-odbc-1.1.4.0, connectionODBC "" produces an uncaught exception without crashing GHCi. Any suggests for getting the package up and running on GHC-6.8.2? Joe --- 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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"data" and classes question
by Chaim Friedman 30 Jan '08

30 Jan '08

Hello, I had a question about type classes and data types. I want to create one class that has a function which depends upon a parameter from another class. I would assume that this can be done, but I can't seem to write code that does it. Can anybody tell me what is wrong? Here is an example of what I want to work: module Main where class Change c class State st where transitions :: (Change c) => st -> [(c, st)] data BinState = On | Off data BinChange = OnToOff | OffToOn -- get a list of (transition, newState) pairs: --binTransitions :: BinState -> [(BinChange, BinState)] -- doesn't work --binTransitions :: (State state, Change change) => state -> -- [(change, state)] -- doesn't work --binTransitions :: (State state1, State state2, Change change) => -- state1 -> [(change, state2)] -- doesn't work binTransitions On = [(OnToOff, Off)] binTransitions Off = [(OffToOn, On)] instance Change BinChange instance State BinState where transitions = binTransitions main = putStrLn "success." -- Thanks, -- -Chaim

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Performance Issues
by Adrian Neumann 30 Jan '08

30 Jan '08

Hello Haskell-Cafe! I've read about Control.Parallel and wanted to give it a try. Here's what I wrote: ----------- import Control.Parallel import Data.List splitList :: [Integer] -> [[Integer]] splitList = unfoldr f where f [] = Nothing f ~x = Just (splitAt 30000 x) map' :: (a->b) -> [a] -> [b] map' f (x:xs) =fx `par` (seq mfxs (fx:mfxs)) where fx = f x mfxs = map' f xs map' f [] = [] product' :: [Integer] -> Integer product' = foldr1 (\x y -> seq x (x*y)) fak :: Integer -> Integer fak n = product' $ map' product' (splitList [1..n]) main = putStrLn $ seq (fak 1000000) "done" ----------- This computes 1000000!. This version takes 8m29.189s to execute. Replace foldr1 with foldr and that goes down to 7m4.315s. Replace product' with the Prelude product and it takes only 6m17.685s. Why is that so? I'm using ghc 6.8.1 on Mac OS X. Regards, Adrian

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Enterprise Haskell AMQP library initial start and would like to pass it off to someone.
by Berlin Brown 29 Jan '08

29 Jan '08

I started a AMQP library; there really isn't a lot there but at least I was able to connect to the server. Here is the code and hopefully someone else can continue with the project. The AMQP protocol is moderately complex. HTTP is simple and stuff like RMI, JMS, Database Protocols are really complicated. AMQP seems to be in the middle. There is a lot of documentation and at least two good implementations. The included java implementation and I used the python implementation as a guide. http://www.iona.com/opensource/amqp/ http://barryp.org/software/py-amqplib/ Here is my source (in subversion): http://openbotlist.googlecode.com/svn/trunk/botlistprojects/botspider/spide… If were to make a suggestion; Barry's python code is really easy to follow. I would suggest using that as a guide. and if you are brave, you can really analyze the protocol and just go off the docs. Either way. My code got as far as connecting to the server, sending the protocol out and getting back an initial response. I didnt build a queue message (frame) or much else. -- Berlin Brown http://botspiritcompany.com/botlist/spring/help/about.html

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How to make GHC 6.6 and 6.8 co-exist -- was: First go at reactive programming
by Steve Lihn 29 Jan '08

29 Jan '08

I want to raise this issue again since I disagreed (see quoted text below) that cabal-install can solve the problem of moving from one version to another completely. For most /pure/ packages, it is probably true. But for those packages that have external depedencies, this is hardly so. For example, HDBC-OBDC package depends on where the odbc driver is installled. You actually have to modify the <package.cabal> file. The other package came to mind is the pcre-regex package depending on pcre lib. On my linux server, it is not obvious pcre-lib can be found in any standard location specified in <package.cabal> file. I believe there are and will be more packages like these. How can cabal-install install this kind of packages without knowing these external dependencies? They can be a path (prefix kind of things) or an environment variable, or even an option to GHC compiler or linker (I am not so sure about this one). In my brief experience with Ocaml's GODI, GODI has a way to specify them in a so-called config package. The install package then reads what it needs from the config package. In perl's CPAN shell, you can specify them in the cpan config file (to some extent). I suggest cabal-install looks into this so that moving from one version to another is indeed painless. 6.8.(1|2) is never the last Haskell version. The fact that Cabal records the 3rd digit version in the path indicates a diligent user has to re-install all the packages on every new 3rd-digit dot release. That is a lot of work! Regards, Steve On Jan 19, 2008 12:38 PM, <gwern0(a)gmail.com> wrote: > ... > > I am asking this question in another thread. The problem is -- I've > > got many modules compiled under 6.6, some with much agony. If I switch > > to 6.8, I have to recompile them again. Two issues I image: > > > > (1) It may take lots of effort to recompile all the modules. I have > > forgetten how I got around some of the modules! Too bad... Got to take > > notes next time... > > These days, every package you'd want to install (with the exception of GHC, Darcs, and the large graphics toolkits) should be available on Hackage or at least in Cabalized form. > > If they aren't, then that's a bug or at least missing feature. The whole point of Cabal was so you don't have to take notes! >

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Why isn't there a FAQ? (was: Re: [Haskell-cafe] anybody can tell me the pronuncation of "haskell"?)
by Tim Chevalier 29 Jan '08

29 Jan '08

I thought that the .wav file that Jeremy linked to should go in the Haskell FAQ, if there was one, but it doesn't seem to exist. There's a comp.lang.functional FAQ (that isn't maintained anymore) with a section on Haskell, and a GHC FAQ, but no general Haskell FAQ. A google search for "haskell faq" turns up this as one of the first ten hits: http://www.mail-archive.com/haskell@haskell.org/msg01153.html which is hardly current. So somebody should write one. Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt "What you call 'lying', other people would call 'abstraction'." -- Alex Aiken

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Crash in Data.ByteString.Lazy.hPut
by Jamie Love 29 Jan '08

29 Jan '08

Hi there, Not sure where to raise bugs in hackage libraries, so I'm posting here. If there is a better place, please let me know. The following code crashes with a divide by zero error when using the package 'binary-0.4.' module Main where import IO import Data.Binary import Data.Binary.Put import qualified Data.ByteString.Lazy as B simpleImage = take tot (map (\x -> x `mod` 256) [1..]) where tot = 640 * 480 main = do output <- openFile "test.tmp" WriteMode B.hPut output $ runPut $ mapM_ putWord8 simpleImage -- Jamie Love Senior Consultant Aviarc Australia Mobile: +61 400 548 048 ------------------------------------------------------------ This message has been scanned for viruses and dangerous content by MailScanner and is believed to be clean.

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anybody can tell me the pronuncation of "haskell"?
by Jeremy Apthorp 29 Jan '08

29 Jan '08

On 29/01/2008, Tim Chevalier <catamorphism(a)gmail.com> wrote: > On 1/28/08, Jeremy Apthorp <nornagon(a)gmail.com> wrote: > > On 29/01/2008, Tim Chevalier <catamorphism(a)gmail.com> wrote: > > > "Haskell", stress on the first syllable; the first syllable is like > > > the word "has" and the second syllable is pronounced with a schwa > > > where the "e" is written. > > > > > > Sometimes you will hear people stress the second syllable, but that is > > > not Preferred. > > > > > > > "Hass" (like in "hassle") "kell" (to rhyme with "fell") > > > > That is not correct. The second syllable does not rhyme with "fell". > In fact, the correct pronunciation sounds like "hassle" with a 'k' > inserted between the two syllables of that word. > > (And when I say it's not correct, I'm comparing to the speech of a few > People Who Should Know.) > The two are pretty similar, but I notice now that when I say it it's more like you describe than like I describe. I blame sleep deprivation :) Jeremy -- - Jem

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question about ghc run-time .a's
by Galchin Vasili 29 Jan '08

29 Jan '08

Hello, I am trying to build Haskell packages on RedHat RHEL 5. How do I tell ghc where the ".a" (Linux archives) are? Is there an enviroment variable? Regards, Vasili

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Haskell maximum stack depth
by istarex 29 Jan '08

29 Jan '08

Hi all, Does Haskell have a maximum stack depth restriction like Java & Python, or is it only limited by the maximum stack available on the system (i.e. what would be available to a C program)? Thanks, Istarex

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How to organize code
by L.Guo 28 Jan '08

28 Jan '08

Hi, How do you organize code ? Here is a sample. Acturally, I am thinking about using this plan. Any suggestions ? > -- BasicalType.hs > type Position = (Int,Int) > data Box = Box { pos :: Position } > data Chain = Chain { pos :: [Position] } > -- Object.hs > import BasicalType > class Object o where > pos :: o -> [Position] > -- Type.hs > import BasicalType > import Object > instance Object Box where > pos = return . BasicalType.pos > instance Object Chain where > pos = BasicalType.pos > -- Main.hs > import Type > ... Regards -------------- L.Guo 2008-01-28

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Re: Case-insensitive lexing with Alex
by Joel Reymont 28 Jan '08

28 Jan '08

On Jan 28, 2008, at 2:44 PM, Geoffrey Mainland wrote: > map toLower onto your input before you pass it to your lexer? Or do > you > only want keywords to be case-insensitive? Just keywords. You can have "Array" or "array" or "aRrAy". -- http://wagerlabs.com

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Fusion of lists and chunky sequences
by Henning Thielemann 28 Jan '08

28 Jan '08

On the one hand I like to use lists in my code because element types can be freely chosen and maximum laziness allows feedback and other tricks. On the other hand there are ByteString.Lazy and one could build chunky sequences from other fast array implementations. They have constraints on the element types like Storable in http://code.haskell.org/~sjanssen/storablevector and IArray multi-parameter type class for UArray. I like to continue writing code for lists, and let the optimizer replace it by operations on chunky sequences whereever possible. E.g. Chunky.fromList (List.map f (List.unfoldr g x)) might be transformed to Chunky.unfoldr (f ... g) x Chunky.fromList serves two purposes: 1. I obtain the result in a data structure that can be quickly accessed by further operations. 2. It tells the optimizer that element granularity for laziness is not needed and that the element type fulfills the constraint of the fast array type, and thus fusion can go on safely. (As far as I can see.) Is there some framework which fuses lists and chunky sequences? When writing fusion rules like the above one by myself, then they interfer with Prelude's fusion rules (and they would probably also interfer with those of an alternative list fusion framework). The 'map' and 'unfoldr' is certainly already fused to the internal 'build' or to another auxiliary function. As far as I know, I cannot disable the Prelude rules and give the List-Chunk rules priority higher than Prelude's ones. I hoped to be able to apply application specific fusion rules by defining a newtype wrapper around the chunky sequence type, while keeping the rest of the list code unchanged. You might argue, that code cannot be application specific if it still relies on the generic list type. Maybe it's the best to wrap the list type in a newtype and lift all of the application relevant list functions to this type and then define fusion rules on the lifted functions.

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Why functional programming matters
by Simon Peyton-Jones 28 Jan '08

28 Jan '08

Friends Over the next few months I'm giving two or three talks to groups of *non* functional programmers about why functional programming is interesting and important. If you like, it's the same general goal as John Hughes's famous paper "Why functional programming matters". Audience: some are technical managers, some are professional programmers; but my base assumption is that none already know anything much about functional programming. Now, I can easily rant on about the glories of functional programming, but I'm a biased witness -- I've been doing this stuff too long. So this message is ask your help, especially if you are someone who has a somewhat-recent recollection of realising "wow, this fp stuff is so cool/useful/powerful/etc". I'm going to say some general things, of course, about purity and effects, modularity, types, testing, reasoning, parallelism and so on. But I hate general waffle, so I want to give concrete evidence, and that is what I particularly want your help with. I'm thinking of two sorts of "evidence": 1. Small examples of actual code. The goal here is (a) to convey a visceral idea of what functional programming *is*, rather than just assume the audience knows (they don't), and (b) to convey an idea of why it might be good. One of my favourite examples is quicksort, for reasons explained here: http://haskell.org/haskellwiki/Introduction#What.27s_good_about_functional_… But I'm sure that you each have a personal favourite or two. Would you like to send them to me, along with a paragraph or two about why you found it compelling? For this purpose, a dozen lines of code or so is probably a maximum. 2. War stories from real life. eg "In company X in 2004 they rewrote their application in Haskell/Caml with result Y". Again, for my purpose I can't tell very long stories; but your message can give a bit more detail than one might actually give in a presentation. The more concrete and specific, the better. E.g. what, exactly, about using a functional language made it a win for you? If you just reply to me, with evidence of either kind, I'll glue it together (regardless of whether I find I can use it in my talks), and put the result on a Wiki page somewhere. In both cases pointers to blog entries are fine. Quite a lot of this is FP-ish rather than Haskell-ish, but I'm consulting the Haskell mailing lists first because I think you'll give me plenty to go on; and because at least one of the talks *is* Haskell-specific. However, feel free to reply in F# or Caml if that's easier for you. Thanks! Simon

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Case-insensitive lexing with Alex
by Joel Reymont 28 Jan '08

28 Jan '08

Any suggestions on how to implement case-insensitive lexing with Alex? Thanks, Joel -- http://wagerlabs.com

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Upgrading ByteString causes (seemingly) impossible RTS linker errs
by Austin Seipp 28 Jan '08

28 Jan '08

Recently I've been developing my IRC bot a little further, and in the midst of it I have come across a very problematic issue that revolves around GHC-dependencies vs. application-dependencies. The central issue is ByteString. Currently, the ghc package depends on bytestring-0.9.0.1. However, the current version of ByteString I have installed is 0.9.0.4. To keep compatability with bytestring-dependent libraries that were built against 0.9.0.1, you have to recompile of all these libs against 0.9.0.4. For example, I had to build a newer version of zlib against 0.9.0.4, because the older version was built for 0.9.0.1, which mean that it was not possible to build cabal-install because of the incompatabilities (it needed bytestring and zlib.) The problem is that if you have a package that depends on ghc as a library, but also depends on bytestring > 0.9.0.1 or any library built against it, you'll get pretty much unstoppable linker errs, since all the symbol names in the ghc package are hardwired for 0.9.0.1 (meaning you can't do 'ghc-pkg update' for it) forcing it to be loaded. The example in my bot is this: it depends on binary which is built against bytestring-0.9.0.4 for serializing the state of plugins. If you configure it with the flag '-fdynamic' and you have hs-plugins for ghc 6.8 installed (http://code.haskell.org/~dons/code/hs-plugins/) then the bot can do in-situ code reloading. However, hs-plugins depends on ghc as a library, which depends on bytestring 0.9.0.1... I think you can see where this is going (actually I can't quite pinpoint it to being hs-plugins importing ghc since, but essentially it could be *any* lib my bot uses which depends on ghc as a library, hs-plugins is just the most obvious one): [austin@continuum 0.4]$ ghci Setup.hs GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. ... *Main> :main configure Loading package array-0.1.0.0 ... linking ... done. Loading package packedstring-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package filepath-1.1.0.0 ... linking ... done. Loading package directory-1.0.0.0 ... linking ... done. Loading package unix-2.3.0.0 ... linking ... done. Loading package process-1.0.0.0 ... linking ... done. Loading package pretty-1.0.0.0 ... linking ... done. Loading package hpc-0.5.0.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package readline-1.0.1.0 ... linking ... done. Loading package Cabal-1.2.3.0 ... linking ... done. Loading package random-1.0.0.0 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package parsec-2.1.0.0 ... linking ... done. Loading package haskell-src-1.0.1.1 ... linking ... done. Loading package network-2.1.0.0 ... linking ... done. Loading package mtl-1.1.0.0 ... linking ... done. Loading package stm-2.1.1.0 ... linking ... done. Loading package irc-0.4 ... linking ... done. Loading package QuickCheck-2.0 ... linking ... done. Loading package bytestring-0.9.0.4 ... linking ... done. <-- needed for binary Loading package binary-0.4.1 ... linking ... done. Loading package bytestring-0.9.0.1 ... <-- needed for ghc GHCi runtime linker: fatal error: I found a duplicate definition for symbol fps_maximum whilst processing object file /usr/local/lib/ghc-6.8.2/lib/bytestring-0.9.0.1/HSbytestring-0.9.0.1.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. [austin@continuum 0.4]$ >From speculation, I would think that any package that uses ghc as a library but also uses bytestring > 0.9.0.1, directly or indirectly, would hit this error. Is there any easy way to resolve this? From the looks of it, the *only* way would really be to just unregister 0.9.0.4, and rebuild everything against 0.9.0.1... Either that or factor out the use of ByteString in the ghc-frontend. Neither seems optimal, though. But I've been working on this too long to just stop, so if it requires recompiling everything against an older version of bytestring, then so be it, I suppose... Thanks for whoever helps. - Austin -- "It was in the days of the rains that their prayers went up, not from the fingering of knotted prayer cords or the spinning of prayer wheels, but from the great pray-machine in the monastery of Ratri, goddess of the Night." Roger Zelazny

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ANNOUNCE: xmonad 0.6 released
by Don Stewart 28 Jan '08

28 Jan '08

The xmonad dev team is pleased to announce the 0.6 release of xmonad! http://xmonad.org xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard: a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating windows * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Easy, powerful customisation and reconfiguration in Haskell * Large extension library * Extensive documentation and support for hacking Get it! Information, screenshots, documentation, tutorials and community resources are available from the xmonad home page: http://xmonad.org The 0.6 release, and its dependencies, are available from hackage.haskell.org, here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad Headlines: This is an incremental release of xmonad, with some new features, and important bug fixes. * Make focus-follows-mouse configurable * Better support for X11 cloned screens * xmonad config dirs can now be shared across architectures * New flags: --version: print xmonad's version --recompile: recompile xmonad.hs if it is out of date --force-recompile: recompile xmonad.hs unconditionally * Improved ManageDocks extension used for docking programs * Improved documentation * Update to Haskell X11-1.4.1, fixes bug with XGetWindowProperty * Close bug #96 relating to window unmapping (fixes gnucash bugs) * 'Testing' mode for developers * Various other fixes and clean ups. Along with many improvements to the extension library. Here are some of the noticeable changes to the extension library: * New: XMonad.Hooks.ManageHelpers : support for hooks * New: XMonad.Util.Search : support for web searching * New: XMonad.Util.Timer : set up timers and to handle them * New: XMonad.Layout.Reflect : reflect layouts horizontally/vertically * New: XMonad.Actions.NoBorders * Improved EDSL for performing actions on windows * Improved window tabbing * ResizableTile works with floating windows * Faster prompts * More EWHM support * Add support for cycling through screens to CycleWS * UrgencyHook support to status bar * Custom zoom levels for magnifier layout * Bug fixes to ManageDocks * Support for _NET_WM_STRUT_PARTIAL * Improved docs for writing new extensions Full changelogs against xmonad 0.5 for the core and extensions here: http://xmonad.org/changelog-0.6.txt http://xmonad.org/changelog-xmc-0.6.txt Extensions: xmonad comes with a huge library of extensions (now more than 5 times the size of xmonad), contributed by viewers like you. Extensions enable pretty much arbitrary window manager behaviour to be implemented by users, in Haskell, in the config files. For more information on using and writing extensions see the webpage. The library of extensions is available from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib Full documentation for using and writing your own extensions: http://xmonad.org/contrib.html This release brought to you by the xmonad dev team: Spencer Janssen Don Stewart Jason Creighton Andrea Rossato David Roundy Brent Yorgey Featuring code contributions from: Aaron Denney Adam Vogt Alec Berryman Alex Tarkovsky Alexandre Buisse Austin Seipp Brandon Allbery Chris Mears Christian Thiemann Clemens Fruhwirth Daniel Neri Daniel Wagner Dave Harrison David Glasser David Lazar Devin Mullins Dmitry Kurochkin Dougal Stanton Eric Mertens Ferenc Wagner Gwern Branwen Hans Philipp Annen Ivan Tarasov Jamie Webb Jeremy Apthorp Joachim Breitner Joachim Fasting Joe Thornber Joel Suovaniemi Juraj Hercek Kai Grossjohann Karsten Schoelzel Klaus Weidner Lucas Mai Mats Jansborg Matsuyama Tomohiro Michael Fellinger Michael Sloan Miikka Koskinen Neil Mitchell Nelson Elhage Nick Burlett Nicolas Pouillard Peter De Wachter Robert Marlow Roman Cheplyaka Sam Hughes Shachaf Ben-Kiki Shae Erisson Simon Peyton Jones Stefan O'Rear Valery V. Vorotyntsev timthelion "intrigeri" As well as the support of many others on the #xmonad and #haskell IRC channels, and the wider Haskell and window manager communities. Thanks to everyone for their support!

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ANN: RPCA and fec
by Adam Langley 28 Jan '08

28 Jan '08

Doubling up an announcement email again to reduce the traffic on the list. First: RPCA is an RPC system. This first release is pretty lacking, simple client-server interactions work but there's no lame-duck handling, load balancing, security etc. RPCA uses codec-libevent's tagged data structures for serialisation. Tagged structures allow you to grow you RPCs incrementally, which is very important in large programs. Very simple tests should that 1000's of RPCs a second isn't a problem. There's also the start of a C library for the protocol, written on libevent and glib. To give a taste, here's what you need to write on the server side: addNumbers :: TestRPC.Addnumbersrequest -> ((Maybe TestRPC.Addnumbersreply) -> STM ()) -> IO () addNumbers (TestRPC.Addnumbersrequest { TestRPC.addnumbersrequest_a = a , TestRPC.addnumbersrequest_b = b }) cb = do atomically $ cb $ Just $ TestRPC.addnumbersreplyEmpty { TestRPC.addnumbersreply_c = a + b } and on the client side... rpc chan "add" (TestRPC.Addnumbersrequest 400 300) 0.1 That's a synchronous call with a timeout. You can, of course, call them asynchronously as well (and from multiple threads etc) It's highly experimental and can be found in Hackage and at http://darcs.imperialviolet.org/darcsweb.cgi?r=network-rpca;a=summary FEC is an erasure coding library, based on zfec by Zooko, based on code by Luigi Rizzo. The most widely known example of an erasure code is the RAID-5 algorithm which makes it so that in the event of the loss of any one hard drive, the stored data can be completely recovered. The algorithm in the fec package has a similar effect, but instead of recovering from the loss of only a single element, it can be parameterized to choose in advance the number of elements whose loss it can tolerate. Hackage: http://hackage.haskell.org/packages/archive/fec/0.1.1/doc/html/Codec-FEC.ht… It's new code, but the library that it's based on is well tested so should be sound. AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

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C# parser written in Haskell
by Joel Reymont 28 Jan '08

28 Jan '08

Does anyone have a C# parser written in Haskell? Thanks, Joel -- http://wagerlabs.com

1 0

Testing one's .ghci
by gwern0＠gmail.com 27 Jan '08

27 Jan '08

So I was recently going over my config files (which are version-controlled in Darcs, of course), and I was adding tests for recording patches. For some of the files, it was easy enough - the application generally provided some mean of loading in the rc file and then it would error or not based on whether the file was at least syntactically correct. For example, to test my .emacs, I have 'emacs -batch -f batch-byte-compile ~/.emacs'. If the .emacs won't byte-compile, it probably won't run! My question is, how many I do the same thing with GHCi? No options specifically for this seem to be listed, and the normal trick doesn't work - I tried: > ghci -read-dot-ghci -e 'let main = return() in main' && echo foo And regardless of whether the .ghci had hideous crippling errors in it, at the end "foo" was still merrily being printed out. Does haskell-cafe have any ideas or should I just go file a feature request? -- gwern chameleon Vale RSA Recon 1080H beanpole advisors Harvard Texas PARKHILL

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ANNOUNCE: HStringTemplate -- An Elegant, Functional, Nifty Templating Engine for Haskell
by Sterling Clover 27 Jan '08

27 Jan '08

HStringTemplate is a port of Terrence Parr’s lovely StringTemplate (http://www.stringtemplate.org) engine to Haskell. It is available, cabalized, at: darcs get http://code.haskell.org/HStringTemplate/ As interest has grown in using Haskell for web applications, there has been an increasing buzz about the need for a good templating engine in Haskell. Why might we need this? After all, Haskell has lovely combinator libraries for generating HTML programmatically, enforcing good form through its type system. But sometimes, we don’t want well-formed HTML. We want the ugly stuff that floats around to deal with eight varieties of browser incompatibilities and the latest silly ajax trick. Or sometimes we’re working with a team of graphic designers, and they want to work in almost-HTML. Or sometimes we just want to be able to change the design of a web application on the fly, completely independent of our program logic, and of, heavens forbid, recompiling and possibly messing with a live application. So template engines are popular, and indeed, considered a key part of most web frameworks out there. One problem — they’re mainly awful, imperatively-conceived behemoths that capriciously mix program logic with display and, consequently, entail a great deal of overhead. Enter StringTemplate, a nifty and fairly-well developed template format that’s both pure and functional, and therefore pretty much the only one of its kind. Indeed, it also seems to be getting heavy use in code generation because its paradigm maps neatly to traversing parse-trees. HStringTemplate is not feature-complete, and indeed is only at version 0.1. But it should implement pretty much everything in the standard StringTemplate 3.0 grammar, only nicer, because it’s in Haskell. There are scads of different recursive constructs and ways to handle inclusion and inheritance. Furthermore, HStringTemplate handles conditionals, and sports also a very Haskellish implementation of custom rendering. Templates can be constructed that return strings, ShowSs, bytestrings, or even pretty printer Docs that handle wrapping, indentation, and fill elegantly. Even better, these templates are parsed and compiled only once, after which point there isn't a syntax tree anymore, just a function that operates on the environment of attributes that have been passed to it. Where I take it from here depends in part on what sort of response I get, so patches, gripes, API comments and feature requests are all more than welcome. Please note that I'm still working in 6.6.1. Everything should be in place to compile properly with the base split, but if it isn't, again, patches more than welcome. Full announcement at: http://fmapfixreturn.wordpress.com/ --SC

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Generic permutations
by Cetin Sert 27 Jan '08

27 Jan '08

How can I make a generic permutations function? -- boolean permutations bpms :: Int → [[Bool]] bpms 0 = [[]] bpms n = map (False:) bss ++ map (True:) bss where bss = bpms (n - 1) -- generic permutations pms a :: Int → [[a]] Best Regards, Cetin Sert

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Re: [Haskell-cafe] Re: Generic permutations with high performance
by Cetin Sert 27 Jan '08

27 Jan '08

Hello again Ryan, I have found out where to import those stuff from and tested your more elegant suggestion and my original performance. -- print ((length ∘ pmsO [0,1]) 24) 9~ seconds -- print ((length ∘ pmsE [0,1]) 24) 23~ seconds -- print ((length ∘ pmsU [0,1]) 24) 23~ seconds -- O: original, E: elegant, U: unreadable I prefer performance over elegance (and for me using monads almost feels like adding an unnecessary dependency to the definition of pms. Though I know I may be dead wrong on that. I just don't quite understand monads yet.) I would love to have you and/or others suggest more performant versions of pms (and maybe also come up with a better name for it). mnt :: [a] → [[a]] → [[a]] mnt [] _ = [] mnt _ [] = [] mnt (x:xs) yss = map (x:) yss ++ mnt xs yss pms :: [a] → Int → [[a]] pms [] _ = [[]] pms _ 0 = [[]] pms xxs n = mnt xxs (pms xxs (n - 1)) I generalized 'pms' from the 'bools' function on page 108 of Programming in Haskell (Hutton, 2007) -- Cetin Sert On 26/01/2008, Ryan Ingram <ryani.spam(a)gmail.com> wrote: > > When you say permuations, I think of reorderings of a list, for example: > > permutations [1,2,3] = > [ [1,2,3], > [1,3,2], > [2,1,3], > [2,3,1], > [3,1,2], > [3,2,1] ] > > Here's an implementation: > > -- split [1,2,3] => [ > -- ( 1, [2,3] ), > -- ( 2, [1,3] ), > -- ( 3, [1,2] ) ] > split :: [a] -> [(a, [a])] > split [] = error "split: empty list" > split [a] = [(a, [])] > split (a:as) = (a, as) : map prefix (split as) > where prefix (x, xs) = (x, a : xs) > > permutations :: [a] -> [[a]] > permutations [] = return [] > permutations xs = do > (first, rest) <- split xs > rest' <- permutations rest > return (first : rest') > > The problem you solved can be solved much more elegantly: > > pms : [a] -> Int -> [[a]] > pms xs n = foldM combine [] (replicate n xs) where > combine rest as = liftM (:rest) as > > or, for the unreadable version: > pms xs n = foldM (map . flip (:)) [] $ replicate n xs > > (note that, in the list monad, liftM = map). > > -- ryan >

1 0

Problem with HGL
by jia wang 27 Jan '08

27 Jan '08

Hi, I'm trying to use HGL-3.2.0.0 with ghc-6.8.2 on my windows system. When I run command: runhaskell Setup.hs build it shows an error D:\HGL-3.2.0.0\HGL-3.2.0.0>runhaskell Setup.hs build Preprocessing library HGL-3.2.0.0... Building HGL-3.2.0.0... Graphics/HGL/Key.hs:57:7: Could not find module `Graphics.Win32': it is a member of package Win32-2.1.1.0, which is hidden so, how to solve this problem, thanks

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running into an unresolved linker error on RHEL 5
by Galchin Vasili 26 Jan '08

26 Jan '08

Hello, I am building the unix-2.2.0.0. The linker is failing with -lgmp. I set "export LD_LIBRARY_ATH=/bin:/usr/lib". I did a "ls -l /usr/lib/*gmp*" and I can see: libgmp.so.3 libgmp.so.3.3.3 libgmpxx.so.3 libgmpxx.so.3.0.5, i.e. either gmp shared objects and whatever the gmp "xx" shared objects are. So why is my link of unix-2.2.0.0 failing in DynamicLinker? Kind regards, Vasili

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ANN: bytestringparser-0.2, a Parsec-like parser for lazy ByteStrings
by Bryan O'Sullivan 26 Jan '08

26 Jan '08

Some time ago, Jeremy Shaw wrote a Parsec clone for lazy ByteStrings. I've been using it for a while, and have made substantial changes to it along the way. It's very fast, using the same manual unpacking trick as the binary package to keep performance nippy. It also integrates with the latest flava in parsing combinators, Control.Applicative, providing instances of the Applicative and Alternative typeclasses. The API is rather smaller than that of Parsec, but it's more than adequate for parsing e.g. text-based network protocols. Download: http://hackage.haskell.org/packages/archive/bytestringparser/0.2/bytestring… Haddocks: http://darcs.serpentine.com/bytestringparser/dist/doc/html/bytestringparser… Darcs repository: darcs get http://darcs.serpentine.com/bytestringparser <b

1 0

HList error with hFoldr
by Denis Bueno 26 Jan '08

26 Jan '08

Hello all, I'm doing some machine learning in Haskell and have run into a problem. I have a generic distance function (declare in the MetricSpace) typeclass that returns the distance between two things as a number. I frequently will be working with heterogeneous collections of data, and if possible I'd like to write the distance function (overloaded) once for all. At the moment, I'm using HList to represent the collections of data uniformly. I wrote the following as a sanity check but I can't even get it to compile. The effect I'd like is that I can call `dist` on any two HLists which have the correct, corresponding element types in the correct positions, and `dist` will be recursively invoked to calculate the typical euclidean distance (squared sum of individual element distances, and at the and taking the square root). My attempt at doing this follows. For the moment my HLists only contain Ints. I'm using GHC 6.8.2 and HList 0.1. import HList class (Num i) => MetricSpace e i where dist :: e -> e -> i instance Num i => MetricSpace Int i where x `dist` y = fromIntegral $ abs (y - x) data ApplyDistSum = ApplyDistSum instance (MetricSpace e r) => Apply ApplyDistSum ((e, e), r) r where apply _ (p, v) = v + uncurry dist p -- Why should the following generate an error? testApplyDistSum = hFoldr ApplyDistSum 0 ((4,4) .*. hNil) {- The error: /Users/denbuen/edu/cornell/meng/classes/cs678/code/practice/Knn.hs:53:19: No instance for (Apply ApplyDistSum ((t, t1), r1) r) arising from a use of `hFoldr' at /Users/denbuen/edu/cornell/meng/classes/cs678/code/practice/Knn.hs:53:19-56 Possible fix: add an instance declaration for (Apply ApplyDistSum ((t, t1), r1) r) In the expression: hFoldr ApplyDistSum 0 ((4, 4) .*. hNil) In the definition of `testApplyDistSum': testApplyDistSum = hFoldr ApplyDistSum 0 ((4, 4) .*. hNil) -} Thanks in advance. -- Denis

1 0

Evolutionary Computing
by PR Stanley 25 Jan '08

25 Jan '08

Hi What does the list think of EC? Genetic algorithm is apparently the latest thing. Is EC mathematically reasonable? Paul

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Fwd: ByteString Parsec clone
by Johan Tibell 25 Jan '08

25 Jan '08

I tried to get Dan's thoughts on cloning parts of the Parsec interface and some of the documentation but none of the emails addresses I've tried seem to work. What's allowed when it comes to duplicating something like an API? What about the documentation? I intend it to be released under BSD3 which I believe is used for Parsec as well. Anyone know how I can reach Dan? -- Johan ---------- Forwarded message ---------- From: Johan Tibell <johan.tibell(a)gmail.com> Date: Jan 22, 2008 2:56 PM Subject: ByteString Parsec clone To: daan(a)cs.uu.nl I've written, but yet not published, a ByteString version of parts of the Parsec API as a part of a web application server project. I've currently used almost the exact wording from your documentation (as the semantics of the functions is intended to be the same as in Parsec.) Are you OK with that? P.S. I intend to give my library a different name as it has different module structure and doesn't support a lot of Parsec's features. It will be BSD licensed. Best regards, Johan Tibell

2 2

Binary IO of a list of ints
by Jamie Love 25 Jan '08

25 Jan '08

Hi there I have a list of ints, with values between 0 and 255 and I need to print them out in little endian form to a file. I've been using Data.Binary for single values, e.g. runPut $ do put 'B' put (0 :: Int32) I'm wondering how I can go about writing a list of Ints out. My thought was to do something like: foldr (\x B.hPut output (runPut $ do put (x :: Word8))) data (where output is my file handle), but apart from giving me type errors, it seems a rather arduous way to do it. Could anyone suggest a better way to do this? -- Jamie Love Senior Consultant Aviarc Australia Mobile: +61 400 548 048 ------------------------------------------------------------ This message has been scanned for viruses and dangerous content by MailScanner and is believed to be clean.

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ANNOUNCE: Harpy 0.4 - Runtime code generation for x86 machine code
by Martin Grabmueller 25 Jan '08

25 Jan '08

We are pleased to announce the release of Harpy 0.4, a library for runtime code generation for x86 machine code. Harpy is available from Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/harpy-0.4 Also see Harpy's homepage, which features two tutorials and access to the darcs repo (and to the Haddock documentation, until it is available from Hackage, too): http://uebb.cs.tu-berlin.de/harpy/ Harpy is supposed to be built with GHC 6.8 and GHC 6.6 (with Cabal 1.2 or greater). Please report any build problems you encounter. This release features the following additions and fixes: - New convenience top-level module "Harpy", which re-exports Harpy.CodeGenMonad, Harpy.Call and Harpy.X86Assembler - It is now possible to override Harpy's automatic code buffer management. The new field 'customCodeBuffer' in the type 'CodeGenConfig' can be set to 'Just (buf, size)', where 'buf' is a pointer to a memory region of 'size' bytes. Harpy will then use the supplied code buffer and will not perform any automatic code buffer allocation on overflow. Overflow checking is still performed and will result in an exception in the CodeGen monad. - When using the high-level assembler in X86Assembler, the code buffer is automatically protected from overflow. - Floating point operations added to X86Assembler (only for double operands yet). - Preliminary support for SSE instructions. Currently, only the packed and scalar floating-point arithmetic operations are supported (both in the low-level module Harpy.X86CodeGen and as methods in Harpy.X86Assembler) - Code buffer default size has been increased from 128 to 4096 bytes. - The CodeGenMonad fails when a label is defined twice. - It is now possible to associate names with labels, using the new operation newNamedLabel. The given names will show up in the disassembly, which makes debugging of generated code much easier. - The doc directory contains a second, slightly larger tutorial now. - The examples/evaluator directory contains a small example interpreter for arithmetic expressions, which translates expressions entered at the keayboard to machine code on the fly. This is the demo program we presented at the Haskell Workshop 2007. Happy Haskell Hacking with Harpy, Martin and Dirk

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Re: [Haskell-cafe] Re: Why functional programming matters
by John Lato 25 Jan '08

25 Jan '08

In addition to STM, another item that should interest serious programmers is forkIO. Lightweight threads that (unlike in Python) can use multiple cpu's. Coming from Python, I personally appreciate this. Using STM to handle concurrency issues often greatly simplifies multithreaded code.

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OT: Isorecursive types and type abstraction
by Edsko de Vries 25 Jan '08

25 Jan '08

Hi, This is rather off-topic but the audience of this list may be the right one; if there is a more appropriate venue for this question, please let me know. Most descriptions of recursive types state that iso-recursive types (with explicit 'fold' and 'unfold' operators) are easy to typecheck, and that equi-recursive types are much more difficult. My question regards using iso-recursive types (explicitly, not implicitly using algebraic data types) together with type abstraction and application. The usual typing rules for fold and unfold are e :: Fix X. t ----------------------------- Unfold unfold e :: [X := Fix X. t] t e :: [X := Fix X. t] t ----------------------------- Fold fold e : Fix X. t This works ok for the following type: ListInt = Fix L. 1 + Int * L (where "1" is the unit type). If x :: ListInt then unfold x :: 1 + Int * ListInt using the Unfold typing rule. However, this breaks when using type abstraction and application. Consider the polymorphic version of list: List = Fix L. /\A. 1 + A * L A Now if we have x :: List Int we can no longer type unfold x because x does not have type (Fix ..), but ((Fix ..) Int) instead. Of course, we can unroll List Int by first unrolling List, and then re-applying the unrolled type to Int to get (/\A. 1 + A * (Fix L. /\A. 1 * L A) A) Int which can be simplified to 1 + Int * List Int but this is not usually mentioned (that I could find; in particular, TAPL does not mention it) and I'm worried that there are subtleties here that I'm missing--nor do I have an exact definition of what this 'extended' unrolling rule should do. Any hints or pointers to relevant literature would be appreciated! Edsko PS. One way to simplify the problem is to redefine List as List = /\A. Fix L. 1 + A * L so that List Int can easily be simplified to the right form (Fix ..); but that can only be done for regular datatypes. For example, the nested type Perfect = Fix L. /\A. A + Perfect (A, A) cannot be so rewritten because the argument to Perfect in the recursive call is different.

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ghc and Red Hat 5 .rpm
by Galchin Vasili 25 Jan '08

25 Jan '08

Hello, I have searching on http://haskell.org/ghc/download_ghc_682.html .. but can't find a ghc .rpm so that I can install ghc. ? Regards, Vasili

1 0

Fwd: Arrow without `>>>'
by Valery V. Vorotyntsev 24 Jan '08

24 Jan '08

The problem has been resolved. Kudos to Ross Paterson. ---------- Forwarded message ---------- From: Valery V. Vorotyntsev <valery.vv(a)gmail.com> Date: Jan 24, 2008 3:34 PM Subject: Re: Arrow without `>>>' To: libraries(a)haskell.org On 1/24/08, Ross Paterson <ross(a)soi.city.ac.uk> wrote: > > The right question: how to build `arrows' in such circumstances? > > To build with the darcs version of the base library (built with GHC), > you'll need the darcs version of the arrows library, which I think is > right now: > > darcs get http://darcs.haskell.org/packages/arrows Yes, after pulling the latest change from repo installation went smoothly. Thank you very much, Ross!

1 0

An idea - Feasibility and effort
by Vimal 24 Jan '08

24 Jan '08

Hi Haskellers, This semester, I am having a software engineering course. I always wanted to do something significant in Haskell. This is probably an opportunity that I can utilize. I have the following ideas, with inspiration from various sources. Please let me know your views/ideas on them. If you feel you are experienced enough, do warn me if the idea might not be feasible to implement! I will have about half a month for getting the Software Requirement Specification ready, and ~ 1 month of time for coding. Idea 1: A Traceroute visualizer. I saw the video on the London Haskell Group on how to create a game in Haskell, and loved the "rotating earth" part of the game. (Space Invaders or something like that). Combine the earth (with a Google earth touch) with a traceroute backend and animate packet routing via different cities live on the screen. I like this idea. More of a fun project than a useful one. Probably Haskell isnt a good language to implement it, but as they say, one should never think about moulding a project to suit the language you code in. Idea 2: A module that generates optimum (in some sense) set of instructions for a given (pure, functional) expression. So, this basically involves concepts like * avoiding recalculating common subexpressions (expression-tree matching?), * register allocation (assuming a finite supply of registers, modeling as graph colouring), * Term rewriting I am just quoting some terms there, I may be wrong. but this might be a project that Haskell might help me beautifully model and execute. I would like to have some opinion on the complexity of the project ideas that I have in mind. Thanks, -- Vimal Indian Institute of Technology Madras Chennai India

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Abstracting ByteStrings
by Chad Scherrer 24 Jan '08

24 Jan '08

A lazy ByteString is an alternative to a String=[Char], where sacrificing some degree of laziness through "chunks" gives much greater performance in many applications. If I remember correctly, we could as well create an IntString, DoubleString, etc by filling the chunk arrays with different types. Now, this might only work for primitive types, but couldn't we extend it by taking a similar approach to NDP? For example, if we have (lazy) ByteString = [Chunk Char], then we could represent, say [Chunk (a,b)] as ([Chunk a], [Chunk b]), etc Is this reasonable? Do you think it would work? Or is all of this subsumed by fusion anyway? -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Ann: Drawing with Cairo: Getting Started
by Hans van Thiel 24 Jan '08

24 Jan '08

Hello All, There is now an appendix to the Gtk2Hs tutorial on getting started with Cairo drawing. It's not actually about drawing, but on how to get drawings on the screen and how to write them to .png, .pdf, .ps. or .svg files. Though this is very easy, the 'recipes' are not self-evident, so this mini-tutorial could save prospective users some time. See: http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/app1.xhtml or: http://j-van-thiel.speedlinq.nl/gtk2hs/app1.html I'd like to add, for Spanish readers, that the Gtk2Hs tutorial itself has been translated into Spanish. See: http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/es-index.xhtml or: http://j-van-thiel.speedlinq.nl/gtk2hs/es-index.html Alex Tarkovsky's Glade tutorial http://haskell.org/gtk2hs/docs/tutorial/glade/ has also been translated into Spanish: http://j-van-thiel.speedlinq.nl/glade/es-index.html The translator of both texts prefers to be known as Laszlo Keuschnig. In case anyone wonders, this is not me, though I wish.. Best Regards, Hans van Thiel

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Has character changed in GHC 6.8?
by Magnus Therning 23 Jan '08

23 Jan '08

I vaguely remember that in GHC 6.6 code like this length $ map ord "a string" being able able to generate a different answer than length "a string" At the time I thought that the encoding (in my case UTF-8) was “leaking through”. After switching to GHC 6.8 the behaviour seems to have changed, and mapping 'ord' on a string results in a list of ints representing the Unicode code point rather than the encoding: > map ord "åäö" [229,228,246] Is this the case, or is there something strange going on with character encodings? I was hoping that this would mean that 'chr . ord' would basically be a no-op, but no such luck: > chr . ord $ 'å' '\229' What would I have to do to get an 'å' from '229'? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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Precedence and associativity in a pretty-printer
by Edsko de Vries 23 Jan '08

23 Jan '08

Hi, Suppose we have some algebraic datatype describing an expression language containing the usual suspects (various binary arithmetic operators such as addition, subtraction, multiplication, division, exponentiation, function abstraction and application, etc.) each with their own precendence (multiplication binds stronger than addition) and associativity (function application is left associative, exponentiation is right associative, addition is associative, etc.) Is there a nice way to pretty-print such an expression with the minimal number of brackets? I can come up with something, but I'm sure somebody thought hard about this problem before and came up with a really nice solution :) Any hints or pointers would be appreciated, Thanks, Edsko

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Darcs Patch Theory and GADTs at London Haskell User Group tonight
by Neil Bartlett 23 Jan '08

23 Jan '08

Hi all, Just a reminder about tonight's London Haskell User Group meeting, at City University from 6:30PM. Ganesh Sittampalam will be giving a talk entitled "Darcs Patch Theory and GADTs". For full details see the website: http://www.londonhug.net/2008/01/07/darcs-patch-theory-and-gadts-with-ganes… For those outside the London area or unable to come, there will be a video posted to the website later, so subscribe to it in your RSS reader now! Regards, Neil

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