December 2007 - Haskell-Cafe

library to read/write audio files
by John Lato 18 Jan '08

18 Jan '08

Hello, I've been working on a library to encode/decode audio files (wave, aiff, etc.) to and from lazy bytestrings, and it's finally in a form where I'm willing to share. It's available at http://mml.music.utexas.edu/jwlato/HSoundFile/, lightly cabalized and haddock-ified. The basic item is a Data.SoundFile datatype and a SndFileCls class. Each file format (wave, etc.) has a datatype with instances of SndFileCls and Data.Binary (thanks to Don S. for suggesting Data.Binary). At least that's the idea, I've only implemented the Wave format so far, but I wanted to make it easy to add new formats. I originally intended to use Erik de Castro Lopo's excellent libsndfile (and there are some remnants of that in darcs), but after realizing I wanted a more functional interface I decided to roll my own. It's likely possible to wrap libsndfile in a functional interface, but that is beyond my current abilities. This library is far from complete. Firstly, only the wave format is implemented (and incompletely). There are also only minimal HUnit tests; I plan to add QuickCheck tests when I have some more time. Secondly there are performance issues. For relatively short clips it's fine, but there seems to be a space leak. Attempting to copy large files while just decoding and encoding the data will quickly exhaust available memory. Other than the space leak; performance is good. Profiling indicates that reading is extremely fast, and the slowest operations during write are the multiple calls to Data.Binary.Put.putWord16le (for 16-bit files). Also, sound data is currently read into a list. I will probably change this at some point in the future, most likely copying the lazy bytestring implementation and using a list of CFloat arrays. If anyone would care to offer suggestions (or patches), they would be greatly appreciated. This is my first real project in Haskell, so please be kind. I've successfully build it on ubuntu linux with GHC 6.6.1 and 6.8.1, and I don't see any reason it won't work on other platforms. Thank you, John Lato PS I'd appreciate knowing if this project is reinventing the wheel. I didn't see anything similar in hoogle or google, but you never know.

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announcing darcs 2.0.0pre2
by David Roundy 17 Jan '08

17 Jan '08

I am pleased to announce the availability of the second prerelease of darcs two, darcs 2.0.0pre2. This release fixes several severe performance bugs that were present in the first prerelease. These issues were identified and fixed thanks to the helpful testing of Simon Marlow and Peter Rockai. We also added support for compilation under ghc 6.4, so even more users should be able to test this release. As before, some information about the prerelease is available at: http://wiki.darcs.net/index.html/DarcsTwo You can either use darcs to grab the latest darcs, or you can download a tarball at: http://darcs.net/darcs2.0.0pre2.tar.gz A few outstanding performance issues are: 1. darcs whatsnew performance has dropped for hashed repositories due to no longer tracking file-modification times. We need to reenable this feature, but it's not quite clear how best to do so. 2. darcs get on a hashed repository is not as fast as the older darcs get --partial. We could fix this by enabling the downloading of a single file _darcs/pristine.hashed.tar.gz. This eliminates the potential benefits of caching file downloads on darcs get (currently a second "get" of the same remote repository is almost free if you've enabled a global cache), so we may want to make this behavior optional. I hope we can get even more testing with this release, and look forward to finding and fixing any remaining performance regressions (or bugs, of course)! David

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Gtk2HS and GHC 6.8.2
by Peter Verswyvelen 12 Jan '08

12 Jan '08

It seems GHC 6.8.2 fixes a couple of bugs in GHC 6.8.1. Gtk2HS does not yet detect my GHC 6.8.2 installation, but I guess it is 100% compatible. Is it possible to get the Gtk2HS installer detect GHC 6.8.2? I'm on Windows. Thanks, Peter

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Haskell-mode 2.4
by Stefan Monnier 11 Jan '08

11 Jan '08

Here's for Xmas, version 2.4 of the Haskell-mode package. Haskell-mode is an Elisp package to support editing Haskell in Emacs. See the home page at http://www.iro.umontreal.ca/~monnier/elisp/ -- Stefan Changes since 2.3: * Update license to GPLv3. * New derived major mode for .hsc files. * Removed the C-c C-r binding to reload a file. You can still call inferior-haskell-reload-file (and/or bind it to your favorite key, including C-c C-r) or you can now use C-u C-c C-l. * C-c C-d looks up the symbol at point in the Haddock docs. * Haddock comments are highlighted with font-lock-doc-face if it exists. * Use `tex' rather than `latex' for haskell-literate. * inf-haskell.el tries to find the root of the module hierarchy to determine the root of a project (either by looking for a Cabal file or relying on the `module' declaration line). If all works well, this will make C-c C-l automatically switch to the root dir, so that dependencies in other directories are automatically found. If it doesn't, complain and/or set inferior-haskell-find-project-root to nil. * The new command haskell-hoogle helps you query Hoogle from Emacs.

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viewing HS files in Firefox
by Isaac Dupree 10 Jan '08

10 Jan '08

When I try to go to one of the Module.hs files, e.g. on darcs.haskell.org, it now has type HS and Firefox refuses to display it (and only lets me download it). Does anyone know how to make Firefox treat certain file types as others (HS as plain text, in particular)? so that I can browse them with any convenience Thanks, Isaac

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Cabal and DB packages
by manu 08 Jan '08

08 Jan '08

Hello, I'm trying to do something that should be fairly simple, installing some DB package so I can use MySQL or SQLite. However I've had troubles building HSQL, HaskellDB and Takusen before giving up (I am using ghc 6.8.1 and Cabal-1.2.2.0). ------------------ with HSQL : Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden so I added 'old-time' to the 'build-depends' line in the .cabal file but then it fails building with Building hsql-1.7... [1 of 2] Compiling Database.HSQL.Types ( Database/HSQL/Types.hs, dist/ build/Database/HSQL/Types.o ) Database/HSQL/Types.hs:134:18: Not in scope: type variable `forall' Database/HSQL/Types.hs:134:25: Not in scope: type variable `a' Database/HSQL/Types.hs:134:27: Illegal operator `.' in type `forall a . (Int -> FieldDef -> (FieldDef -> CString -> Int -> IO a) -> IO a)' (Use -XTypeOperators to allow operators in types) etc... At this point, it goes over my head... --------------- What the hell, I'll use HaskellDB and FlatDB, but then I get this message while building HaskellDB : src/Database/HaskellDB/HDBRec.hs:106:11: Illegal signature in pattern: l f a Use -XPatternSignatures to permit it when I add 'PatternSignatures' to the 'Extensions:' line in .cabal, it fails configuring with : Setup.hs: haskelldb.cabal:8: Parse of field 'extensions' failed: Jeeeesus, nevermind, I'll try Takusen --------------- with Takusen : $ runhaskell Setup.hs configure Setup.hs:26:7: Could not find module `Distribution.Program': Use -v to see a list of the files searched for. I give up... ---------------- Is there any simple way to do it though ? Is there any problems with Cabal that I need to work around ? Help !! Manu

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Wikipedia on first-class object
by Cristian Baboi 07 Jan '08

07 Jan '08

http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions. ________ Information from NOD32 ________ This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com

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PHP/code generation libraries?
by Justin Bailey 07 Jan '08

07 Jan '08

I'm working on a project which would generate a PHP data-access layer from a Haskell model. I'm wondering what libraries might be already be available for generating PHP or other types of code. The pretty-printing library is one option. Any other suggestions? Thanks in advance. Justin

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map/reduce example
by Bulat Ziganshin 06 Jan '08

06 Jan '08

Hello Haskell-Cafe, i've written small program which demonstrates how map/reduce may be implemented in Haskell. it counts amount of words in file, splitting it into 64kb blocks processed by two threads. their results are combined by another two threads. how it may be made better? in particular, is it strict in values send over channels? {-# LANGUAGE BangPatterns #-} import Control.Concurrent import Control.Concurrent.Chan import Control.Monad import Data.IORef import Data.ByteString.Char8 as B hiding (length) import System.Environment import System.IO main = do (file:_) <- getArgs h <- openBinaryFile file ReadMode map <- newChan reduce <- newChan result <- newChan replicateM_ 2 (forkIO$ mapThread map reduce) replicateM_ 2 (forkIO$ reduceThread reduce result) jobs <- new 0 untilM (hIsEOF h) $ do str <- B.hGet h 65536 writeChan map str jobs += 1 jobs' <- val jobs writeChan reduce (0,-jobs') res <- readChan result print res mapThread map reduce = forever $ do str <- readChan map let !sum = length (B.words str) writeChan reduce (sum,1) reduceThread reduce result = forever $ do (sum1,n1) <- readChan reduce (sum2,n2) <- readChan reduce let (!sum,!n) = (sum1+sum2,n1+n2) case n of 0 -> writeChan result sum _ -> writeChan reduce (sum,n) untilM cond action = do deny <- cond unless deny $ do action untilM cond action forever action = action >> forever action infixl 0 =:, += new = newIORef val = readIORef a=:b = writeIORef a b a+=b = modifyIORef a (\a->a+b) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell-cafe] what does @ mean?.....
by Serge LE HUITOUZE 06 Jan '08

06 Jan '08

Jedaï wrote: > [...] > Yes, but in the second version, it has to reconstruct (S a) before > comparing it to "one" where in the first it could do the comparison > directly. In this cas there may be some optimisation involved that > negate this difference but in many case it can do a real performance > difference. > The "as-pattern" (@ means as) is both practical and performant in most cases. Chris wrote: > The first example takes apart the (Just n) and later reconstructs (Just n). > Unless the compiler is fairly clever, this will cause the new (Just n) to be a > new allocation instead of reusing the input value. The second form uses an I tend to believe that the '@' notation is mere syntactic sugar. Indeed, it seems to me that Haskell compilers need not to be very clever to share the identical sub-expressions, for one very simple reason implied by Haskell semantics: referential transparency. Am I right, or am I missing something? And, of course, I'm not saying that syntactic sugar is bad. As Alfonso noted, it saves some typing and avoids screwing up while trying to modify multiple occurrences of the same sub-expression. And, an even more important I think, a software engineering benefit: It emphasizes for the reader that we really are talking about the very same thing at two (or more) different places. --Serge No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.17.9/1198 - Release Date: 26/12/2007 17:26

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