December 2008 - Haskell-Cafe

'#' in literate haskell
by John MacFarlane 01 Dec '08

01 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 01 Dec '08

01 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

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Scala job in Boston writing quantitative finance software
by Paul Chiusano 01 Dec '08

01 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

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Parallel alpha-beta search
by Colin Paul Adams 01 Dec '08

01 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

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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

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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

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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

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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

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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

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