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ANNOUNCE: Haskell Communities and Activities Report (15th ed., November 2008)
by Janis Voigtlaender 02 Dec '08

02 Dec '08

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (15th edition, November 2008) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2009: target deadline for contributions to the May 2009 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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What causes <<loop>>?
by Martin Hofmann 02 Dec '08

02 Dec '08

I am picking up a discussion with the same topic from haskell-users on 8th November. Thunks with reference on themselves was mentioned as main reason for <<loop>>. > A safe recursive definition would be > let x = Foo (x+1) > However, if you leave out the constructor, > let x = x + 1 > you get a <<loop>> (or a deadlock). > Are there any other reasons? I am trying to debug monadic code which stores state information in a record maintaining several Data.Maps, but in vein so far. A state is modified/changed in several steps by a compound function i.e. changeA $ changeB $ changeC state Is code using mtl, records or Maps more prone to <<loop>> or does this not matter at all. How can I identify self-referencing thunks more easily? Is there any rule of thumb to prevent <<loop>>? Are there any less obvious newbie mistakes causing <<loop> then the one above? Any comments, experience, and tips are highly appreciated. Thanks, Martin

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serious defect in system-uuid
by Jason Dusek 02 Dec '08

02 Dec '08

I discovered that my UUID type causes a <<loop>> when tested for equality. Anyone who is using `system-uuid`, please upgrade to version 1.0.2. -- _jsn

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System.IO.UTF8 Problem
by Ben George 02 Dec '08

02 Dec '08

Hi, so I've been working with System.IO.UTF8 trying to get it to render Japanese characters correctly. Using just IO I've gotten it to output the correct unicode values, however, replacing the IO.putStrLn commands with System.IO.UTF8 yielded some strange errors. I went through the program and replaced the System.IO.UTF8 putStrLn and hGetLine commands with their "normal" IO counterparts, however, simply having the "import System.IO.UTF8" line at the beginning of the file causes an error during compilation. $ghc file.hs compilation IS NOT required byteindex.o: In function `sKE_info': (.text+0x8bf): undefined reference to `__stginit_utf8zmstringzm0zi3zi3_SystemziIOziUTF8_' collect2: ld returned 1 exit status Below is the output of ghc -v. I am running Ubuntu 7.10 gutsy. I have been getting the same error whether GHC was compiled from binaries or source. Currently I am running GHC (version 6.8.3) compiled from source. $ ghc -v file.hs Glasgow Haskell Compiler, Version 6.8.3, for Haskell 98, stage 2 booted by GHC version 6.8.3 Using package config file: /usr/local/lib/ghc-6.8.3/package.conf wired-in package base mapped to base-3.0.2.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0.1.0 wired-in package template-haskell mapped to template-haskell-2.2.0.0 wired-in package ndp not found. Hsc static flags: -static Created temporary directory: /tmp/ghc6586_0 *** Checking old interface for main:Main: compilation IS NOT required *** Touching object file: touch byteindex.o *** Linker: gcc -v -o a.out -DDONT_WANT_WIN32_DLL_SUPPORT byteindex.o -L/usr/local/lib/ghc-6.8.3/lib/haskell98-1.0.1.0 -L/usr/local/lib/ghc-6.8.3/lib/array-0.1.0.0 -L/usr/local/lib/ghc-6.8.3/lib/process-1.0.0.1 -L/usr/local/lib/ghc-6.8.3/lib/unix-2.3.0.1 -L/usr/local/lib/ghc-6.8.3/lib/random-1.0.0.0 -L/usr/local/lib/ghc-6.8.3/lib/directory-1.0.0.1 -L/usr/local/lib/ghc-6.8.3/lib/filepath-1.1.0.0 -L/usr/local/lib/ghc-6.8.3/lib/old-time-1.0.0.0 -L/usr/local/lib/ghc-6.8.3/lib/old-locale-1.0.0.0 -L/usr/local/lib/ghc-6.8.3/lib/base-3.0.2.0 -L/usr/local/lib/ghc-6.8.3 -lHShaskell98-1.0.1.0 -lHSarray-0.1.0.0 -lHSprocess-1.0.0.1 -lHSunix-2.3.0.1 -lutil -ldl -lHSrandom-1.0.0.0 -lHSdirectory-1.0.0.1 -lHSfilepath-1.1.0.0 -lHSold-time-1.0.0.0 -lHSold-locale-1.0.0.0 -lHSbase-3.0.2.0 -lHSrts -lm -lgmp -ldl -lrt -u base_GHCziBase_Izh_static_info -u base_GHCziBase_Czh_static_info -u base_GHCziFloat_Fzh_static_info -u base_GHCziFloat_Dzh_static_info -u base_GHCziPtr_Ptr_static_info -u base_GHCziWord_Wzh_static_info -u base_GHCziInt_I8zh_static_info -u base_GHCziInt_I16zh_static_info -u base_GHCziInt_I32zh_static_info -u base_GHCziInt_I64zh_static_info -u base_GHCziWord_W8zh_static_info -u base_GHCziWord_W16zh_static_info -u base_GHCziWord_W32zh_static_info -u base_GHCziWord_W64zh_static_info -u base_GHCziStable_StablePtr_static_info -u base_GHCziBase_Izh_con_info -u base_GHCziBase_Czh_con_info -u base_GHCziFloat_Fzh_con_info -u base_GHCziFloat_Dzh_con_info -u base_GHCziPtr_Ptr_con_info -u base_GHCziPtr_FunPtr_con_info -u base_GHCziStable_StablePtr_con_info -u base_GHCziBase_False_closure -u base_GHCziBase_True_closure -u base_GHCziPack_unpackCString_closure -u base_GHCziIOBase_stackOverflow_closure -u base_GHCziIOBase_heapOverflow_closure -u base_GHCziIOBase_NonTermination_closure -u base_GHCziIOBase_BlockedOnDeadMVar_closure -u base_GHCziIOBase_BlockedIndefinitely_closure -u base_GHCziIOBase_Deadlock_closure -u base_GHCziIOBase_NestedAtomically_closure -u base_GHCziWeak_runFinalizzerBatch_closure -u base_GHCziConc_ensureIOManagerIsRunning_closure Using built-in specs. Target: i486-linux-gnu Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.1.3 --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --enable-checking=release i486-linux-gnu Thread model: posix gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2) /usr/lib/gcc/i486-linux-gnu/4.1.3/collect2 --eh-frame-hdr -m elf_i386 --hash-style=both -dynamic-linker /lib/ld-linux.so.2 -o a.out -u base_GHCziBase_Izh_static_info -u base_GHCziBase_Czh_static_info -u base_GHCziFloat_Fzh_static_info -u base_GHCziFloat_Dzh_static_info -u base_GHCziPtr_Ptr_static_info -u base_GHCziWord_Wzh_static_info -u base_GHCziInt_I8zh_static_info -u base_GHCziInt_I16zh_static_info -u base_GHCziInt_I32zh_static_info -u base_GHCziInt_I64zh_static_info -u base_GHCziWord_W8zh_static_info -u base_GHCziWord_W16zh_static_info -u base_GHCziWord_W32zh_static_info -u base_GHCziWord_W64zh_static_info -u base_GHCziStable_StablePtr_static_info -u base_GHCziBase_Izh_con_info -u base_GHCziBase_Czh_con_info -u base_GHCziFloat_Fzh_con_info -u base_GHCziFloat_Dzh_con_info -u base_GHCziPtr_Ptr_con_info -u base_GHCziPtr_FunPtr_con_info -u base_GHCziStable_StablePtr_con_info -u base_GHCziBase_False_closure -u base_GHCziBase_True_closure -u base_GHCziPack_unpackCString_closure -u base_GHCziIOBase_stackOverflow_closure -u base_GHCziIOBase_heapOverflow_closure -u base_GHCziIOBase_NonTermination_closure -u base_GHCziIOBase_BlockedOnDeadMVar_closure -u base_GHCziIOBase_BlockedIndefinitely_closure -u base_GHCziIOBase_Deadlock_closure -u base_GHCziIOBase_NestedAtomically_closure -u base_GHCziWeak_runFinalizzerBatch_closure -u base_GHCziConc_ensureIOManagerIsRunning_closure /usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crt1.o /usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crti.o /usr/lib/gcc/i486-linux-gnu/4.1.3/crtbegin.o -L/usr/local/lib/ghc-6.8.3/lib/haskell98-1.0.1.0 -L/usr/local/lib/ghc-6.8.3/lib/array-0.1.0.0 -L/usr/local/lib/ghc-6.8.3/lib/process-1.0.0.1 -L/usr/local/lib/ghc-6.8.3/lib/unix-2.3.0.1 -L/usr/local/lib/ghc-6.8.3/lib/random-1.0.0.0 -L/usr/local/lib/ghc-6.8.3/lib/directory-1.0.0.1 -L/usr/local/lib/ghc-6.8.3/lib/filepath-1.1.0.0 -L/usr/local/lib/ghc-6.8.3/lib/old-time-1.0.0.0 -L/usr/local/lib/ghc-6.8.3/lib/old-locale-1.0.0.0 -L/usr/local/lib/ghc-6.8.3/lib/base-3.0.2.0 -L/usr/local/lib/ghc-6.8.3 -L/usr/lib/gcc/i486-linux-gnu/4.1.3 -L/usr/lib/gcc/i486-linux-gnu/4.1.3 -L/usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib -L/lib/../lib -L/usr/lib/../lib byteindex.o -lHShaskell98-1.0.1.0 -lHSarray-0.1.0.0 -lHSprocess-1.0.0.1 -lHSunix-2.3.0.1 -lutil -ldl -lHSrandom-1.0.0.0 -lHSdirectory-1.0.0.1 -lHSfilepath-1.1.0.0 -lHSold-time-1.0.0.0 -lHSold-locale-1.0.0.0 -lHSbase-3.0.2.0 -lHSrts -lm -lgmp -ldl -lrt -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/i486-linux-gnu/4.1.3/crtend.o /usr/lib/gcc/i486-linux-gnu/4.1.3/../../../../lib/crtn.o byteindex.o: In function `sKE_info': (.text+0x8bf): undefined reference to `__stginit_utf8zmstringzm0zi3zi3_SystemziIOziUTF8_' collect2: ld returned 1 exit status *** Deleting temp files: Deleting: /tmp/ghc6586_0/ghc6586_0.s Warning: deleting non-existent /tmp/ghc6586_0/ghc6586_0.s *** Deleting temp dirs: Deleting: /tmp/ghc6586_0 Any help would be greatly appreciated. -- Ben George blgeorge(a)fastmail.fm -- http://www.fastmail.fm - The way an email service should be

2 1

bug in time libs?
by Jason Dusek 02 Dec '08

02 Dec '08

I'm on 6.10.1, using the libs provided with the binary of GHC for Mac OS X. I was fooling around with the time package in GHCi and something seems to be off. I have included a transcript. If I take a time diff, then the seconds are positive when the picos are negative and vice versa. -- _jsn |...transcript...| Prelude System.Time> :m + System.Time Prelude System.Time> t0 <- System.Time.getClockTime Prelude System.Time> t1 <- System.Time.getClockTime Prelude System.Time> diffClockTimes t0 t1 TimeDiff { tdYear = 0 , tdMonth = 0 , tdDay = 0 , tdHour = 0 , tdMin = 0 , tdSec = -7 , tdPicosec = 238846000000 } Prelude System.Time> diffClockTimes t1 t0 TimeDiff { tdYear = 0 , tdMonth = 0 , tdDay = 0 , tdHour = 0 , tdMin = 0 , tdSec = 7 , tdPicosec = -238846000000 }

2 1

Re: Binary Trees missing on hackage
by Andrew Wagner 02 Dec '08

02 Dec '08

The reasons I've always heard for this is that 1.) It's so easy to define a tree and 2.) There are tons of different variations of trees and what you can do with them. Not that I 100% agree, just what I've always heard. On Mon, Dec 1, 2008 at 6:09 AM, Christian Maeder <Christian.Maeder(a)dfki.de>wrote: > Hi, > > I was surprised that I could not find a "classical" binary tree data > structure on hackage, mainly for teaching purposes, like: > > data BinTree a = Nil | Node (BinTree a) a (BinTree a) > > with a couple of utility functions and instances (i.e. in-order traversal). > > Surely, one may argue, that such a tree can always be defined on the fly > when needed, but nobody would argue so for lists or tuples. (Although > I've rarely seen someone redefining lists, it is quite common to > introduce user-defined products or enumerations.) > > There are rose trees in Data.Tree and many other variants of trees are > conceivable, ie. > > data Boom a = Leaf a | Node (Boom a) (Boom a) > > or a kind of combination: > > data BinLeafTree a b = > Leaf b > | Node (BinLeafTree a b) a (BinLeafTree a b) > > I don't know, why I find the above BinTree most important. I'm not even > sure if such a tree could be re-used for Search- or AVL-trees that need > strict fields with redundant size or height counters for efficiency > reasons. > > In any case I would find such a data type at least as useful as > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OneTuple > > Who would supply and maintain such a package? Or did I simply not search > hard enough? > > Cheers Christian > > P.S. I took the (non-empty) "Boom" from the Boom-Hierarchy described in > "An Exploration of the Bird-Meertens Formalism" by Roland Backhouse > 1988, Groningen > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries >

3 3

Can anybody give me some advice on this?
by Georgel Calin 02 Dec '08

02 Dec '08

Hello everybody, I have a piece of code that gives me headaches for some time now. Simply put, I would like to know which is the best way to overpass a "Couldn't match expected type * against inferred type *"-error and an "Occurs check: cannot construct the infinite type:"-error in the following situation: {-# OPTIONS -fglasgow-exts #-} > module Simple where > import Text.ParserCombinators.Parsec > > data HData a = O | C a deriving (Eq,Ord,Show) > data IN l = IN Int (HData l) deriving (Eq,Ord,Show) > data CH l = CH Char (HData l) deriving (Eq,Ord,Show) > -- data type is well-defined: > sample = C(IN 0 (C(CH 'a' (C(IN 1 (C(CH 'b' (C(IN 2 O))))))))) > > embeddedParser types = do string "end"; spaces; return O > {- > <|> do let h = head types > let t = tail types > case h of > 1 -> do aux <- pInt > rest <- embeddedParser $t++[h] > return $ C (IN aux rest) > 2 -> do aux <- pCh > rest <- embeddedParser $t++[h] > return $ C (CH aux rest) > _ -> error "unallowed type" > -} > pInt = do n <- fmap read $ many1 digit; return $ fromInteger n > pCh = do c <- letter; return $ c > simple = embeddedParser [1,2] > > -- the above result from sample I would like to get by running > -- parseTest simple "0a1b2end" > The way I see it, the defined datatype works but I am a bit clueless about how to modify the parser to accept things of the type (e.g.): HData (IN (CH (IN (CH (IN a))))) (and in general of any finite type embedded like this). Thanks in advance for your help, George

2 2

'#' in literate haskell
by John MacFarlane 02 Dec '08

02 Dec '08

Can anyone explain why ghc does not treat the following as a valid literate haskell program? --------- test.lhs ---- # This is a test > foo = reverse . words ------------------------ When I try to load this in ghci (or compile it using ghc), I get: test.lhs:1:2: lexical error at character 'T' It seems that the problem is the '#' character in the first column. Replacing it with something else, or moving it to the right one space, solves the problem. The following literate haskell program, from http://notvincenz.blogspot.com/2008/01/literate-haskell-and-c.html, also fails to load for me, for the same reason (the leading '#' in line 8). ---- literate-haskell-and-c.lhs --- /* c and lhs file > > module Foo where > main = print "Haskell" > */ #include int main() { printf("C\n"); return 0; } ------------------------------------ I've reproduced this with ghc 6.10.1 and ghc 6.8.3 (linux binaries from haskell.org) and with ghc 6.8.2 (Ubuntu intrepid). Interestingly, hugs (September 2006 version) has no trouble with test.lhs. I haven't tried ghc 6.6. I care about this because I'd like to use markdown conventions to format the comment parts of literate haskell programs. Markdown supports atx-style headers, which begin with strings of '#' characters starting in the first column. I know that some people use markdown with literate haskell, so there must be something basic here that I'm missing! John

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Re: The Knight's Tour: solutions please
by Diego Echeverri 02 Dec '08

02 Dec '08

>>I've created a wiki page, >>http://haskell.org/haskellwiki/The_Knights_Tour >>I note the LogicT version is the shortest so far. >>-- Don Probably noob question. I was looking into the first solution in the page and tried to replace sortOn f = map snd . sortBy (comparing fst) . map (f &&& id) for sortOn' f = sortBy (comparing fst) The first one was much faster? Why? Thanks! Diego

2 1

Scala job in Boston writing quantitative finance software
by Paul Chiusano 02 Dec '08

02 Dec '08

Hi folks, The company I work for, ClariFI (http://clarifi.com/) is looking to hire developers with a strong background in functional programming to do a mixture of Scala and Java programming. It's fine if you don't happen to know Scala but are strong in Haskell - we are looking for people who really "get" FP and who we could turn loose designing and implementing clean, functional code in Scala once up to speed with the language. You'd also work on extending existing Scala modules and integrating the library code you write with application code written in Java. For more info, check out our official job posting: http://www.clarifi.com/About-ClariFI-Careers.php#SoftwareEngineer Some background on us: ClariFI is a small company (about 15 developers) that specializes in software for quantitative investment management. Our chief product, ModelStation, is used by portfolio managers for handling all stages of the quant process: building and backtesting of factors, portfolio optimization, simulation of trading strategies, performance and risk attribution, overall data management (including organizing huge amounts of time series data pulled from a diverse set of raw sources), and lots more. We don't expect you to have a background in finance but you should be willing and able to learn a lot once you start. This position is in our Boston office, so you must live or be willing to relocate here in order to be considered. If you're interested, send an email to myself or my boss, Scott McFarland (smcfarland(a)clarifi.com) along with your resume. Also, feel free to contact either of us if you just have questions about the position. Cheers, Paul

1 0

Parallel alpha-beta search
by Colin Paul Adams 02 Dec '08

02 Dec '08

Has anyone ever attempted this in Haskell? If so, what sort of speed-up have you had (with which algorithm)? -- Colin Adams Preston Lancashire

1 0

howto tuple fold to do n-ary cross product?
by Larry Evans 01 Dec '08

01 Dec '08

http://www.muitovar.com/monad/moncow.xhtml#list contains a cross function which calculates the cross product of two lists. That attached does the same but then used cross on 3 lists. Naturally, I thought use of fold could generalize that to n lists; however, I'm getting error: {-- cut here Compilation started at Sun Nov 23 13:15:24 make runghc -XMultiParamTypeClasses -XFunctionalDependencies -XFlexibleInstances cross.hs cross.hs:23:30: Occurs check: cannot construct the infinite type: a = (a, a) Expected type: [a] -> [a] -> [a] Inferred type: [a] -> [a] -> [(a, a)] In the first argument of `Data.Foldable.foldl1', namely `cross' In the first argument of `print', namely `(Data.Foldable.foldl1 cross (list2, list3, list1))' make: *** [run] Error 1 }-- cut here How do I apply fold to cross and n lists, for n>=0? TIA. -regards, Larry

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workarounds for Codec.Compression.Zlib errors in darcs
by Eric Kow 01 Dec '08

01 Dec '08

Hi everybody, This advisory is for people who have installed darcs 2.1.2 via the Cabal build method. As you may have noticed, the cabalised darcs sometimes fails with errors like Codec.Compression.Zlib: incorrect data check Why this happens ---------------- Older versions of darcs can to produce gzipped files with broken CRCs. We never noticed this because our homegrown wrapper around the C libz library does not pick up these errors. Lately, we have been working on adopting the Haskell zlib wrapper, which is made available by default in darcs.cabal. This new wrapper is more stringent and fails when it encounters these files. Workaround 1 : use C libz instead of Haskell zlib ------------------------------------------------- So how can you work around these errors? If you are building darcs on any Unix-y operating system (e.g. Linux or MacOS X), you can cabal configure darcs to use the old C libz binding: cabal configure -f external-zlib This will restore our homegrown wrapper which ignores the broken CRCs (note that the darcs head no longer *produces* these broken files, thanks to debugging by Matthias Andree and to a bugfix by David Roundy; http://bugs.darcs.net/issue844 for details). In principle, the same advice applies for Windows users, with more details hopefully to follow on how the C libz in a GHC-accesible location. Details to follow. In the meantime, you can either build darcs using the old configure and make technique (assuming you have MSYS and related tools), or use a binary that does not use the Haskell zlib wrapper (for example, by downgrading to http://www.haskell.org/~simonmar/darcs-2.0.2+75.zip ) Workaround 2 : fix your broken gzipped files -------------------------------------------- If you have control over the repositories with broken gzipped files, it should be possible to repair these files by gunzipping them and then redo-ing the gzip. We think that the attached script should help. Please report back if this is not the case. How we will fix this problem in the long term --------------------------------------------- I'm very sorry for the grief this has caused. To begin with, we will ensure that the 2.2 release gets more testing before releasing it in January. It will also handle these broken CRCs more gracefully. Our plan is to - either extend darcs repair or provide a Haskell script to fix these broken files - detect the broken files and advise users to run darcs repair (or the script) as needed - somewhere in the future, disallow broken CRC files whilst still advising users on how to fix their files. Many thanks! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

10 15

Problem with directory-1.0.0.0
by Bartosz Wójcik 01 Dec '08

01 Dec '08

Hi Folks, I'm facing problem after I've reinstalled directory-1.0.0.0 (setup configure/build/install). Since then I cannot complie anything that needs this library. It fails with following messages: Preprocessing library haddock-2.4.0... Preprocessing executables for haddock-2.4.0... Building haddock-2.4.0... /usr/bin/ar: creating dist/build/libHShaddock-2.4.0.a Linking dist/build/haddock/haddock ... /usr/local/lib/ghc-6.8.2/libHSghc.a(Coverage.o): In function `scXR_info': (.text+0x17b7c): undefined reference to `directoryzm1zi0zi0zi0_SystemziDirectory_lvl29_closure' /usr/local/lib/ghc-6.8.2/libHSghc.a(Coverage.o): In function `s8xU_info': (.text+0x1792c): undefined reference to `directoryzm1zi0zi0zi0_SystemziDirectory_a43_info' /usr/local/lib/ghc-6.8.2/libHSghc.a(Coverage.o): In function `r7aC_closure': (.data+0xd18): undefined reference to `directoryzm1zi0zi0zi0_SystemziDirectory_a43_closure' collect2: ld returned 1 exit status Situation is following: Old directory-1.0.0.0 resides in /usr/local/lib/ghc-6.8.2/lib/directory-1.0.0.0. New one in /usr/local/lib/directory-1.0.0.0. Why new one doesn't work? How to force linker to use old one? Best, Bartek

2 2

getLine and ^C on Windows
by Lyle Kopnicky 01 Dec '08

01 Dec '08

Hi folks, I'm using System.IO.getLine to read input in my program. I've compiled it on Windows Vista with ghc-6.10.1. I've noticed that if I press Ctrl+C while the program is waiting for input, it will get stuck. No keypresses can get me out of the program - I have to kill the shell. I've tried cmd.exe, Powershell and cygwin - same behavior. I'm sure editline would work better, but it's overkill. I just want to read lines of text and don't need fancy edit behavior. And I think editline complicates the build process. I also want my program to work right in non-interactive mode, reading redirected input. Am I missing something? Thanks, Lyle

4 3

catting to cat gets stuck at > 135K
by Jason Dusek 01 Dec '08

01 Dec '08

I've put together a simple test case for a rather annoying problem. I've got a program that drives other programs. For example, it can drive `cat`: :; Simple cat a-file When the file is a little bit greater than 135060 bytes, this program fails to produce any output at all -- I need to use ^C to get my console back. I have a similar program that encounters the same issue a little after 139192 bytes. The inconsistency is one baffling feature of this bug. If I remove the `hClose`, the example program just hangs, no matter the size of the input. It is entirely possible that I have overlooked some subtle feature of pipe semantics -- indeed, I hope that is the case. -- _jsn import Data.ByteString.Lazy import System.Process import System.Environment import System.IO (hClose) import Prelude hiding (writeFile, readFile) main = do exe:file:_ <- getArgs bytes <- readFile file foo <- simple exe bytes [] writeFile (file ++ ".foo") foo -- Manufactures a simple stream handler from a command line utility. simple :: String -> ByteString -> [String] -> IO ByteString simple exe bytes args = do (i, o, e, p) <- runInteractiveProcess exe args Nothing Nothing hPut i bytes s <- hGetContents o hClose i return s

7 13

Combining licences
by Emil Axelsson 01 Dec '08

01 Dec '08

Hello, I know very little about licensing, so I'm hoping to get some advice from you guys. In the next release of Wired, I plan to include an open-source standard cell library (or rather data derived from it). This cell library has a custom license (found at the bottom of http://www.nangate.com/openlibrary) It appears to be very permissive, but it states that the license text "must be included in all copies of the Library, in whole or in part, and all derivative works of the Library". Wired itself is BSD3-licensed. How do I arrange this in the package? Can I still have a BSD3 license for Wired? Where do I put the respective license texts? Any other things I should have in mind? Or perhaps it's better to put the cell library in its own package? I'm a bit reluctant to do this, because it means that Wired will be essentially useless on its own. Thanks, / Emil

2 2

Re: [Haskell-cafe] Cabal
by Henning Thielemann 01 Dec '08

01 Dec '08

On Sun, 30 Nov 2008, Don Stewart wrote: > lemming: >> >> Maybe you like to add a pointer in cabal-install.cabal/Homepage field to >> this page. > > Good idea. Duncan? After I finished that article, I also found: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall

2 2

The Knight's Tour: solutions please
by oleg＠okmij.org 01 Dec '08

01 Dec '08

It seems the following pure functional (except for the final printout) version of the search has almost the same performance as the Dan Doel's latest version with the unboxed arrays and callCC. For the board of size 40, Dan Doel's version takes 0.047s on my computer; the version below takes 0.048s. For smaller boards, the difference is imperceptible. Interestingly, the file sizes of the compiled executables (ghc -O2, ghc 6.8.2) are similar too: 606093 bytes for Dan Doel's version, and 605938 bytes for the version below. The version below is essentially Dan Doel's earlier version. Since the problem involves only pure search (rather than committed choice), I took the liberty of substituting FBackTrack (efficient MonadPlus) for LogicT. FBackTrack can too be made the instance of LogicT; there has not been any demand for that though. import Data.List import Data.Ord import qualified Data.IntMap as Map import System.Environment import FBackTrack import Control.Monad -- Emulate the 2-dimensional map as a nested 1-dimensional map initmap n = Map.fromList $ (1,Map.singleton 1 1):[ (k,Map.empty) | k <- [2..n] ] notMember (i,j) m = Map.notMember j $ Map.findWithDefault undefined i m insrt (i,j) v m = Map.update (Just . Map.insert j v) i m lkup (i,j) m = Map.findWithDefault undefined j $ Map.findWithDefault undefined i m successors n b = sortWith (length . succs) . succs where sortWith f = map fst . sortBy (comparing snd) . map (\x -> (x, f x)) succs (i,j) = [ (i', j') | (dx,dy) <- [(1,2),(2,1)] , i' <- [i+dx,i-dx] , j' <- [j+dy, j-dy] , i' >= 1, j' >= 1, i' <= n, j' <= n , notMember (i',j') b ] tour n k s b | k > n*n = return b | otherwise = do next <- foldl1 mplus.map return $ successors n b s tour n (k+1) next $ insrt next k b showBoard n b = unlines . map (\i -> unwords . map (\j -> pad $ lkup (i,j) b) $ [1..n]) $ [1..n] where k = length . show $ n*n + 1 pad i = let s = show i in replicate (k-length s) ' ' ++ s main = do (n:_) <- map read `fmap` getArgs let (b:_) = runM Nothing . tour n 2 (1,1) $ initmap n putStrLn $ showBoard n b

1 0

Haskell Weekly News: Issue 95 - November 30, 2008
by Brent Yorgey 01 Dec '08

01 Dec '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20081130 Issue 95 - November 30, 2008 --------------------------------------------------------------------------- Welcome to issue 95 of HWN, a newsletter covering developments in the [1]Haskell community. Real World Haskell is [2]finally here! [3]Read it online, and/or [4]get your own dead tree copy. Better yet, get two copies, one for yourself and one for a friend. The fifteenth [5]Haskell Communities and Activities Report is also here---check out all the exciting stuff being worked on in the Haskell world! Announcements HCAR. Janis Voigtlaender [6]announced the 15th edition of the [7]Haskell Communities and Activities Report (HCAR) is now available! Not quite another Haskell tutorial, but .... Janis Voigtlaender [8]announced that he submitted his [9]Habilitation thesis last week. The first few chapters of it try to give an introduction to Haskell with emphasis on types and reasoning principles. hledger 0.2. Simon Michael [10]announced version 0.2 of [http://joyful.com/hledger hledger, a minimal haskell clone of John Wiegley's [11]"ledger" text-based accounting tool. darcs zlib error workaround. Eric Kow [12]outlined workarounds and future plans for a darcs bug relating to broken CRCs in gzipped patch files. You should read this if you have installed darcs 2.1.2 via the Cabal build method. Turbinado 0.2. Alson Kemp [13]announced version 0.2 of [14]Turbinado, an easy-to-use, fast web application framework. Fun with type functions. Simon Peyton-Jones [15]requests examples of compelling use cases for type functions: "can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies? Simple is good. It doesn't have to be elaborate: just something that does something useful you could not have done otherwise." Jobs PhD Positions in Language-based Security at Chalmers. Andrei Sabelfeld [16]announced the availability of [17]PhD student positions in programming language-based security in the Department of Computer Science and Engineering at Chalmers University of Technology, Sweden. The application deadline is January 30, 2009. FP Jobs. Julien Sylvestre [18]announced several new permanent positions, based in Paris, with MLstate -- an IT company whose functional programming approach to SaaS and cloud computing has been recently recognized by the French Ministry of Research Innovation Award. Blog noise [19]Haskell news from the [20]blogosphere. * Dan Piponi (sigfpe): [21]An Approach to Algorithm Parallelisation. Dan illustrates a clever, general framework for parallelizing some not-so-obviously-parallelizable algorithms, based on linear operations over rings. * Real-World Haskell: [22]First European Orders Now Landing. * Braden Shepherdson: [23]Pimp Your XMonad #3: Prompt. Braden continues his series on getting the most out of your xmonad configuration with a post describing the Prompt family of extensions. * Real-World Haskell: [24]Amazon orders now arriving. * Real-World Haskell: [25]Orders Now Arriving on the US West Coast. * Luke Palmer: [26]Relative time FRP. A new, more elegant semantics for FRP -- relative time instead of absolute time? * JP Moresmau: [27]Predictable random for testing. * Osfameron: [28]London Perl Workshop tomorrow!. * >>> Phil Ratzsch: [29]Initial Haskell Impressions. * Chung-chieh Shan: [30]Metalinguistics. * Alex McLean: [31]Babble. A simple vocable synthesiser that runs in a web browser, written in HaXe. * Jonathan Tang: [32]Sum types vs. typeclasses. * Magnus Therning: [33]Re: Redesigning GHC's build system. Magnus's response to a recent post about redoing GHC's build system. * Real-World Haskell: [34]Real World Haskell is shipping!. * Twan van Laarhoven: [35]Knight in n, part 1: moves. Twan explores knight-move problems in Haskell. * Conal Elliott: [36]Semantic editor combinators. An elegant explanation of semantic editor combinators -- a principled framework for building up ways to "edit" components of structured values. * David Sankel: [37]Why is the Reactive Behavior tutorial taking so long? splitB. * Chung-chieh Shan: [38]Bowling balls. A nice solution to the bowling-ball problem in Haskell. * >>> Cory: [39]Developing in "impractical" languages. * Chris Double: [40]Dynamic Compilation and Loading of Modules in Haskell. A mini-tutorial on using the GHC API. * Ashish Hanwadikar: [41]More on Haskell DSL. A Haskell DSL for replacing 'make'. * Luke Palmer: [42]Screw laziness (w.r.t. Fran semantics). Quotes of the Week * quicksilver: I ACCIDENTALLY THE WHOLE VERB * dons: [on ghc's new code generation] <byorgey> so how's the new code gen better? <dons> it's got 98% less dumbs. * adu: source code is transient, dreams are forever. * monochrom: n is the nth English letter. * nomeata: Ah, it seems I'm creating a tuple with more than 62 elements somewhere... * dons: we had 15 years building ivory towers - time to throw rocks from the top! About the Haskell Weekly News New editions are posted to [43]the Haskell mailing list as well as to [44]the Haskell Sequence and [45]Planet Haskell. [46]RSS is also available, and headlines appear on [47]haskell.org. To help create new editions of this newsletter, please see the information on [48]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [49]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.realworldhaskell.org/blog/2008/11/25/real-world-haskell-is-shipp… 3. http://book.realworldhaskell.org/read/ 4. http://book.realworldhaskell.org/#buy 5. http://www.haskell.org/communities/ 6. http://www.haskell.org//pipermail/haskell-cafe/2008-November/051112.html 7. http://www.haskell.org/communities/ 8. http://article.gmane.org/gmane.comp.lang.haskell.general/16640 9. http://wwwtcs.inf.tu-dresden.de/~voigt/habil.pdf 10. http://www.haskell.org//pipermail/haskell-cafe/2008-November/050920.html] 11. http://newartisans.com/software/ledger.html 12. http://www.haskell.org//pipermail/haskell-cafe/2008-November/051021.html 13. http://www.haskell.org//pipermail/haskell/2008-November/020836.html 14. http://www.turbinado.org/ 15. http://www.haskell.org//pipermail/haskell-cafe/2008-November/051069.html 16. http://article.gmane.org/gmane.comp.lang.haskell.general/16637 17. http://www.chalmers.se/cse/EN/news/vacancies/positions/ph-d-positions-in 18. http://article.gmane.org/gmane.comp.lang.haskell.general/16636 19. http://planet.haskell.org/ 20. http://haskell.org/haskellwiki/Blog_articles 21. http://sigfpe.blogspot.com/2008/11/approach-to-algorithm-parallelisation.ht… 22. http://www.realworldhaskell.org/blog/2008/11/29/first-european-orders-now-l… 23. http://braincrater.wordpress.com/2008/11/29/pimp-your-xmonad-3-prompt/ 24. http://www.realworldhaskell.org/blog/2008/11/26/amazon-orders-now-arriving/ 25. http://www.realworldhaskell.org/blog/2008/11/28/orders-now-arriving-on-the-… 26. http://lukepalmer.wordpress.com/2008/11/28/relative-time-frp/ 27. http://jpmoresmau.blogspot.com/2008/11/predictable-random-for-testing.html 28. http://greenokapi.net/blog/2008/11/28/london-perl-workshop-tomorrow/ 29. http://www.geeklair.net/~pratzsch/blog/2008/11/initial-haskell-impressions.… 30. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Metalinguistics/ 31. http://yaxu.org/babble/ 32. http://eve-language.blogspot.com/2008/11/sum-types-vs-typeclasses.html 33. http://therning.org/magnus/archives/417 34. http://www.realworldhaskell.org/blog/2008/11/25/real-world-haskell-is-shipp… 35. http://twan.home.fmf.nl/blog/haskell/Knight1.details 36. http://conal.net/blog/posts/semantic-editor-combinators/ 37. http://netsuperbrain.com/blog/posts/why-is-the-reactive-behavior-tutorial-t… 38. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Bowling_balls/ 39. http://onag.blogspot.com/2008/11/developing-in-impractical-languages.html 40. http://www.bluishcoder.co.nz/2008/11/dynamic-compilation-and-loading-of.html 41. http://feedproxy.google.com/~r/typepad/ahanwadi/ashishs_niti/~3/n41w6S_82SE… 42. http://lukepalmer.wordpress.com/2008/11/23/screw-laziness/ 43. http://www.haskell.org/mailman/listinfo/haskell 44. http://sequence.complete.org/ 45. http://planet.haskell.org/ 46. http://sequence.complete.org/node/feed 47. http://haskell.org/ 48. http://haskell.org/haskellwiki/HWN 49. http://code.haskell.org/~byorgey/code/hwn/

1 0

Instances that shouldn't overlap
by Paul Johnson 01 Dec '08

01 Dec '08

Hi, I'm trying to set up some operators for applicative versions of prelude types. For instance: -- | Applicative Equality. class (Eq a) => AppEq f a where (.==.), (./=.) :: f a -> f a -> f Bool instance (Applicative f, Eq a) => AppEq f a where (.==.) = liftA2 (==) (./=.) = liftA2 (/=) Hopefully the intention is fairly straightforward: if "f" is an instance of Applicative then the lifted implementation of the underlying type. Otherwise I can just give my own instance, which is useful for things that "wrap" prelude types but where "fmap" doesn't work. For instance: data (Ord a) => Interval a = Interval a a instance (Ord a) => AppEq Interval a where i1@(Interval _ u1) .==. i2@(Interval _ u2) | isSingleton i1 && isSingleton i2 && u1 == u2 = Interval True True | has i1 u2 || has i2 u1 = Interval False True | otherwise = Interval False False i1 ./=. i2 = let Interval b1 b2 = (i1 .==. i2) in Interval (not b2) (not b1) isSingleton :: (Ord a) => Interval a -> Bool isSingleton (Interval lower upper) = lower == upper has :: (Ord a) => Interval a -> a -> Bool has (Interval lower upper) v = v >= lower && v <= upper You can't (easily) define fmap for Interval because the function given as an argument might not be monotonic. So instead you have to write custom implementations for each lifted function, as shown here for (.==.) and (./=.) . The same principle works for AppOrd, AppNum etc, but I'm trying to solve the problem for just AppEq for now. This compiles, but when I try to use it I get this in ghci: *Interval> let i1 = Interval 4 5 *Interval> let i2 = Interval 4 6 *Interval> i1 .==. i2 <interactive>:1:0: Overlapping instances for AppEq Interval Integer arising from a use of `.==.' at <interactive>:1:0-9 Matching instances: instance (Ord a) => AppEq Interval a -- Defined at Interval.hs:(22,0)-(27,78) instance (Control.Applicative.Applicative f, Eq a) => AppEq f a -- Defined at AppPrelude.hs:(32,0)-(34,23) In the expression: i1 .==. i2 In the definition of `it': it = i1 .==. i2 I'm puzzled, because Interval is not an instance of Applicative, so the second instance doesn't apply. Can anyone help me out? I'm using ghc 6.8.3, so its possible that this was a bug fixed in 6.10. Paul.

5 5

Permutations
by Andrew Coppin 01 Dec '08

01 Dec '08

OK, so here's something just for fun: Given a list of items, find all possible *unique* permutations of that list. (E.g., the input list is explicitly _allowed_ to contain duplicates. The output list should not contain any duplicate permutations.) I've found one simple way to do this, but I'm sure there are multiple valid approaches. So let's see what people come up with. ;-)

6 13

Re: [Haskell-cafe] Re: Go Haskell! -> array libraries
by Andrew Coppin 01 Dec '08

01 Dec '08

Henning Thielemann wrote: > I suspect that this particular function is less useful than you think. > It safes one allocation and might be faster since it uses less cache, > but on the other hand, it cannot be fused. If the array is "seriously large", you don't want to have five or six versions of it sitting in memory until the GC comes along to remove it. (OTOH, presumably an unboxed array can be allocated or freed quite quickly...) At a minimum, you've going to end up with two copies in memory - the existing one, and the one being built. > I think in-place array > updates are only sensible for writing array elements in really random > order. As long as you can formulate your algorithm the way "read from > random indices, but write a complete array from left to right", there is > almost no need for mutable arrays. > Does quick-sort fit that pattern? (I'm guessing yes, since all you need to do is filtering and merging...) Yeah, generally you only need arrays if you really want random access. Except in Haskell, where an array also happens to be the only way of storing large numbers of values unboxed. So sometimes you'll use an array because you want to save space. (E.g., parsing text is usually a sequential process with no random access, but you probably want to use ByteString all the same!) I sometimes also use unboxed arrays for the increase in strictness. I guess the thing to do would be to measure the difference...

5 5

"Could not find module ... which is hidden" problem
by Belka 30 Nov '08

30 Nov '08

Hello! ----------------------------START---------------------------------- $ sudo runghc Setup configure --user Configuring HCL-1.2... $ sudo runghc Setup build Preprocessing library HCL-1.2... Preprocessing executables for HCL-1.2... Building HCL-1.2... HCL.hs:302:7: Could not find module `System.Random': it is a member of package random-1.0.0.1, which is hidden $ sudo ghc-pkg expose random Saving old package config file... done. Writing new package config file... done. $ sudo runghc Setup configure --user Configuring HCL-1.2... $ sudo runghc Setup build Preprocessing library HCL-1.2... Preprocessing executables for HCL-1.2... Building HCL-1.2... HCL.hs:302:7: Could not find module `System.Random': it is a member of package random-1.0.0.1, which is hidden ---------------------THE-END---------------------------------------- Perhaps, i'm 1000th, who asks how to solve this. Sorry that didn't search better, but it's really tiring for a newby. :-( [ http://www.haskell.org/haskellwiki/Upgrading_packages/Updating_to_GHC_6.8 Here ] found something, which maybe called "a gentle explaination" of what's going on. I have ghc ver: 6.8.2, base ver: 3.0.1.0. So there are 2 questions of mine: (1) Is there any centralized and well organized Haskell Knowledge Base, where it would be much easier to find solutions for "popular" problems? Anybody, please, your recommendations. (2) As for the topic: the only option for me would be obtaining older GHC (base?) - is that correct? So that require me reinstalling every Haskell package I have installed so far? Thanks in advance. -- View this message in context: http://www.nabble.com/%22Could-not-find-module-...-which-is-hidden%22-probl… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 3

Data parallelism doesn't seem to work on windows...
by Sebastian Sylvan 30 Nov '08

30 Nov '08

Hi,I can't seem to get DPH to work on 6.10.1 on Vista-64. I run the executables with +RTS -N2, and to verify that I'm doing it correctly I checked with a simple benchmark using forkIO and that does indeed use both my cores: -- compiler command line: ghc --make -O2 -threaded parr.hs -- execution command line: parr.exe +RTS -N2 main = do forkIO $ print [ True | n <- [ 1000 .. 3000], fac n == 0 ] forkIO $ print [ True | n <- [ 1000 .. 3000], fac n == 0 ] getLine return () This, on the other hand does not use more than one core: -- compiler command line (from shootout code): ghc --make -fcpr-off -threaded -fdph-par -package dph-base -Odph -XPArr parr2.hs -- execution as before main = print $ [: True | n <- [: 1000 .. 5000 :], fac n == 0 :] That's 4000 items of work there, so surely it should kick off plenty of sparks to overcome the "sparks bug"? -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862

2 1

Control.Exception Funny
by Dominic Steinitz 30 Nov '08

30 Nov '08

I'm probably doing something wrong but this example doesn't compile for me under ghc 6.10.1 (http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exceptio…) > catch (openFile f ReadMode) > (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e)) > Run.hs:77:24: > Couldn't match expected type `Handle' against inferred type `()' > Expected type: IO Handle > Inferred type: IO () > In the expression: > hPutStr stderr ("Couldn't open " ++ d ++ ": " ++ show e) > In the second argument of `CE.catch', namely > `(\ e -> hPutStr stderr ("Couldn't open " ++ d ++ ": " ++ show e))' Fair enough because openFile returns a Handle and hPutStr returns () so they don't match as the compiler says. > CE.catch :: (CE.Exception e) => IO a -> (e -> IO a) -> IO a So if I fix the example thus: > foo d = CE.catch (openFile d ReadMode >> return ()) > (\e -> hPutStr stderr ("Couldn't open "++ d ++": " ++ show e)) I get > Run.hs:70:8: > Ambiguous type variable `e' in the constraint: > `CE.Exception e' > arising from a use of `CE.catch' at Run.hs:(70,8)-(71,78) > Probable fix: add a type signature that fixes these type variable(s) Now I think I never used to get this under 6.8.2 but I don't easily have a 6.8.2 to try it out on. Doing what the compiler suggests doesn't work for obvious reasons: > foo :: CE.Exception e => FilePath -> IO () > foo d = CE.catch (openFile d ReadMode >> return ()) > (\e -> hPutStr stderr ("Couldn't open "++ d ++": " ++ show e)) > Run.hs:69:0: > Ambiguous constraint `CE.Exception e' > At least one of the forall'd type variables mentioned by the constraint > must be reachable from the type after the '=>' > In the type signature for `foo': > foo :: (CE.Exception e) => FilePath -> IO () There seems to be a ticket for it (http://hackage.haskell.org/trac/ghc/ticket/2819) but this doesn't give a suggested example that compiles. Dominic.

2 2

Re: Go Haskell! -> array libraries
by John Lato 30 Nov '08

30 Nov '08

> From: Andrew Coppin <andrewcoppin(a)btinternet.com> > >>> My view would be to let the free market of developers decide what is >>> best. No bottlenecks -- there's too many Haskell libraries already (~1000 now). >>> >>> And this approach has yielded more code than ever before, more libraries >>> than ever before, and library authors are competing. >>> >>> So let the market decide. We're a bazaar, not a cathedral. >>> > > I find this kind of attitude disturbing. > > Are you seriously asserting that it's "bad" for people to stop and think > about their designs before building? That it's "bad" for people to get > together and coordinate their efforts? Would you really prefer each and > every developer to reinvent the wheel until we have 50,000 equivilent > but slightly different wheel implementations? Certainly you seem > obsessed with the notion that "more packages on Hackage == better". Well > in my book, quantity /= quality. (The latter being vastly more important > than the former - while admittedly far harder to measure objectively.) I > would far prefer to see one well-written library that solves the problem > properly than see 25 incompatible libraries that all solve small > fragments of the problem poorly. In the latter case, there will be no > "competition" between libraries; everybody will just give up and not use > *any* of them. You _can_ have too much choice! > > I really hope I'm not the only person here who sees it this way... > I would love to see a perfect, unified array library in Haskell. I think everyone would. However, the problem Don, Roman, and others have raised is that there is no single consensus on what that library would look like, or how it would be implemented. It might be impossible for one library to fill the entire design space. Don's point is that, since this isn't yet a solved problem, having multiple implementations available to see what works well (and what doesn't) is a necessary step in finding a solution. I also think this is a exactly why Hackage needs a way to indicate that packages are deprecated/superceded by other packages. There was some talk about this recently, and IIRC even a submitted patch. I hope that gets adopted soon. John

3 2

GHC 6.10.1 multi-threaded broken?
by Jeroen Baekelandt 30 Nov '08

30 Nov '08

Hi, Since I upgraded from ghc 6.8.3 to 6.10.1, I noticed that my programs do not run multi-threaded anymore. I tried simplifying my code, till I just took one of the par/pseq demo's to verify if it wasn't my fault. When I compile this code on 6.8.3 (both ubuntu and OS X), top shows something like 180% CPU usage and the elapsed time is almost halved. When compiled with 6.10.1, top shows max 100% and no speedup is noticable at all. Sample run: $ ghc --make -threaded paralleltest $ time ./paralleltest +RTS -N1 -RTS 119201850 real 0m26.024s user 0m25.494s sys 0m0.220s $ time ./paralleltest +RTS -N2 -RTS 119201850 real 0m25.770s user 0m25.539s sys 0m0.167s I have the same on OS X and Ubuntu. I even compiled the Ubuntu ghc from source myself and verified that GMP was used. Any ideas? Thanks, Jeroen Here's the code: module Main where import Control.Parallel fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib (n - 1) + fib (n - 2) mkList :: Int-> [Int] mkList n = [1..n-1] relprime :: Int -> Int -> Bool relprime x y = gcd x y == 1 euler :: Int -> Int euler n = length (filter (relprime n) (mkList n)) sumEuler :: Int -> Int sumEuler = sum . (map euler) . mkList parSumFibEuler :: Int -> Int -> Int parSumFibEuler a b = f `par` (e `pseq` (e + f)) where f = fib a e = sumEuler b main = do print $ parSumFibEuler 40 7450

2 1

Extensible Exceptions
by David F. Place 29 Nov '08

29 Nov '08

Hi, All. I am trying to understand the new exceptions package in base-4 Control.Exceptions. The documentation for catchJust is the same as in Control.OldException including this example: result <- catchJust errorCalls thing_to_try handler Control.OldException provides the predicate errorCalls, but the new one does not. I don't see how to write it. Thanks for reading. Cheers, David

6 11

Using Parsec with other monads
by Roly Perera 29 Nov '08

29 Nov '08

Hi, I've spent some time writing a parser using the Parsec library and was looking forward to being able to plug in some side-behaviour once I'd got the parser working. Now it seems I can't actually do that in a nice way because Parsec appears to be "fixed" to a simple State monad. I found this mentioned in the Cafe archives, but not much discussion. Is there a reason Parsec wasn't implemented using the monad transformer approach? Are there any plans to open it up? It's a nice powerful library and a natural thing to want to do for example would be to plug in something like a Reader to track a variable context. I guess one could abuse the State monad to achieve the goal but that feels like the wrong tool for the job. Any insights appreciated. thanks, Roly

3 2

definitive list of editline bindings anywhere?
by Paul Brown 29 Nov '08

29 Nov '08

All -- Anyone have a definitive list of editline keybindings available? I find myself missing some of the capabilities of readline, and there doesn't seem to be documentation. -- Paul

2 1

Question about fastcgi
by Mauricio 29 Nov '08

29 Nov '08

Hi, I'm learnng to use fastcgi and, reading the examples, I see the main "loop" is like this: main = runFastCGI my_work However, isn't a fastcgi program supposed to choose a port where to listen to calls? For instance, in this C example: xzdev.com/nginx_fastcgi.html doesn't the line listen_socket = FCGX_OpenSocket(":8002", 2000); says it's listening to port 8002? I read the code for fastcgi, from hackage, and I can't find anything related to ports like, for instance, a default port. Am I understanding something the wrong way? Thanks, Maurício

3 4

Shared modification inside lambdas
by Andrew Hunter 28 Nov '08

28 Nov '08

I'm developing an embedded DSL in Haskell (for exact real arithmetic work, but that's irrelevant right now.) Among other things, the language contains functions and application, as well as various terms. In one iteration, the abstract syntax looked kinda like this (in spirit, don't worry about the actually available terms...) data Expr = Var String | Const Int | Plus Expr Expr | Quantified Range String Expr | Lambda String Expr | App Expr Expr Here, lambda-terms just have the name of their bound variable and the RHS. Quantified terms are much like specialized lambdas; one note is that you *will* have to evaluate the RHS of the quantified term with many values for the argument. It's obvious how to write an evaluator on these--just keep an environment, etc. All well and good, but it seems to me that if I'm embedding the DSl, shouldn't I be able to use the host language's facilities--for example, function abstractions and applications--directly? Well, I tried this, and it seems it works OK, like so: data Expr = Var String | Const Int | Plus Expr Expr | Quantified Range (Int -> Expr) I replaced Lambda terms and applications with Haskell-level functions. Note that my quantifiers still need internal functions, but I replaced them in the same fashion. However, I have a problem; one thing I often have to do with expressions is refine them, where refine has type: refine :: Expr -> Expr and transforms expressions into more interesting ones via certain, mostly simple rules. The problem comes with functions, either just plain lambdas or quantified ones. The rule for refining functions boils down to just refining (recursively) the RHS; in the old version, that was easy! refine (Lambda var rhs) = Lambda var (refine rhs) I only refine while evaluating terms of type Expr, so top level functions aren't important--by the time I'm evaluating them, I've applied them to arguments--but quantified terms are a problem. I could write something like: refine (Quantified range pred) = Quantified range pred' where pred' = \c -> refine (pred c) But the problem is that this refines the term, again, every time I apply an argument to pred': I know I'm going to apply arguments many times to that new pred, and I want to refine it /once/: refinement is argument-agnostic, it only rearranges the structure of the AST, so in theory it'd be nice if I could refine that structure, just duplicating references to the appropriate free variable where I would have duplicated refences to the appropriate (Var String) before. Am I sore out of luck here? Is there a reasonable way I can implement my internal functions via Haskell functions, but apply argument-agnostic transformations to the RHS in a shared fashion? Or, is there some optimization in GHC that means I don't need to worry about this? Thanks, AHH

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Re: Suggestion: Syntactic sugar for Maps!
by Don Stewart 28 Nov '08

28 Nov '08

bulat.ziganshin: > Hello circ, > > Thursday, November 27, 2008, 9:59:08 PM, you wrote: > > So why not {"hello": 1, "there": 2} ? > > mymap "hello:1 there:2" > > where mymap implementation is left to the reader :) Hey, well, even easier: {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE FlexibleInstances #-} import Data.Map import Data.String import Text.JSON instance IsString (Map Int Bool) where fromString = fromList . read -- or, say, JSON syntax for assoc lists. {- fromString s = case resultToEither (decode s) of Right a -> a Left s -> error s -} test :: Map Int Bool test = "[(7, True), (1, False)]" main = print test -- Don

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I have forgotten .. ghci question
by Galchin, Vasili 28 Nov '08

28 Nov '08

Hello, I have an experimental version of a package ~/FTP/Haskell/blah. Who I "point" ghci ath this experimental package version so I can poke around? Kind regards, Vasili

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Need machine for DPH benchmarking
by Roman Leshchinskiy 28 Nov '08

28 Nov '08

Hi all, we, the DPH team, are at the moment in the very unfortunate situation of not having a proper machine for running our benchmarks on. Could a kind soul maybe give us (i.e., me) access to a quadcore or 2xquadcore x86 Linux or OS X machine? I only need to build ghc on it and run small benchmarks which never take more than a couple of minutes, maybe once every couple of days or so. We do need to use all cores, though, so no other CPU-intensive processes can be running during benchmarking. This is only for a week or two, until we get our own machine. We would be eternally grateful and won't forget you when DPH takes over the world. Roman

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RE: [Haskell-cafe] followedBy parser in Parsec
by Paul Keir 28 Nov '08

28 Nov '08

'lookAhead' is exactly what I needed: try $ array_element >>= \ae -> lookAhead (reservedOp "(") >> return ae Many thanks, Paul Maybe you're looking for 'lookAhead'? > > [...] //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Building QuickCheck 2 Under GHC 6.10.1
by Dominic Steinitz 28 Nov '08

28 Nov '08

Having been a happy user of QuickCheck 2 for many years, I now find it won't build under ghc 6.10.1. Before I investigate further, has anyone encountered this problem and has a fix? Thanks, Dominic. > C:\Users\Dom\QuickCheck>Setup build > Preprocessing library QuickCheck-2.0... > Building QuickCheck-2.0... > [2 of 9] Compiling Test.QuickCheck.Exception ( Test\QuickCheck\Exception.hs, dis > t\build\Test\QuickCheck\Exception.o ) > > Test\QuickCheck\Exception.hs:12:31: > Class `Exception' used as a type > In the type `Exception' > In the type `Either Exception a' > In the type `IO (Either Exception a)' > > Test\QuickCheck\Exception.hs:15:36: > Class `Exception' used as a type > In the type `Exception' > In the type `Either Exception a' > In the type `IO (Either Exception a)'

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followedBy parser in Parsec
by Paul Keir 28 Nov '08

28 Nov '08

Hi, Is there a way in Parsec to check what the next token is, and if it is what you're hoping for, leave it there. This is an example of something which doesn't work at all: testpar = try $ do ae <- array_element option [] $ try $ satisfy (\c -> c /= '(') >> unexpected "" return ae I'm finding this totally confusing by now %0 Can I invert "notFollowedBy" somehow, or maybe there's a "peek" function I don't know about? Help! Thanks, Paul

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HaskellWiki image upload fails because of - probably - the filesize.
by And1 28 Nov '08

28 Nov '08

Hey dear Haskell-Cafe-Readers, I tried several times to upload several images - screenshots that is - to my HaskellWiki/Xmonad site: http://haskell.org/haskellwiki/Image:And1_xmonad.png I always get the error, that the connection to the site/server was reset. I mailed both administrators of the HaskellWiki but I got, for about 2-4 weeks now, no reply. I tried to upload the following images, maybe that's important: http://and1.donnergurgler.net/misc/screen_2008-09-26.png http://and1.donnergurgler.net/misc/screen_2008-11-02.png http://and1.donnergurgler.net/misc/screen_2008-11-10.png I was told, that the problem is maybe the filesize of the new images, because it is bigger, then the filesize of the image already uploaded. Thanks for reading this and hopefully finding the problem, -- Regards, Andi R.

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Possible issue with Hoogle and Haddock?
by Stephen Hicks 27 Nov '08

27 Nov '08

Hi, I was noticing recently that there seems to be a problem with Hoogle and Haddock. In particular, I just hoogled "bracket" and got the following result: bracket :: IO a -> a -> IO b -> a -> IO c -> IO c Clearly this is the wrong type, as it should be bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c The Haddock-generated documentation at least has newlines to group the terms, but this is still far from correct. I noticed the same thing a while back in Parsec's documentation for token, token :: Stream s Identity t => t -> String -> t -> SourcePos -> t -> Maybe a -> Parsec s u a which again should have parens. Is this a bug? Is is something that's well-known? Cheers, steve

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haddock question
by allan 27 Nov '08

27 Nov '08

Dear all I'm trying to locally build the documentation for the haskell-src-exts package and running into a bit of bother. If I run: cabal haddock I get the error: haddock: parse error in doc string so: cabal haddock -v Doesn't really provide any extra information, it gives me the exact haddock command-line used but there is still no way (I can see) of obtaining the source file containing the parse error. So I downloaded via darcs the latest haddock source code and installed that but I get the exact same error message. I tried grepping in the haddock source code for that particular error message (and portions of it) but I couldn't find it. Finally I tried running haddock on a single source file: haddock Language/Haskell/Exts.hs and even this gets the exact same error message, which is somewhat bizarre since that file does not contain any documentation strings. Anyone have any ideas? regards allan -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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Philip Wadler video on Howard-Curry Correspondence ???
by Galchin, Vasili 27 Nov '08

27 Nov '08

Hello, I am reading re-reading Prof. Wadler paper Proofs are Programs: 19th Century Logic and 21st Century Computing but also want to re-read watch his video on same subject..... ??? Very kind thanks, Vasili

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Problem getting code from AFP08 parallel tutorial to run in parallel
by Olivier Boudry 27 Nov '08

27 Nov '08

Hi all, I'm reading the following tutorial: http://research.microsoft.com/~simonpj/papers/parallel/AFP08-notes.pdf "A Tutorial on Parallel and Concurrent Programming in Haskell" and have problems getting the expected speed improvement from running two tasks in parallel. With any version of the code present in pages 1-7 of the tutorial I keep getting the CPU stick to 50%. I did not forget to compile the code with `-threaded` and run it with `+RTS -N2` and it runs on a dual core machine on which I already used the Control.Parallel.Strategies.parMap function and got 100% CPU usage. The first version of the parallel function in the tutorial (page 6) is: parSumFibEuler :: Int −> Int −> Int parSumFibEuler a b = f 'par' (f + e) where f = fib a e = sumEuler b In the tutorial, swapping f and e on the 3rd line does the job, but in my case it doesn't change anything. C:\Temp\haskell>ghc --make -threaded SumEulerP6.hs [1 of 1] Compiling Main ( SumEulerP6.hs, SumEulerP6.o ) Linking SumEulerP6.exe ... C:\Temp\haskell>SumEulerP6 +RTS -N1 sum: 119201850 time: 36.890625 seconds C:\Temp\haskell>SumEulerP6 +RTS -N2 sum: 119201850 time: 36.859375 seconds Next page of the tutorial the tasks are explicitly sequenced so the code does not depend on the ordering of the two `+` operands: parSumFibEuler :: Int -> Int -> Int parSumFibEuler a b = f `par` (e `pseq` (f + e)) where f = fib a e = sumEuler b With once again a disappointing result: C:\Temp\haskell>ghc --make -threaded SumEulerP7.hs [1 of 1] Compiling Main ( SumEulerP7.hs, SumEulerP7.o ) Linking SumEulerP7.exe ... C:\Temp\haskell>SumEulerP7 +RTS -N1 sum: 119201850 time: 36.875 seconds C:\Temp\haskell>SumEulerP7 +RTS -N2 sum: 119201850 time: 36.75 seconds I tried this on a Windows XP Dell Dual Core with GHC 6.10.1 and on a iMac Dual Core with GHC 6.8.3 and got the same result on both. I'm probably missing something really stupid but I'm lacking the "parallel thinking" skills to understand how to look at the problem and resolve it. Any pointers? Thanks, Olivier.

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Need machine for DPH benchmarking
by Dominic Steinitz 27 Nov '08

27 Nov '08

> we, the DPH team, are at the moment in the very unfortunate situation > of not having a proper machine for running our benchmarks on. Could a > kind soul maybe give us (i.e., me) access to a quadcore or 2xquadcore > x86 Linux or OS X machine? I only need to build ghc on it and run > small benchmarks which never take more than a couple of minutes, maybe > once every couple of days or so. We do need to use all cores, though, > so no other CPU-intensive processes can be running during > benchmarking. This is only for a week or two, until we get our own > machine. We would be eternally grateful and won't forget you when DPH > takes over the world. > > Roman > Roman, Graeme can set you up on an 8 core machine if you email him. Dominic.

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Instances that shouldn't overlap
by oleg＠okmij.org 27 Nov '08

27 Nov '08

Paul Johnson wrote: > class (Eq a) => AppEq f a where > instance (Applicative f, Eq a) => AppEq f a where > instance (Ord a) => AppEq Interval a where In Haskell, instances are selected based solely on the types in the head. Constraints like `Applicative f' are not consulted when the instance is being selected. Rather, constraints are checked after the instance has been selected. The above two instances are overlapping since Interval is a particular case of `f'. That said, it is possible to select an instance based on constraints. The approach is described at http://haskell.org/haskellwiki/GHC/AdvancedOverlap

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Finalisers
by Andrew Coppin 27 Nov '08

27 Nov '08

Here's an interesting question... Is it possible to attach finalisers to a value? (That is, have some Haskell code executed when the item in question is reclaimed by the GC.) I'm interested in knowing whether a particular data structure is shared (i.e., whether it's safe to mutate it or whether it must be copied first), and a simple reference-counting scheme looks like the easiest option.

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build tools and Cabal
by John Lato 27 Nov '08

27 Nov '08

Hello, Cabal allows specifying arguments for tools it recognizes on the command line, e.g. runhaskell Setup.hs configure --c2hs-option=some_option Unfortunately, I can't find a way to make this work with .cabal (or .buildinfo) files, except for the specific cases of ghc, hugs, and nhc98 options. Am I missing something? If not, could this be added easily? It would be very helpful as I'm trying to work around the broken cpp Apple provides. Thanks, John

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Re: [Haskell] Wait for *either* MVar to be set
by Isaac Dupree 27 Nov '08

27 Nov '08

Peter Verswyvelen wrote: > ... by spawning and killing two threads (which might be an expensive operation, I'm not sure) pretty cheap in GHC -- they're not system threads > Am I wrong in this? If so, is this something that might be considered as a future enhancement in the GHC libraries and runtime? Also, look at STM (e.g. TVars), which is designed to do what you want more directly, I think. (timeouts might still use the GHC-thread thing. Don't worry about its performance unless you're measurably suffering from it...) -Isaac

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monads with take-out options
by Greg Meredith 27 Nov '08

27 Nov '08

Haskellians, Some monads come with take-out options, e.g. - List - Set In the sense that if unit : A -> List A is given by unit a = [a], then taking the head of a list can be used to retrieve values from inside the monad. Some monads do not come with take-out options, IO being a notorious example. Some monads, like Maybe, sit on the fence about take-out. They'll provide it when it's available. Now, are there references for a theory of monads and take-out options? For example, it seems that all sensible notions of containers have take-out. Can we make the leap and define a container as a monad with a notion of take-out? Has this been done? Are there reasons for not doing? Can we say what conditions are necessary to ensure a notion of take-out? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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