- Haskell-Cafe - Haskell.org

Hugs running on cygwin
by Galchin Vasili 31 Oct '07

31 Oct '07

Hello, When I was downloading and installing the cygwin tool set on my laptop, I noticed that Hugs doesn't appear to be in the toolset. What would it take to get Hugs running on top of cygwin? I guess the Hugs Makefile would have to be modified to correctly link in any of the cygwin "Unix" libraries. Regards, Bill Halchin

1 0

pi
by Jonathan Cast 30 Oct '07

30 Oct '07

I just noticed that pi doesn't have a default definition in the standard prelude, according to the Haddock docs. Why is this? jcc

19 58

PNG files
by Tim Newsham 30 Oct '07

30 Oct '07

I needed something small for writing out png files and didn't see anything, so I wrote my own. It's not really large or general enough yet to warrant a full package. It wouldn't require much work to support other variant formats, such as color. ---- Png.hs ---- {- A small library for creating monochrome PNG files. This file is placed into the public domain. Dependencies: Zlib. -} module Png (png) where import Data.Array import Data.Bits import Data.List import Data.Word import qualified Codec.Compression.Zlib as Z import qualified Data.ByteString.Lazy as B be8 :: Word8 -> B.ByteString be8 x = B.singleton x be32 :: Word32 -> B.ByteString be32 x = B.pack [fromIntegral (x `shiftR` sh) | sh <- [24,16,8,0]] pack :: String -> B.ByteString pack xs = B.pack $ map (fromIntegral.fromEnum) xs unpack :: B.ByteString -> String unpack xs = map (toEnum.fromIntegral) (B.unpack xs) hdr, iHDR, iDAT, iEND :: B.ByteString hdr = pack "\137\80\78\71\13\10\26\10" iHDR = pack "IHDR" iDAT = pack "IDAT" iEND = pack "IEND" chunk :: B.ByteString -> B.ByteString -> [B.ByteString] chunk tag xs = [be32 (fromIntegral $ B.length xs), dat, be32 (crc dat)] where dat = B.append tag xs -- | Return a monochrome PNG file from a two dimensional bitmap -- stored in a list of lines represented as a list of booleans. png :: [[Bool]] -> String png dat = unpack $ B.concat $ hdr : concat [ihdr, imgdat, iend] where height = fromIntegral $ length dat width = fromIntegral $ length (head dat) ihdr = chunk iHDR (B.concat [ be32 width, be32 height, be8 1, be8 0, be8 0, be8 0, be8 0]) imgdat = chunk iDAT (Z.compress imgbits) imgbits = B.concat $ map scanline dat iend = chunk iEND B.empty scanline :: [Bool] -> B.ByteString scanline dat = 0 `B.cons` bitpack dat bitpack' :: [Bool] -> Word8 -> Word8 -> B.ByteString bitpack' [] n b = if b /= 0x80 then B.singleton n else B.empty bitpack' (x:xs) n b = if b == 1 then v `B.cons` bitpack' xs 0 0x80 else bitpack' xs v (b `shiftR` 1) where v = if x then n else n .|. b bitpack :: [Bool] -> B.ByteString bitpack xs = bitpack' xs 0 0x80 crc :: B.ByteString -> Word32 crc xs = updateCrc 0xffffffff xs `xor` 0xffffffff updateCrc :: Word32 -> B.ByteString -> Word32 updateCrc = B.foldl' crcStep crcStep :: Word32 -> Word8 -> Word32 crcStep crc ch = (crcTab ! n) `xor` (crc `shiftR` 8) where n = fromIntegral (crc `xor` fromIntegral ch) crcTab :: Array Word8 Word32 crcTab = array (0,255) $ zip [0..255] $ flip map [0..255] (\n -> foldl' (\c k -> if c .&. 1 == 1 then 0xedb88320 `xor` (c `shiftR` 1) else c `shiftR` 1) n [0..7])

1 0

Newbie Question on Setting the GHC Search Path
by Benjamin L. Russell 30 Oct '07

30 Oct '07

Please pardon this intrusion for an elementary question on setting the GHC search path. I have installed GHC on my work Windows XP machine, and would like to be able to search for files in the following directory: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC However, when I type the following command into the GHC interpreter: :cd "D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC" I get the following error message: *** Exception: "D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC": setCurrentDirectory: invalid argument (Invalid argument) Yet, for testing purposes, when I type the following command: :cd cygwin I do not get any error message. There seems to be a problem with the spaces in the filename. However, I would like to be able to use the "D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC" directory for GHC programming practice, because I keep my practice work for my other programming languages in the same super-super-directory. Does anybody know a way to specify "D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC" as a directory in the search path for GHC? Benjamin L. Russell

3 4

Re: [Haskell] Image manipulation
by Tim Chevalier 30 Oct '07

30 Oct '07

[redirecting to haskell-cafe] On 10/29/07, Brent Yorgey <byorgey(a)gmail.com> wrote: > Haskell is a wonderful language, so I hate to say this...but personally I > don't see the benefit of using Haskell here, unless the manipulations you > want to do are very complex. Some simple shell (or > perl/python/ruby/whatever) scripts to glue together some calls to convert > (or possibly other ImageMagick utilities) ought to do the job just fine. > If you're doing many image transformations, invoking an external program like convert will get expensive due to context-switching. So that's why something like the (not-yet-existent) bindings for ImageMagick-as-a-library might be useful. I agree that laziness probably isn't going to be helpful, but there may be interesting things to be done that are enabled by purity and higher-order-functions. I'm working on the ImageMagick bindings because I'm curious to find out. There are obvious things like being able to define compositions of transformations -- which is easy to do when you're calling library functions in-memory, and less so if you're using a separate executable -- but maybe there are more interesting applications too. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "Thus spake the Master Programmer: When you have learned to snatch the error code from the trap frame, it will be time for you to leave."--J. Geoffrey

2 2

Tim Sweeney and multi-cores .... and Haskell
by Galchin Vasili 30 Oct '07

30 Oct '07

http://www.americanscientist.org/content/AMSCI/AMSCI/ArticleAltFormat/20071…

3 3

placing modules in the module hierarchy
by Dimitry Golubovsky 29 Oct '07

29 Oct '07

Hi, Long ago (back in 2001), there was a message in the Haskell mailing list, archived e. g. here: http://www.mail-archive.com/haskell@haskell.org/msg08187.html and what it refers to is here: http://www.cs.york.ac.uk/fp/libraries/layout.html which goes for "Graphics.UI. etc" I used this approach in my script to cabalize Fudgets: everything was placed below "Graphics.UI.Fudgets" (original Fudgets library used plain module space). I am currently working on another stuff related to Haskell GUI for web browser, and us the same approach: everything goes under Graphics.UI.<library name> So, I think, despite this creates longer paths in the hierarchy, the more specialized the stuff is, the deeper in the hierarchy it should be pushed. Slightly off topic, this is IMHO the same problem that we have in Linux packages/filesystem layout: every package containing binaries, places them into say /usr/bin. Then, if the two packages have a binary with the same name, this leads to conflict (HSSFFIG has a program named "splitter". Another package in Debian also had a binary with the same name. they conflicted. To resolve it' I'd have to rename my binary to say hsffig-splitter. Should every package be installed into its own subtree, such conflicts wouldn't arise). So, to my personal opinion (which is in agreement with the proposal mentioned) each package has to have its own tree not crossing with other packages' tree. So, I'd suggest for the Grapefruit library: whatever is specific to this library, goes under Graphics.UI.Grapefruit. Whatever may be commonly used elsewhere (say some useful data structures) might go under "Data". So, if FRP signals are usable outside the Grapefruit, they might go to Control. Thanks. -- Dimitry Golubovsky Anywhere on the Web

2 1

Re: [Haskell] placing modules in the module hierarchy
by Isaac Dupree 29 Oct '07

29 Oct '07

Wolfgang Jeltsch wrote: > Since the Control.Grapefruit subtree would probably only consist of > Control.Grapefruit.Signal and submodules, I’d prefer to just use > Control.Signal and Control.Signal.*. But this would pose the problem that no > other FRP library could use Control.Signal without conflicting with > Grapefruit. One _ideal_ is to develop a consensus in the community for a FRP Signal library, one that is good enough that Grapefruit could use it. (If the library is inherently Grapefruit-specific, do use Control.Signal.Grapefruit or Control.Grapefruit.Signal.) Of course that ideal is not always possible, and best-library consensuses do change over the years -- but we've been doing it a lot anyway. Isaac

1 0

newbie optimization question
by Prabhakar Ragde 29 Oct '07

29 Oct '07

For the purposes of learning, I am trying to optimize some variation of the following code for computing all perfect numbers less than 10000. divisors i = [j | j<-[1..i-1], i `mod` j == 0] main = print [i | i<-[1..10000], i == sum (divisors i)] I know this is mathematically stupid, but the point is to do a moderate nested-loops computation. On my 2.33GHz dual-core MacBookPro, the obvious C program takes about .3 seconds, and a compiled OCaML program (tail recursion, no lists) about .33 seconds. The above takes about 4 seconds. I've tried using foldl', and doing explicit tail recursion with strict accumulators, but I can't get the running time below 3 seconds. Is it possible to come within striking distance of the other languages? Thanks. --PR

16 39

Quasiquotations part of GHC?
by Joel Reymont 29 Oct '07

29 Oct '07

Folks, Did quasiquotations ever make it into the GHC tree? They were implemented as a patch to 6.7. Thanks, Joel -- http://wagerlabs.com

1 0

newbie question about list performance
by John Lato 29 Oct '07

29 Oct '07

Hello, I've been following the list optimization thread with great interest, as it pertains to something I'm working on at the moment. I'm working with moderate-sized files (tens to hundreds of MBs) that have some ascii header data followed by a bunch of 32-bit ints. I can read the files into a lazy ByteString (and parse the header), but I'd like some advice as to the best data type to convert the ints into. Ideally, I'd have some functions like this: decode :: ByteString -> (FileFormat, [Int32]) encode :: FileFormat -> [Int32] -> ByteString but I don't know if Int32 is actually the best choice. It seems to me that something like a lazy list of strict arrays (analogous to a lazy bytestring) would be better. Is there a library like this already? Or is this a case of premature optimization, and I should just try the list and see if it's good enough? Any suggestions would be appreciated. Also, I'd like to let the maintainers and implementors know that I really appreciate the work that's been done on optimizing Haskell. I haven't used Haskell much yet, but I've fallen in love with the language and it's great to see that performance even for heavy I/O tasks can be comparable to or exceed C. Thank you, John Lato

3 2

ANNOUNCE / POST MORTEM: hswm, version ()
by Remi Turk 29 Oct '07

29 Oct '07

Hi everyone, HSWM was my attempt at a Haskell Window Manager, mostly written during the first half of 2006 as a personal research project, and out of frustration with some not to be named other window managers. Although I have been running it myself for almost two years, I never got around to polishing it into something releasable due to lack of time. [1] Since, as of today, its number of users is officially back to zero [2], this seems like a good moment to release version () of HSWM: The first and last version of my own window manager. Features are: - includes a lambda mouse cursor - multiple desktops - sticky windows - about 2300 lines, about half of which is X boilerplate and user configuration - somewhat based on evilwm - still regularly dies due to unhandled X errors, so a script to automatically restart in that case is included - my first Xlib program ever The basic idea is an event loop inside an X monad providing three services to "plugins": - registering X Event handlers - registering X Error handlers - registering/requesting "services", to be used by other "plugins" (a global registry of named Dynamics, basically) Compared to XMonad: - no tiling - much less stable - no extensions - needs no external libs So you might want to look at it, but even _I_ don't use it anymore. HSWM only was its working name, so if anybody ever feels like writing another window manager and calling it HSWM, I certainly won't mind. The BSD-licensed code: darcs get http://student.science.uva.nl/~rturk/hswm/ `make' compiles, and that's it. Greetings, Remi [1] Technical detail: The one feature I really wanted before releasing it, but never got around to implementing, is having the WM add a frame for each managed window and reparenting the window below that frame. This way, focusing windows can go right even when the WM dies, among others. [2] It once had 3 users: Me at home, me at the University of Amsterdam and me at Utrecht University. Utrecht now runs KDE and the rest xmonad.

1 0

Re: [Haskell-cafe] viewing HS files in Firefox
by Richard Kelsall 29 Oct '07

29 Oct '07

Shachaf Ben-Kiki wrote: > Richard Kelsall wrote: >> Thomas Schilling wrote: >>> On Sat, 2007-10-27 at 18:48 -0400, Isaac Dupree wrote: >>>> 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 >>> I believe those kinds of problem have to do with the MIME-encoding on >>> the server side: The server uses text/x-haskell. For Firefox to >>> display the document inline it probably has to be text/plain. Not sure >>> what the proper fix is, though. >> It should probably be fixed in one of the Apache config files. In the >> HTTP headers from darcs.haskell.org (viewed with the Live HTTP headers >> extension in FireFox) I'm getting > > Fixed? It's already fixed. text/x-haskell is the right Content-Type to > send; this is Haskell after all, not just plaintext. It's your > browser's responsibility to display it any way it likes. People have > had this discussion in #xmonad, and it seems that MIME edit > (<https://addons.mozilla.org/en-US/firefox/addon/4498>) is a workable > solution (I don't use it myself, though). Of course, Sterling Clover's > suggestion (Open in Browser) would also work well. Hello Shachaf, I hope you intended this for the list, as I haven't seen it come through, if not my apologies for posting it. Ah, yes I see what you mean, text/x-haskell is the best media-type, however I think it ought to be possible to use 'content negotiation' http://httpd.apache.org/docs/2.2/content-negotiation.html with more than one representation to supply the same file as text/plain to non-Haskell-capable browsers. I haven't actually done this though so can't be certain of it or provide configuration details. If somebody really wanted to get clever with it it could also supply pretty coloured Haskell for a text/html representation. Sorry, I couldn't find the #xmonad discussion, is this published somewhere? Richard.

1 0

Recipes for organizing HUnit tests
by Mushfeq Khan 29 Oct '07

29 Oct '07

I'm new to Haskell and am trying to find a good way to organize my HUnit tests. Having used some of the other XUnit frameworks, I tended towards trying to organize them all in a parallel "test" folder structure, but this seems to be a bad fit for Haskell, since the test modules cannot see the source modules unless they are either in the same folder or a folder above it. That's without modifying the module search path when I run the tests, which I would like to avoid. So I tried looking for examples of HUnit-tested source code out there and found that the cryptographic library Crypto has some unit tests. However, Crypto seems to place all its test code at the top of the folder hierarchy for the reasons I mentioned above. This turns out to be fine for Crypto since there are few tests. But I can foresee the tests for my project multiplying to the point where it would become very unsightly to put them all in the top-level folder. My questions to you are: how do you organize your HUnit/QuickCheck tests? Is there a convention? Is there some library out there that embodies this convention? Mushfeq.

3 2

Problem with PDF/PS backend in GTK2HS
by Peter Verswyvelen 29 Oct '07

29 Oct '07

I have a strange problem, which is so elementary that I think I must be missing something... In GTK2HS, when I draw text using using textPath, the text is located at different locations depending on which backend is used. I'm not talking about a difference of a couple of pixels , but in my case it's half a page off. The PNG and Win32 backend work fine, but the PDF/PS backends get it wrong. For example, I modified the Text.hs demo in the demos/Cairo subdirectory so it also outputs PDF. Here the text is also at different locations, so I guess it's not just my code. Code is pasted below. I'm using Windows, and GTK2HS version 0.9.12 from http://www.haskell.org/gtk2hs Maybe someone could give this a quick test on Linux? I guess this is most likely a Cairo problem, and has nothing to do with the Haskell wrapper? Still this is hard to believe, since these kinds of bugs would be quickly found. Thanks, Peter ---------------------------------------- import Graphics.Rendering.Cairo import qualified Graphics.Rendering.Cairo.Matrix as M boxText :: String -> Double -> Double -> Render () boxText text x y = do save lineWidth <- getLineWidth (TextExtents xb yb w h _ _) <- textExtents text rectangle (x + xb - lineWidth) (y + yb - lineWidth) (w + 2 * lineWidth) (h + 2 * lineWidth) stroke moveTo x y textPath text fillPreserve setSourceRGBA 0 0 1 0.5 setLineWidth 3.0 stroke restore transpSurface :: Double -> Double -> Render () transpSurface w h = do save rectangle 0 0 w h setSourceRGBA 0 0 0 0 setOperator OperatorSource fill restore width = 400 height = 300 main :: IO () main = withImageSurface FormatARGB32 width height $ \surface -> do let render = do setSourceRGB 0.0 0.0 0.0 setLineWidth 2.0 transpSurface (fromIntegral width) (fromIntegral height) selectFontFace "sans" FontSlantNormal FontWeightNormal setFontSize 40 extents <- fontExtents let fontHeight = fontExtentsHeight extents boxText "Howdy, world!" 10 fontHeight translate 0 fontHeight save translate 10 fontHeight rotate (10.0 * pi / 180.0) boxText "Yay for Haskell!" 0 0 restore translate 0 (3 * fontHeight) save setFontMatrix $ M.rotate ((-10.0) * pi / 180.0) $ M.scale 40.0 40.0 M.identity boxText "...and Cairo!" 10 fontHeight restore renderWith surface render surfaceWriteToPNG surface "Text.png" withPDFSurface "Text.pdf" (realToFrac width) (realToFrac height) (flip renderWith $ render >> showPage) return ()

3 2

Good news for Windows developers?
by Peter Verswyvelen 28 Oct '07

28 Oct '07

If I understand this blog correctly, F# leaves the research lab and becomes an "official" Visual Studio language? http://blogs.msdn.com/somasegar/archive/2007/10/17/f-a-functional-programmi… Okay it's not Haskell, but I think it's good news anyway... Cheers, Peter PS: Shouldn't the Super Simons do some more PR at Microsoft so this also happens to Haskell? ;-) ;-)

2 1

lazily traversing a foreign data structure
by Graham Fawcett 27 Oct '07

27 Oct '07

Hi folks, I'm writing a Gnu DBM module as an exercise for learning Haskell and its FFI. I'm wondering how I might write a function that returns the database keys as a lazy list. I've wrapped the two relevant foreign functions: firstKey :: Ptr Db -> IO (Maybe String) nextKey :: Ptr Db -> String -> IO (Maybe String) NextKey takes a key, and returns the next one. Either function could return Nothing, since the db may have 0 or 1 keys. Given these, is it possible to write a (simple) function allKeys :: Ptr Db -> IO [String] that lazily fetches the keys? (Or, an idiomatic way of achieving the same end?) Thanks, Graham

5 10

Linking problems with a fresh gtk2hs build
by Olivier Boudry 27 Oct '07

27 Oct '07

Hello, I just built gtk2hs 0.9.12 using MinGW, GTK_2.0 and ghc-6.8.0.20071016. I just changed some EXTERNALDEPS in the Makefile based on info found in the following page http://haskell.org/haskellwiki/Grapefruit 25a126,128 > if HAVE_SPLIT_BASE > tools_c2hs_c2hsLocal_EXTERNALDEPS += pretty containers array > endif 358a362,364 > if HAVE_SPLIT_BASE > libHSgtk_a_EXTERNALDEPS += containers array > endif 1578a1585,1587 > if HAVE_SPLIT_BASE > libHSsoegtk_a_EXTERNALDEPS += old-time > endif I had no problem building and installing the library but when trying to compile the HelloWorld example to check the installation I get this error: C:\Temp>ghc --make HelloGtk.hs Linking HelloGtk.exe ... C:/Progra~1/Haskell/lib/gtk2hs/libHSgtk.a(Gtk__1.o)(.text+0x419):fake: undefined reference to `__stginit_gtkzm0zi9zi12_GraphicsziUIziGtkziLayoutziHPaned_' collect2: ld returned 1 exit status I found an irc log talking of using nm to track the problem. Tried nm and got this (different from the problem specified in the irc log). nm libHSgtk.a | grep __stginit_gtkzm0zi9zi12_GraphicsziUIziGtkziLayoutziHPaned_ U ___stginit_gtkzm0zi9zi12_GraphicsziUIziGtkziLayoutziHPaned_ c:\MinGW\bin\nm.exe: HPaned__4.o: File format not recognized c:\MinGW\bin\nm.exe: MenuBar__48.o: File format not recognized The "missing symbol" is found but I have a problem with the HPaned__4.o and MenuBar__48.o of the libHSgtk.a file. Any idea of what could have gone wrong? Thanks, Olivier.

2 2

latest hdbc-odbc
by jeff p 26 Oct '07

26 Oct '07

Hello, When building the latest hdbc-odbc (1.1.2.0) on a linux box with ghc6.6.1, I get the following warnings: [7 of 7] Compiling Database.HDBC.ODBC ( Database/HDBC/ODBC.hs, dist/build/Database/HDBC/ODBC.o ) hdbc-odbc-helper.c: In function â: hdbc-odbc-helper.c:131:0: warning: pointer targets in passing argument 6 of â differ in signedness hdbc-odbc-helper.c:131:0: warning: pointer targets in passing argument 8 of â differ in signedness hdbc-odbc-helper.c: In function â: hdbc-odbc-helper.c:136:0: warning: pointer targets in passing argument 8 of â differ in signedness /usr/bin/ar: creating dist/build/libHSHDBC-odbc-1.1.2.0.a then, after installing the package I get the following error/strange behavior: Prelude> :t Database.HDBC.ODBC.connectODBC /usr/local/lib/HDBC-odbc-1.1.2.0/ghc-6.6.1/Database/HDBC/ODBC/Connection.hi Declaration for connectODBC: Failed to load interface for `Database.HDBC.ODBC.ConnectionImpl': Use -v to see a list of the files searched for. Cannot continue after interface file error Can anyone shed light on what is going wrong? Is it something I can fix, or is the distribution buggy? thanks, Jeff

8 13

Re: [Haskell-cafe] Haskell to math
by Peter Verswyvelen 26 Oct '07

26 Oct '07

Jerzy K wrote >wants. He wants a specific pretty-printer *in Haskell*. This was most >probably a subject of many students' exercices... Yes Jerzy, exactly. I wrote some Haskell code to create plots of binary relations and functions on the Cartesian plane, using GTK2HS (Cairo). I have to pretty print the math formulas used in these plots. Now I'm retyping the math formulas in Word, but being a really lazy programmer, I want to automate this task. So I wanted to write some code that renders math formulas (maybe using GTK, or to MathML; I must admit I don't know LaTex at all, sorry...), but since this feels like a lot of work to get right, I was wandering if it existed allready. Okay, I could have skipped all the above and just use Maple or whatever other tool, but I'm trying to use Haskell whenever possible, because I'm kind of addicated to it. Also I still feel a bit stuck in newbie land (although I think I'm approaching escape velocity ;-), so I grab every opportunity to practice in Haskell... Thanks, Peter

1 0

Haskell to math
by Peter Verswyvelen 26 Oct '07

26 Oct '07

Some packages like Open Office and Microsoft Word contain a math expression writer. The same can be done with Maple, etc. Standard formats such as MathML, LaTex etc exist. Now what I would like to do, is to create a mathematical expression in Haskell (using e.g. an expression tree), which can be evaluated in Haskell, and be converted to something pretty-math-printable (MathML, PS, PDF, LaTex, whatever). Does something like this already exist? Thanks a lot, Peter

3 2

Re: [Haskell-cafe] Existential types (Was: Type vs TypeClass duality)
by apfelmus＠quantentunnel.de 26 Oct '07

26 Oct '07

Ryan Ingram wrote: > On 10/24/07, apfelmus <apfelmus(a)quantentunnel.de> wrote: >> So, instead of storing a list [∃a. Show a => a], you may as well >> store a >> list of strings [String]. > > I've seen this before, and to some extent I agree with it, but it > breaks down for bigger examples due to sharing. > > In most cases, "x" has a more compact representation than the result > of evaluating "show x". So if you keep a list of [show x, show y, > show z] around for a long period of time, you're leaking space. > Consider instead: > > data StringFn where > StringFn :: forall a. a -> (a -> String) -> StringFn > > applyStringFn :: StringFn -> String > applyStringFn (StringFn a f) = f a > > [ StringFn x show, StringFn y show, StringFn z show ] > > Now, you use more time every time you compute the result, but StringFn > has a compact representation; you're not leaking the space used for > the string throughout the execution of the program, but only > allocating it while it's used. Indeed. (Although the effect will be marginal in this particular example since the unevaluated thunk (show x) is represented as compactly as x . But the time-space trade-off would matter when strings from the list are used several times.) In a sense, that's also the reason why stream fusion à la Duncan + Roman + Don uses an existential type data Stream a where Stream :: ∀s. s -> (s -> Step a s) -> Stream a data Step a s = Done | Yield a s | Skip s I mean, every stream can be represented by the list of its results but the observation for fusion is that there is a much more compact representation for the state (like an integer for [1..] or the source list in map f . map g $ [x,y,...]) Regards, apfelmus

3 2

Haskell Weekly News: October 25, 2007
by Don Stewart 26 Oct '07

26 Oct '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20071025 Issue 66 - October 25, 2007 --------------------------------------------------------------------------- Welcome to issue 66 of HWN, a newsletter covering developments in the [1]Haskell community. A huge month for the Haskell community, with the Haskell Workshop, ICFP and CUFP conferences, the second international Haskell Hackathon, and 63 libraries and tools uploaded to hackage! A round of applause to everyone involved! 1. http://haskell.org/ Hackage This week's new libraries in [2]the Hackage library database. 2. http://hackage.haskell.org/ * SDL 0.5.0. Uploaded by Lemmih. [3]SDL, a binding to libSDL. * Stream 0.2.1. Uploaded by Wouter Swierstra. [4]Stream, functions, analogous to those from Data.List, to create and manipulate infinite lists * bktrees 0.1.1. Uploaded by Josef Svenningsson. [5]bktrees, Burhard-Keller trees provide an implementation of sets which apart from the ordinary operations also has an approximate member search, allowing you to search for elements that are of a certain distance from the element you are searching for. * happy 1.17. Uploaded by Simon Marlow. [6]happy, a parser generator for Haskell. * HaXml 1.19. Uploaded by Malcolm Wallace. [7]HaXml, utilities for parsing, filtering, transforming and generating XML documents. 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-0.5.0 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Stream-0.2.1 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bktrees-0.1.1 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happy-1.17 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml-1.19 * polyparse 1.1. Uploaded by Malcolm Wallace. [8]polyparse, A variety of alternative parser combinator libraries, including the original HuttonMeijer set. The Poly sets have features like good error reporting, arbitrary token type, running state, lazy parsing, and so on. Finally, Text.Parse is a proposed replacement for the standard Read class, for better deserialisation of Haskell values from Strings. * bzlib 0.4.0.1 . Uploaded by Duncan Coutts. [9]bzlib, compression and decompression in the bzip2 format. * zlib 0.4.0.1. Uploaded by Duncan Coutts. [10]zlib, compression and decompression in the gzip and zlib formats * tar 0.1.1.1. Uploaded by Bjorn Bringert. [11]tar, a library for reading and writing TAR archives. * unix-compat 0.1.2.0. Uploaded by Bjorn Bringert. [12]unix-compat, provides portable implementations of parts of the unix package. This package re-exports the unix package when available. When it isn't available, portable implementations are used. 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/polyparse-1.1 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bzlib-0.4.0.1 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib-0.4.0.1 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tar-0.1.1.1 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unix-compat-0.1.… * oeis 0.1. Uploaded by Brent Yorgey. [13]oeis, Haskell interface to the Online Encyclopedia of Integer Sequences. * dataenc 0.9. Uploaded by Magnus Therning. [14]dataenc, Data encoding library currently providing Uuencode, Base64, Base64Url, Base32, Base32Hex, and Base16. * cabal-setup 1.2.1. Uploaded by Simon Marlow. [15]cabal-setup, cabal-setup is a user interface to Cabal. It provides the basic commands for configuring, building, and installing Cabal packages. * cabal-install 0.4.0. Uploaded by cabal-devel(a)haskell.org. [16]cabal-install, apt-get like tool for Haskell. The 'cabal' command-line program simplifies the process of managing Haskell software by automating the fetching, configuration, compilation and installation of Haskell libraries and programs. * HTTP 3001.0.0. Uploaded by Bjorn Bringert. [17]HTTP, A library for client-side HTTP. 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/oeis-0.1 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc-0.9 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-setup-1.2.1 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-install-0.… 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP-3001.0.0 * iconv 0.4. Uploaded by Duncan Coutts. [18]iconv, provides an interface to the POSIX iconv library functions for string encoding conversion. * binary 0.4.1. Uploaded by the Binary Strike Team. [19]binary, efficient, pure binary serialisation using lazy ByteStrings. Haskell values may be encoded to and form binary formats, written to disk as binary, or sent over the network. Serialisation speeds of over 1 G/sec have been observed, so this library should be suitable for high performance scenarios. * Cabal 1.2.1. Uploaded by cabal-devel(a)haskell.org. [20]Cabal: the Haskell Common Architecture for Building Applications and libraries = Libraries: a framework defining a common interface for authors to more easily build their Haskell applications in a portable way. The Haskell Cabal is meant to be a part of a larger infrastructure for distributing, organizing, and cataloging Haskell libraries and tools. * bytestring 0.9. Uploaded by Don Stewart and Duncan Coutts. [21]bytestring. fast, packed, strict and lazy byte arrays with a list interface. * arrows 0.3. Uploaded by Ross Paterson. [22]arrows, Several classes that extend the Arrow class, and some transformers that implement or lift these classes. 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/iconv-0.4 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-0.4.1 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Cabal-1.2.1 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-0.9 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/arrows-0.3 * sat 1.0. Uploaded by Andrii Zvorygin. [23]sat, CNF(Clausal Normal Form) SATisfiability Solver and Generator. * pcap 0.4. Uploaded by Bryan O'Sullivan. [24]pcap, system-independent interface for user-level packet capture * hslogger4j 0.1.1. Uploaded by Bjorn Buckwalter. [25]hslogger4j, provides handlers for hslogger (John Goerzen's Haskell logging library) that are compatible with log4j's XMLLayout. In particular log messages created by the handlers can be published directly to the GUI-based log viewer Chainsaw v2. * IndentParser 0.2.1. Uploaded by Piyush P Kurur. [26]IndentParser, provides two modules, Text.ParserCombinators.Parsec.IndentParser and Text.ParserCombinators.Parsec.IndentToken, for constructing parser combinators for indentation based syntactic structures. The former exports the basic indentation parser combinators and the later together with Text.ParserCombinators.Parsec.Language and Text.ParserCombinators.Parsec.Token can be used to define tokenisers for programming languages. * lazysmallcheck 0.1. Uploaded by Matthew Naylor and Fredrik Lindblad. [27]lazysmallcheck, a library for exhaustive, demand-driven testing of Haskell programs. It is based on the idea that if a property holds for a partially-defined input then it must also hold for all fully-defined instantiations of the that input. Compared to `eager' input generation as in SmallCheck, Lazy SmallCheck may require significantly fewer test-cases to verify a property for all inputs up to a given depth. 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sat-1.0 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcap-0.4 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslogger4j-0.1.1 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/IndentParser-0.2… 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lazysmallcheck-0… * HPDF 1.2. Uploaded by alpheccar. [28]HPDF, A PDF library with support for several pages, page transitions, outlines, annotations, compression, colors, shapes, patterns, jpegs, fonts, typesetting ... * xmonad. Uploaded by Spencer Janssen and Don Stewart. [29]xmonad, xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. All features of the window manager are accessible from the keyboard: a mouse is strictly optional. xmonad is written and extensible in Haskell. Custom layout algorithms, and other extensions, may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several screens. * HCL 1.2. Uploaded by Justin Bailey. [30]HCL, provides a set of functions for building simple command-line interfaces. It allows interfaces which collect values (such as Integers, Dates, or other structured values), build lists of values, and use simple menus. It is not intended to build complex interfaces with full cursor control. It is oriented towards line-based interfaces. * safecopy 0.2. Uploaded by David Himmelstrup. [31]safecopy, an extension to Data.Binary with built-in version control. * selenium 0.2.1. Uploaded by Aaron Tomb. [32]selenium, Haskell bindings to communicate with a Selenium Remote Control server. This package makes it possible to use Haskell to write test scripts that exercise web applications through a web browser. 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF-1.2 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-0.4 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HCL-1.2 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/safecopy-0.2 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/selenium-0.2.1 * HsOpenSSL 0.2. Uploaded by PHO. [33]HsOpenSSL, a (part of) OpenSSL binding for Haskell. It can generate RSA and DSA keys, read and write PEM files, generate message digests, sign and verify messages, encrypt and decrypt messages. * X11-extras 0.4. Uploaded by Spencer Janssen. [34]X11-extras, missing bindings to the X11 graphics library. * X11-1.2.3. Uploaded by Don Stewart. [35]X11, a Haskell binding to the X11 graphics library. * flow2dot-0.2. Uploaded by Dmitry Astapov. [36]flow2dot, generates sequence diagrams from textual descriptions with help of Graphviz graph drawing tool. * hsSqlite3-0.0.1. Uploaded by Evgeny Jukov. [37]hsSqlite, Bindings for Sqlite3 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsOpenSSL-0.2 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-extras-0.4 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-1.2.3 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/flow2dot-0.2 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsSqlite3-0.0.1 * X11-xft-0.1. Uploaded by Clemens Fruhwirth. [38]X11-xft, Bindings to the Xft, X Free Type interface library, and some Xrender parts. * metaplug-0.1.1. Uploaded by Austin Seipp. [39]metaplug, a ghc-api wrapper, designed to make the api more transparent across releases and offer eval and plugin esque facilities in the form of a simple, easy to modify library. * dimensional. Uploaded by Bjorn Buckwalter. [40]dimensional, a library providing data types for performing arithmetic with physical quantities and units. Information about the physical dimensions of the quantities and units is embedded in their types and the validity of operations is verified by the type checker at compile time. The boxing and unboxing of numerical values as quantities is done by multiplication and division with units. The library is designed to, as far as is practical, enforce/encourage best practices of unit usage. * ProbabilityMonads. Uploaded by Eric Kidd. [41]ProbabilityMonads, tools for random sampling, explicit enumeration of possible outcomes, and applying Bayes' rule. Highly experimental, and subject to change. In particular, the Data.Probability API is rather poor and could stand an overhaul. * MonadRandom. Uploaded by Eric Kidd . [42]MonadRandom, support for computations which consume random values. 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-xft-0.1 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/metaplug-0.1.1 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dimensional-0.7 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ProbabilityMonad… 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MonadRandom-0.1.0 * MaybeT. Uploaded by Eric Kidd. [43]MaybeT, Support for computations with failures. * network-bytestring. Uploaded by Johan Tibell. [44]network-bytestring, Faster and more memory efficient low-level socket functions using Data.ByteStrings instead of Strings. * irc. Uploaded by Trevor Elliott. [45]irc, a set of combinators and types for parsing IRC messages. * clevercss. Uploaded by Georg Brandl. [46]clevercss, a CSS preprocessing library that allows defining variables and nesting selectors so that you don't need to Repeat Yourself. * HsHyperEstraier. Uploaded by PHO. [47]HsHyperEstraier, a HyperEstraier binding for Haskell. HyperEstraier is an embeddable full text search engine which is supposed to be independent to any particular natural languages. 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MaybeT-0.1.0 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-bytestri… 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/irc-0.2.3 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/clevercss-0.1 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsHyperEstraier-… * libmpd. Uploaded by Ben Sinclair. [48]libmpd, client library for MPD, the Music Player Daemon. * hS3. Uploaded by Greg Heartsfield. [49]hS3, provides an interface to Amazon's Simple Storage Service (S3), allowing Haskell developers to reliably store and retrieve arbitrary amounts of data from anywhere on the Internet. * infinity. Uploaded by Austin Seipp. [50]infinity, tiny IRC bot, extendable through plugins written in haskell * WURFL. Uploaded by alpheccar. [51]WURFL, support for the WURLF file format * hburg. Uploaded by Igor Boehm. [52]hburg, a program that generates tree parsers for cost-augmented tree grammars. It is useful for writing code generators for compilers. Given a mapping of a tree structured intermediate representation onto target machine instructions, HBURG generates a code generator that can be plugged into the instruction selection phase of a compiler. 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/libmpd-0.1.3 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hS3-0.2 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/infinity-0.1.1 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/WURFL-0.1 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hburg-1.1 * ipprint. Uploaded by Gleb Alexeyev. [53]ipprint, tiny helper for pretty-printing values in ghci console * numbers. Uploaded by Lennart Augustsson. [54]numbers, instances of the numerical classes for a variety of different numbers: (computable) real numbers, arbitrary precision fixed numbers, arbitrary precision floating point numbers, differentiable numbers, symbolic numbers, natural numbers, interval arithmetic. * numeric-quest. Uploaded by Henning Thielemann. [55]numeric-quest, List based linear algebra, similtaneous linear equations, eigenvalues and eigenvectors, roots of polynomials, transcendent functions with arbitrary precision implemented by continued fractions, quantum operations, tensors * shell-pipe. Uploaded by Henning Thielemann. [56]shell-pipe, shell scripting stuff. * hstats. Uploaded by Marshall Beddoe. [57]hstats, library of commonly used statistical functions. 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ipprint-0.3 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/numbers-2007.9.25 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/numeric-quest-0.1 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/shell-pipe-0.1 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hstats-0.2 Libraries This week's proposals and extensions to the [58]standard libraries. * [59]In base, for Data.Version, change the meaning of comparisons * [60]Add Compositor class as superclass of Arrow 58. http://haskell.org/haskellwiki/Library_submissions 59. http://article.gmane.org/gmane.comp.lang.haskell.libraries/7847 60. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/7663 Conference roundup Haskell papers presented at the Haskell Workshp, ICFP and CUFP, in Freiburg, Germany. * [61]Andy Gill and Colin Runciman. Haskell Program Coverage (pdf) 61. http://www.unsafeperformio.com/papers/Hpc07.pdf * [62]Simon Marlow, José Iborra, Bernard Pope and Andy Gill. A Lightweight Interactive Debugger for Haskell (pdf) 62. http://www.haskell.org/~simonmar/papers/ghci-debug.pdf * [63]Wouter Swierstra and Thorsten Altenkirch. Beauty in the Beast: A Functional Semantics of the Awkward Squad (pdf) 63. http://www.cs.nott.ac.uk/~txa/publ/beast.pdf * [64]Matthew Naylor, Emil Axelsson and Colin Runciman. A Functional-Logic Library for Wired (acm.org) 64. http://tinyurl.com/2mw6jo * [65]Neil Mitchell and Colin Runciman. Uniform Boilerplate and List Processing; Or: Scrap Your Scary Types (pdf) 65. http://tinyurl.com/yulkt4 * [66]Philip Wadler and Simon Peyton Jones. Comprehensions with `Order by' and `Group by' 66. http://research.microsoft.com/~simonpj/papers/list-comp/index.htm * [67]Geoffrey Mainland. Why It's Nice to be Quoted: Quasiquoting for Haskell 67. http://www.eecs.harvard.edu/~mainland/ghc-quasiquoting/ * [68]Louis-Julien Guillemette and Stefan Monnier. A Type-Preserving Closure Conversion in Haskell (pdf) 68. http://www.iro.umontreal.ca/~monnier/tcm.pdf * [69]Demo 1: George Giorgidze and Henrik Nilsson. Programming Modular Synthesizer in Haskell (.mov) 69. http://www.ludd.ltu.se/~pj/hw2007/modular%20synthesis.mov * [70]Demo 2: Martin Grabmüller and Dirk Kleeblatt. Run-time Code Generation in Haskell (.mov) 70. http://www.ludd.ltu.se/~pj/hw2007/harpy.mov * [71]Joao Fernandes, Alberto Pardo and João Saraiva. A Shortcut Fusion Rule for Circular Program Calculation (pdf) 71. http://www.fing.edu.uy/~pardo/papers/hw07.pdf * [72]Peng Li, Andrew Tolmach, Simon Marlow and Simon Peyton Jones. Lightweight concurrency primitives for GHC (pdf) 72. http://web.cecs.pdx.edu/~apt/hw07.pdf * [73]Demo 3: Don Stewart XMonad (.mov) 73. http://www.ludd.ltu.se/~pj/hw2007/xmonad.mov * [74]Videos of all the Haskell Workshop talks 74. http://www.ludd.ltu.se/~pj/hw2007/HaskellWorkshop.html * [75]Selected videos from ICFP and IFL 75. http://www.ludd.ltu.se/~pj/icfp2007/ICFP2007.html Jobs Finance. An NYC finance company [76]requires an expert level Haskell user; must be comfortable with monads, monad transformers, type level programming (i.e. MPTC, overlapping and undecidable instances), and lazy evaluation (i.e. know how to find and eliminate space leaks). 76. http://article.gmane.org/gmane.comp.lang.haskell.cafe/30228 Web Developer. RedNucleus Ltd [77]requires a highly motivated programmer for a full or part-time posititon developing social web applications. Initially you will develop and maintain applications using RubyOnRails or a similar framework, but there will be opportunities to explore new web programming paradigms with declarative languages. The successful application will have exposure to functional programming methodologies, e.g. in Haskell, Lisp or Erlang 77. http://article.gmane.org/gmane.comp.lang.haskell.cafe/30685 Blog noise [78]Haskell news from the [79]blogosphere. 78. http://planet.haskell.org/ 79. http://haskell.org/haskellwiki/Blog_articles * [80]Strongly Specified Functions * [81]Amazon S3 binding * [82]Octane Mech, OpenGL Haskell based mech game; early code and some haskell notes * [83]Fast, parallel log file processing in Haskell * [84]Haskell in the Hallway: an interview with Simon Peyton Jones * [85]Agda Exercise: Sized Mergesort * [86]Agda Exercise: Proving that Mergesort Returns Ordered Lists * [87]Lazy functions in C# LlNQ * [88]Functional Javascript * [89]Seemingly impossible functional programs * [90]For most programmers learning Haskell will be no picnic : a tutorial 80. http://sequence.complete.org/node/363 81. http://scsibug.com/2007/09/23/haskell-s3-library/ 82. http://berlinbrowndev.blogspot.com/2007/09/octane-mech-opengl-haskell-based… 83. http://www.serpentine.com/blog/2007/09/25/what-the-heck-is-a-wide-finder-an… 84. http://port25.technet.com/archive/2007/09/26/haskell-in-the-hallway-sam-int… 85. http://www.iis.sinica.edu.tw/~scm/?p=36 86. http://www.iis.sinica.edu.tw/~scm/?p=37 87. http://www.aboutcode.net/2007/09/27/Lazy+Functions+In+LINQ.aspx 88. http://osteele.com/sources/javascript/functional/ 89. http://math.andrej.com/2007/09/28/seemingly-impossible-functional-programs/ 90. http://www.lisperati.com/haskell/ht5.html * [91]Haskell in industry: a surprising encounter! * [92]Exhaustive search over infinite binary trees * [93]How to Install HAppS * [94]Simple Haskell Web Programming with HAppS * [95]Haskell Web Spider, Part 1: HaXML * [96]Haskell Web Spider, Part 2: HXT, or I Was Promised There Would Be No Side Effects * [97]In praise of implementation-defined languages * [98]Arboreal Isomorphisms from Nuclear Pennies * [99]Higher-dimensional enumeration * [100]Haskell and my preception of programming languages 91. http://pinderkent.blogsavy.com/archives/133 92. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Exhaustive/ 93. http://bluebones.net/2007/09/how-to-install-happs/ 94. http://bluebones.net/2007/09/simple-haskell-web-programming-with-happs/ 95. http://kevin.scaldeferri.com/blog/2007/09/29/UrlSpider1HaXML.html 96. http://kevin.scaldeferri.com/blog/2007/09/30/UrlSpider2HXT.html 97. http://pozorvlak.livejournal.com/70452.html 98. http://sigfpe.blogspot.com/2007/09/arboreal-isomorphisms-from-nuclear.html 99. http://byorgey.wordpress.com/2007/10/01/higher-dimensional-enumeration/ 100. http://serishema.livejournal.com/28327.html * [101]Monads in Scala * [102]Total stream processors and quantification over infinite number of infinite streams * [103]The xmonad experience * [104]Wiring Haskell into a FastCGI Web Server * [105]A Link-Resolving Library * [106]Immutable data structures are the way of the future in C# * [107]What do you get when you curry partial application? * [108]Universal Architecture * [109]lambdacats * [110]A venture into functional programming (by a Python programmer) 101. http://james-iry.blogspot.com/2007/10/monads-are-elephants-part-2.html 102. http://conway.rutgers.edu/~ccshan/wiki/blog/posts/StreamPEval/ 103. http://braindump.dk/tech/2007/10/01/the-xmonad-experience/ 104. http://mult.ifario.us/articles/2007/10/03/wiring-haskell-into-a-fastcgi-web… 105. http://freeshells.ch/~revence/Link.lhs 106. http://blogs.msdn.com/ericlippert/archive/2007/10/04/path-finding-using-a-i… 107. http://tinyurl.com/2mdo93 108. http://luqui.org/blog/archives/2007/10/05/universal-architecture/ 109. http://arcanux.org/lambdacats.html 110. http://pyside.blogspot.com/2007/10/venture-into-functional-programming.html * [111]Is Java Dying? * [112]Tracing your code... the dirty way! * [113]Using Emacs to insert SCC annotations in Haskell code * [114]Erlang and Haskell Books: First Impressions * [115]Update on F# and Haskell, especially monads * [116]Back from ICFP * [117]A Simple RPN Calculator in Haskell * [118]Another simple RPN calculator in Haskell * [119]A simple regex engine in Haskell * [120]The square of the Catalan sequence 111. http://fiatdev.com/2007/10/09/is-java-dying 112. http://monadicheadaches.blogspot.com/2007/10/tracing-your-code-dirty-way.ht… 113. http://www.serpentine.com/blog/2007/10/09/using-emacs-to-insert-scc-annotat… 114. http://on-ruby.blogspot.com/2007/04/erlang-and-haskell-books-first.html 115. http://jan.varwig.org/archiv/update-on-f-and-haskell-especially-monads 116. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=75 117. http://jcreigh.blogspot.com/2006/11/simple-rpn-calculator-in-haskell.html 118. http://meta-meta.blogspot.com/2007/10/simple-infix-calculator-in-haskell.ht… 119. http://jcreigh.blogspot.com/2006/12/simple-regex-engine-in-haskell.html 120. http://blog.plover.com/2007/10/12/#catalan-squared * [121]Haskell changing your thinking * [122]How I started off with Haskell and wound up with Lisp * [123]Phantom type problems * [124]My first trip to the phantom zone * [125]Writing Your Last For-Loop * [126]The Curry-Howard Correspondence in Haskell * [127]Tuppence Tour of Haskell Concurrency Constructs * [128]Is Haskell really expressive? * [129]A Quickstart to Haskell * [130]Respect C Programmers 121. http://ephemient.livejournal.com/43696.html 122. http://blog.vagmim.com/2007/10/how-i-started-off-with-haskell-and.html 123. http://therning.org/magnus/archives/322 124. http://therning.org/magnus/archives/323 125. http://beautifulcode.oreillynet.com/2007/10/writing_your_last_forloop.php 126. http://www.thenewsh.com/~newsham/formal/curryhoward/ 127. http://mult.ifario.us/articles/2007/10/21/tuppence-tour-of-haskell-concurre… 128. http://monadicheadaches.blogspot.com/2007/10/is-haskell-reallt-expressive.h… 129. http://www.builderau.com.au/program/soa/A-Quickstart-to-Haskell/0,339024614… 130. http://kontsevoy.blogspot.com/2007/10/respect-c-programmers.html * [131]Building Functions from Functions, part 2: Function Composition * [132]On the Importance of Purity * [133]Functional programming - Back to the Roots * [134]Simpler, Easier! Or, how to write a simple dependent type checker * [135]Is Visual Basic 9 'Haskell for the Masses?' 131. http://diditwith.net/2007/10/23/BuildingFunctionsFromFunctionsPart2Function… 132. http://higherlogics.blogspot.com/2007/10/on-importance-of-purity.html 133. http://blog.codingnaked.com/?p=3 134. http://augustss.blogspot.com/2007/10/simpler-easier-in-recent-paper-simply.… 135. http://blogs.teamb.com/craigstuntz/2007/10/25/37762 Quotes of the Week * As someone who has written production code in functional languages (Ive written Haskell for the U.S. Navy) and in other languages (Perl and Ruby for several startups), I have to say that Haskell enabled me to be way more productive than the untyped scripting languages. Ruby is fun, but Haskell lets me get the work done faster and better. * Perhaps if C wants to be taken seriously it should provide portability, which has been present in Haskell since the beginning * However, since starting learning Haskell I?ve had aha-moments that manifest themselves in a single line of code. This has never happened before. Ever! * Cale: Inheritance? Inheritance is broken, anyway * DRMacIver: I dread to think what category theory would look like after the software engineering world had got their grubby paws on it. Enterprise variant functors. Commutative UML diagrams. * DukeDave: Haskell has the greatest unlearning curve * Jon Harrop: As Haskell has shown, laziness cannot be implemented efficient at all. * Logan Capaldo: All I want for christmas is monad comprehensions * Olathe: We can't be totally sure, though. There might be some value of 1 that wasn't checked. * Pseudonym: Smart programmers naturally write monadic code, even if they don't realise it. * augustss: Haskell already has enterprise monads; there is a fail method. * bitwize: The combinator known as compose; Makes me extremely morose; The full stop is better, than writing in letters; Which makes it extremely verbose * glguy: map became not overloaded in the great polymorphic scare of haskell 98 * mauke: haskell software should move from alpha to beta to eta, then lambda * pl0nk: I wonder what SPJ sees when he closes his eyes before answering a question. About the Haskell Weekly News New editions are posted to [136]the Haskell mailing list as well as to [137]the Haskell Sequence and [138]Planet Haskell. [139]RSS is also available, and headlines appear on [140]haskell.org. Headlines are available as [141]PDF. To help create new editions of this newsletter, please see the [142]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [143]http://www.cse.unsw.edu.au/~dons/code/hwn 136. http://www.haskell.org/mailman/listinfo/haskell 137. http://sequence.complete.org/ 138. http://planet.haskell.org/ 139. http://sequence.complete.org/node/feed 140. http://haskell.org/ 141. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20071025.pdf 142. http://haskell.org/haskellwiki/HWN 143. http://www.cse.unsw.edu.au/~dons/code/hwn

1 0

Unfoldr love
by Justin Bailey 26 Oct '07

26 Oct '07

In my best Homer Simposon voice - "unfoldr - is there anything it can't do?" I have strange unfoldr love right now. I'm probably too impressed by this function, but it takes a string and splits it into a list of words, while keeping "quoted" phrases together: import Data.List phrasesAndWords :: String -> [String] phrasesAndWords = let startsWith w c = [c] `isPrefixOf` w endsWith w c = [c] `isSuffixOf` w buildPhrases [] = Nothing buildPhrases (w:ws) | w `startsWith` '"' = -- A word starts with a '"', now look for ending '"' let (p, rest) = break (`endsWith` '"') (w:ws) in case () of () | null p -> Just ((init . tail) w, ws) -- single word in quotes | null rest -> Just ((tail . unwords) p, rest) -- No ending quote -- Some amount of words in the phrase and some not | otherwise -> Just ((init . tail) (unwords (p ++ [head rest])), tail rest) | otherwise = Just (w, ws) in -- Use of words makes sure any valid quotes appear at beginning or -- end of a word. unfoldr buildPhrases . words Of course it's not a full parser (no nested quotes or escaping) but who would think unfoldr could do that? Justin p.s. Style comments, improvements welcome.

1 0

Binary constants in Haskell
by Maurício 26 Oct '07

26 Oct '07

Hi, Are there binary constants in Haskell, as we have, for instance, 0o232 for octal and 0xD29A for hexadecimal? Thanks, Maurício

12 19

List comprehension order of evaluation
by Maurício 26 Oct '07

26 Oct '07

Hi, Today, if I write: [a:[b] | a<-"ab" , b<-"12"] I get: ["a1","a2","b1","b2"] Are there any guarantees that I'll never get ["a1","b1","a2","b2"] instead, i.e., that the first list will always be the last one to be fully transversed? Even if I use a different compiler or a future version of Haskell? Reading how list comprehensions are translated in the Haskell report it seems the answer is yes. Is that written in stone? Can compilers do it in their own different way? Thanks, Maurício

3 2

ANN: Math.OEIS 0.1
by Brent Yorgey 26 Oct '07

26 Oct '07

Hi all, I'm pleased to announce the release of a somewhat silly -- yet perhaps somewhat useful -- library module, Math.OEIS<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/oeis-0.1>, intended for the enjoyment of combinatorial dilettantes, Project Euler addicts, and the merely curious alike. It provides a Haskell interface to the Online Encyclopedia of Integer Sequences, with support for looking up sequences by ID number or partial list. You can use it to look up any and all information on a sequence (references, formulas, comments, keywords...), or just return the sequence data as a list. My favorite feature -- and the initial impetus for the library -- is that it can "guess" the rest of a sequence by using the first search result from the OEIS to extend it. Some examples of use: Prelude Math.OEIS> extendSequence [5,7,11,13,17] [5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71... Prelude Math.OEIS> extendSequence [2,4,8,16,32] [2,4,8,16,32,64,128,256,512,1024,2048,4096,8192... Prelude Math.OEIS> description `fmap` lookupSequence [1,2,5,14,42] Just "Catalan numbers: C(n) = binomial(2n,n)/(n+1) = (2n)!/(n!(n+1)!). Also called Segner numbers." Math.OEIS can be obtained from hackage<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/oeis-0.1>, and additional documentation is included with the module in Haddock form. Features planned for future versions include: * support for returning multiple search results instead of just one. * support for automatically returning additional terms if available in a b****.txt file. * hack GHC to provide [1,2,5,14,42..] syntax for extendSequence. * returning a lazy infinite list for infinite sequences via an embedded general AI and Mathematica interpreter. Enjoy! -Brent (byorgey)

9 11

freebsd-7.0BETA1 and ghc
by brad clawsie 25 Oct '07

25 Oct '07

a note recently went out regarding the first beta release of freebsd7 note that ghc is still marked as broken in the 7-branch ports tree due to issues regarding the move from gcc3x to gcc4x. if you rely on ghc, you may want to hold off on an upgrade

3 3

Haskell Paralellism
by Dominik Luecke 25 Oct '07

25 Oct '07

Hello, I am trying to use the code rels list = let o1 = (map makeCompEntry) $ head $ splitList list o2 = (map makeCompEntry) $ head $ tail $ splitList list o3 = (map makeCompEntry) $ head $ tail $ tail $ splitList list in case (head $ tail $ tail $ tail $ splitList list) of [] -> o1 `par` o2 `par` o3 `seq` o1 : o2 : o3 : [] _ -> let o4 = rels (head $ tail $ tail $ tail $ splitList list) in o1 `par` o2 `par` o3 `par` o4 `seq` o1 : o2 : o3 : o4 to apply some operation on some lists in parallel. The input list "list" is quite long (about 10^17) entries, and I split it into chunks of 1000 entries each. I am compiling with the options " -O2 --make -smp -threaded" and when running the file with "+RTS -N2 -sstderr -RTS", I see that only one worker thread is really doing anything. I am using a vanilla build of ghc-6.6.1 running on Ubuntu 7.10. I am currently wondering about what I am doing wrong. Regards, Dominik -- Dominik Luecke Phone +49-421-218-64265 Dept. of Computer Science Fax +49-421-218-9864265 University of Bremen luecke(a)informatik.uni-bremen.de P.O.Box 330440, D-28334 Bremen PGP-Key ID 0x2D82571B

3 3

Type vs TypeClass duality
by TJ 25 Oct '07

25 Oct '07

Hi again, Following up on my previous thread, I have figured out why it bothered me that we cannot have a list such as the following: ["abc", 123, (1, 2)] :: Show a => [a] It seems to me that there is an annoying duality in treating simple algebraic data type vs type classes. As it stands, I can only have a list where all the elements are of the same basic, ADT, type. There is no way to express directly a list where all the elements satisfy a given type class constraint. If I am not mistaken, type classes are currently implemented in GHC like so: Given a function "show" of type Show a => a -> string, and the expression "show 10", GHC will pass the Int dictionary for class Show's methods and the integer 10 to the function "show". In other words, for each type class constraint in the function type, there will be a hidden dictionary parameter managed entirely by the compiler. What I find strange is, if we can have functions with hidden parameters, why can't we have the same for, say, elements of a list? Suppose that I have a list of type Show a => [a], I imagine that it would not be particularly difficult to have GHC insert a hidden item along with each value I cons onto the list, in effect making the concrete type of the list [(Dictionary Show, a)]. I'm assuming that it will not be particularly difficult because GHC will know the types of the values I cons onto it, so it will most definitely be able to find the Show dictionary implemented by that type, or report a type mismatch error. No dynamic type information is necessary. I am not an OO programming zealot, but I'd like to note here that this is also how (class based) OO languages would allow the programmer to mix types. e.g. I can have a List<Show> where the elements can be instances of Show, or instances of subclasses of Show. Why does this second rate treatment of type classes exist in Haskell? TJ

13 21

XML parser recommendation?
by Ketil Malde 25 Oct '07

25 Oct '07

Hi, I'm struggling to get my HXT-based parser to parse a largish file (<300MB), even after breaking into reasonably-sized chunks. The culprit appears to be parsing one element comprising 25K lines of text, which apparently requires more memory than the 2Gb my computer is equipped with. I'm wondering what approach others use for non-toy XML data. Is the problem due to some error I have made, or should I just ignore the XML, and just parse it "manually" by dissecting bytestrings, or will another XML library serve better? -k -- If I haven't seen further, it is by standing in the footprints of giants

10 20

Haskell Report??
by Galchin Vasili 24 Oct '07

24 Oct '07

Hello, I am trying to use a Haskell shell (Hsh.hs). I am a little frustrated. There are two modules that are imported: import LibPosix import LibSystem I am running WinHugs (Windows) and hugs/ghci can't find LibSystem. I am confused about 1) what are currently standard libraries, etc. in Haskell, 2) what is deprecated and 3) what are "home grown tomatoes" ;^) How current and definitive is the Haskell Report? Kind regards, Bill Halchin

2 1

ANNOUNCE: dataenc 0.9
by Magnus Therning 24 Oct '07

24 Oct '07

My collection of data encoding functions are now available at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc-0.9: Dependencies base License LGPL Copyright Magnus Therning, 2007 Author Magnus Therning Maintainer magnus(a)therning.org Exposed modules Codec.Binary.DataEncoding, Codec.Binary.Base16, Codec.Binary.Base32Hex, Codec.Binary.Base32, Codec.Binary.Uu, Codec.Binary.Base64Url, Codec.Binary.Base64 There's a short description of the library on the wiki: http://haskell.org/haskellwiki/Library/Data_encoding The source darcs repo is available on code.haskell.org. Hopefully the haddockuments will be generated on hackage soon-ish. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus

2 2

Re: [Haskell-cafe] will the real quicksort please stand up? (or: sorting a > million element list)
by Brent Yorgey 24 Oct '07

24 Oct '07

On 10/23/07, Thomas Hartman <thomas.hartman(a)db.com> wrote: > > > Actually I can't compile it, with or without -O2. loads fine into ghci, > but when I try to create an executable I get > > ghc quicksort.hs -o quicksort > quicksort.o: In function `r1Nc_info': undefined reference to > `QuickCheckzm1zi0zi1_TestziQuickCheck_vector_closure' > quicksort.o: In function `r1Nc_info': undefined reference to > `QuickCheckzm1zi0zi1_TestziQuickCheck_zdf16_closure' > quicksort.o: In function `s1Ws_info': undefined reference to > `QuickCheckzm1zi0zi1_TestziQuickCheck_generate_closure' > quicksort.o: In function `r1Nc_srt': undefined reference to > `QuickCheckzm1zi0zi1_TestziQuickCheck_vector_closure' > quicksort.o: In function `r1Nc_srt': undefined reference to > `QuickCheckzm1zi0zi1_TestziQuickCheck_zdf16_closure' > quicksort.o: In function `rzC_srt': undefined reference to > `QuickCheckzm1zi0zi1_TestziQuickCheck_generate_closure' > collect2: ld returned 1 exit status hmm, try adding --make to the ghc command line? Unfortunately, GHC doesn't automatically chase down dependencies for linking; you must specify them explicitly with -package, or tell it to auto-chase dependencies with --make. -Brent

1 0

Job: web development in Haskell
by red nucleus 24 Oct '07

24 Oct '07

Anyone who is serious about doing web development in Haskell is welcome to apply for the job outlined below. If we found a good functional programmer, we can ignore the bits about Rails. Web Developer RedNucleus Ltd (rednucleus.co.uk) is seeking a highly motivated programmer for a full or part-time posititon developing social web applications. Initially you will develop and maintain applications using RubyOnRails or a similar framework, but there will be opportunities to explore new web programming paradigms with declarative languages. The successful application will have: -a good degree in computer science or a quantitative topic. -good independent programming and quantitative analysis skills. -some familiarity with HTML, CSS, JavaScript and SQL, and with network programming. -exposure to functional programming methodologies, e.g. in Haskell, Lisp or Erlang. Graduate level qualifications in computer science and expertise in web development including RubyOnRails would be a plus. RedNucleus is based in Beeston, Nottingham. The successful application could work here or mostly from home. Salary depends on experience and qualifications, starting from £22,000 plus performance bonus. To apply, please prepare a single pdf file containing your cover letter and CV, and name the file with your surname, e.g. smith.pdf. Then send the file to: rednucleusltd(a)gmail.com. Informal enquiries can be directed to the same address. The closing date for applications is 22nd November 2007

1 0

ANN: new version of matrix library (GSLHaskell)
by Alberto Ruiz 24 Oct '07

24 Oct '07

Dear Haskellers, I am happy to announce a new version of GSLHaskell, a basic library for matrix computations and other numeric algorithms based on LAPACK, BLAS and GSL. The goal is that simple problems involving singular values, linear systems, etc, can be easily solved using Haskell. The library is horribly incomplete, I don't recommend it for any serious work. But at least I enjoy using it in my projects... Some features of this release are: * Improved internal implementation, most code has been refactored. * It works on Linux, Windows, and MacOS X (see the README of the distribution). * Automatic bindings to 200+ GSL special functions. Source code, documentation, updated tutorial, and more information is available from: http://dis.um.es/~alberto/GSLHaskell I'd like to upload a decent version to hackage, so any kind of feedback is very welcome! Thanks, Alberto

1 0

A possibly stupid doubt about the GHC's overlapping instances flag's
by Rodrigo Geraldo 24 Oct '07

24 Oct '07

Hi! Suppose that the GHC's flag -fallow-incoherent-instances is enabled. In this situation, when a instance will be rejected? And if the flag -fallow-overlapping-instances is enabled. When a instance will be rejected? Thanks! Rodrigo

3 3

string literals and haskell'
by David Roundy 24 Oct '07

24 Oct '07

I've just been annoyed with errors ghc reports when I use a string literal such as ":\/:" (which is a contructor in darcs). Of course, it wants ":\\/:", but I'd rather type the former. Is there any reason why the language couldn't be modified (e.g. in haskell') to make the former legal? i.e. to treat string literals with '\\' followed by a character that doesn't describe an escape as a literal backslash? It makes the rules a bit more complicated, but doesn't modify the meaning of any currently-legal code, and removes a potential error. -- David Roundy Department of Physics Oregon State University

5 4

OS Abstraction module??
by Galchin Vasili 24 Oct '07

24 Oct '07

Hello, In the Haskell libraries, is there an OS abstraction module, that would "hide" the POSIX API and Win-32 API? If not, this would be nice so that Haskell "programs" could be written in an OS independent manner! Kind regards, Bill Halchin

9 20

Using Haddock to document ADTs
by Alfonso Acosta 24 Oct '07

24 Oct '07

Hi all, I'm beginning to get familiar with Haddock and I want to document a library which, as usually happens, has some ADT definitions. I'd like to document the ADTs both for the end-user (who shouldn't be told about its internal implementation) and future developers. Let's imagine my library is called FooLib, defines an ADT whose implementation is hidden by an outer module. =========== FooLib/ADT1.hs ===== -- | This module defines my type ADT1 module ADT1 -- | This type implements whatever and. As you can see it's implemented -- using blah blah so that is more efficient because of blah blah data ADT1 a = ... ========== Foolib.hs ============ -- | This is Foolib which is aimed at whatever module Foolib (ADT) where import ADT =================== Generating documentation for Foolib.hs with this Haddock annotations will unfortunatelly reveal implementation details since the annotation of ADT1 is forwarded. My question is, How can I generate documentation for both developers and the end-users using the same Haddock annotations? I'm beginning to think it would be a good idea to distinguish those two cases (documentation for end-users and internal developers) in Haddock, but I'm probably wrong since it seems a common thing and wasn't implemented yet. Best Regards, Alfonso

3 4

Back on opengl win32 again? no window
by bbrown 23 Oct '07

23 Oct '07

Ok, sorry to bring this back up again. I asked before and kind of went to a working linux system. My question; On Win32, has anyone seen where you try to run the application but the window will not stay open. There are no errors, the window with 3D objects will not stay open. I was using a recent haskell 6.6.1 (ghc-6.6.1- i386-windows.exe). I asked before it seemed like the others suggested I use freeglut and use that. But I dont really need freeglut extensions. I am using basic opengl. I noticed that opengl is included in the ghc (mingw) I downloaded. Also, the code compiles, just doesnt run. The machine itself is a 3.2ghz, nvidia machine (win2k). I know opengl code should work. You can see the source here: http://octanemech.googlecode.com/svn/trunk/octanemech/src/misc/camera/ I compiled with: ghc --make MechCamera.hs -prof -auto-all -o $@ Basically the initialization aspect: import Graphics.Rendering.OpenGL import Graphics.UI.GLUT import Data.IORef mechLaunchGLWindow = do getArgsAndInitialize -- Initialize with the following settings -- Double buffer, RGBA color -- Alpha components supported, Depth buffer initialDisplayMode $= [ DoubleBuffered, RGBAMode, WithDepthBuffer, WithAlphaComponent ] initialWindowSize $= Size mechScreenWidth mechScreenHeight createWindow mechWindowTitle mechInitGL let mainCamera = initCamera (1.4, 4.2, -4.5) (0,0,0) (0,0,0) mainMech = initMech (0, 0, 0) (0, 0, 0) mainCameraRef <- newIORef(mainCamera) mainMechRef <- newIORef(mainMech) -- Set the OpenGL callbacks displayCallback $= displayGameLoop mainCameraRef mainMechRef -- Set the keyboard callback for handling actions keyboardMouseCallback $= Just (mechKeyPressed mainCameraRef) idleCallback $= Just idle mainLoop -- Berlin Brown [berlin dot brown at gmail dot com] http://botspiritcompany.com/botlist/?

1 1

ANNOUNCE: zlib and bzlib 0.4 releases
by Duncan Coutts 23 Oct '07

23 Oct '07

I'm pleased to announce updates to the zlib and bzlib packages. The releases are on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bzlib What's new in these releases is that the packages work with a wider range of systems and versions of dependent packages. In particular: * Works "out of the box" on Windows It uses a bundled copy of the zlib C library (version 1.2.3) on Windows and uses the system zlib on all other platforms. * Works with ghc-6.4, 6.6, 6.8 * Works with new bytestring versions * Works with older versions of zlib (eg zlib 1.1 on MacOS X) They require Cabal-1.2.1 (which is also available on hackage and works with all ghc versions). The zlib and bzlib packages provide functions for compression and decompression in the gzip and bzip2 formats. Both provide pure functions on streams of data represented by lazy ByteStrings: compress, decompress :: ByteString -> ByteString This makes it easy to use either in memory or with disk or network IO. For example a simple gzip compression program is just: > import qualified Data.ByteString.Lazy as ByteString > import qualified Codec.Compression.GZip as GZip > > main = ByteString.interact GZip.compress Or you could lazily read in and decompress .gz file using: > content <- fmap GZip.decompress (ByteString.readFile file) Both packages are bindings to the corresponding C libs, so they depend on those C libraries. Fortunately both zlib and bzlib2 are available on every OS. It also means that the compression speed is as you would expect since it's the C lib that is doing all the work. The zlib package is now being used in cabal-install to work with .tar.gz files. So it has actually been tested on Windows. The development versions have new homes on code.haskell.org. I'm very happy to get feedback on the API, the documentation or of course any bug reports. Duncan

2 2

Re: [Haskell] [Fwd: undecidable & overlapping instances: a bug?]
by Mark P Jones 23 Oct '07

23 Oct '07

[Sorry, I guess this should have been in the cafe ...] Simon Peyton-Jones wrote: > The trouble is that > a) the coverage condition ensures that everything is well behaved > b) but it's too restrictive for some uses of FDs, notably the MTL library > c) there are many possibilities for more generous conditions, but > the useful ones all seem complicated > > Concerning the last point I've dumped the current brand leader > for (c) into http://hackage.haskell.org/trac/ghc/ticket/1241#comment:15. > > Better ideas for (c) would be welcome. Let's take the declaration: "instance P => C t where ..." The version of the "coverage condition" in my paper [1] requires that TV(t_Y) \subseteq TV(t_X), for each dependency (X->Y) \in F_C. (I'm using the notation from the paper; let me know if you need more help to parse it.) This formulation is simple and sound, but it doesn't use any dependency information that could be extracted from P. To remedy this, calculate L = F_P, the set of functional dependencies induced by P, and then expand the right hand side of the set inequality above by taking the closure of TV(t_X) with respect to L. In symbols, we have to check that: TV(t_Y) \subseteq TV(t_X)^+_L, for each (X->Y) \in F_C. I believe (but haven't formally proved) that this is sound; I don't know how to make a more general "coverage condition" without losing that. I don't know if it's sufficient for examples like MTL (because I'm not sure where to look for details of what that requires), but if it isn't then I'd be very suspicious ... All the best, Mark [1] http://www.cs.pdx.edu/~mpj/pubs/fundeps-esop2000.pdf

5 10

Learning hierarchy
by Lutz Donnerhacke 23 Oct '07

23 Oct '07

Hi, in order to upload my teaching programms to hackage, I look for an accepted hierarchy to put the modules into. My current version is Learning.Donnerhacke.Algorithms.Sort ... etc. pp. Would such a hierarchy make sense? Is the a hope for inclusion of other modules? The main purpose of this moduls in my special case are: Implementation of the algorithms from Sedgewick's famous book. They are combined with a ver short program

1 1

Beginners problem with the type system
by Henrik Tramberend 23 Oct '07

23 Oct '07

Dear Hasekellers, I am trying to build some abstractions on top of the module Control.Concurrent and run into a problem with the type system that I do not understand. In particular, I want to define two classes 'Channel' and 'Port' (see below) that define a common interface for several concrete implementations with different synchronization characteristics. The following code is a simplified excerpt that demonstrates the problem: > module Main where > import Control.Concurrent.MVar > class Channel c where > port :: Port p => c a -> IO (p a) > class Port p where > take :: p a -> IO a > put :: p a -> a -> IO () The problem arises when I instantiate the 'Channel' class and implement the 'port' function. > data C a = C (P a) > instance Channel C where > port (C p) = return p Couldn't match expected type `p' (a rigid variable) against inferred type `P' `p' is bound by the type signature for `port' Expected type: p a Inferred type: P a In the first argument of `return', namely `p' In the expression: return p I am quite new to Haskell and my knowledge of how the type system works is fairly limited. Any help on this particular problem will be greatly appreciated, but any pointers to reading material that brings me closer to enlightenment are also very welcome. Thanks, Henrik > newC = do > p <- newP > return (C p) > data P a = P (MVar a) > newP = do > v <- newEmptyMVar > return (P v) > instance Port P where > take (P mv) = takeMVar mv > main = do > c <- newC > p <- port c > put p 1

3 3

Polymorphic (typeclass) values in a list?
by TJ 23 Oct '07

23 Oct '07

Why is it illegal to store values of differing types, but which instance the same class, into a list? e.g. a = [ 1, 2.0 ] :: Num a => [a] After all, sometimes all you need to know about a list is that all the elements support a common set of operations. If I'm implementing a 3d renderer for example, I'd like to have class Renderable a where render :: a -> RasterImage scene :: Renderable a => [a] Instead of hardcoding a bunch of types as being Renderable, as in data Renderable = Point Something | Line Something | Polygon Something scene :: [Renderable] Or maybe data Point = Point Something data Line = Line Something data Polygon = Polygon Something scene :: { points :: [Point], lines :: [Line], polygons :: [Polygons] } Is there a way of achieving what I want to do? Existentials maybe? I'm still learning the basic stuff and don't grok existentials at all, but I even if I use those, I'll still have to wrap things up in a constructor, won't I? Thanks a bunch, TJ

9 16

Function Types
by PR Stanley 23 Oct '07

23 Oct '07

Hi What are the rules for calculating function types? Is there a set procedure ? Thanks, Paul

3 2

Transformation sequence
by Andrew Coppin 23 Oct '07

23 Oct '07

I'm writing some code where I take an expression tree and transform it into another equivilent one. Now it's moderately easy to write the code that does the transformation. But what I *really* want is to print out the transformation *sequence*. This appears to be much more awkward. What I have is a function like this: transform :: Expression -> [Expression] The trouble is, if you want to apply the transformation recursively, things get very messy. Oh, it *works* and everything. It's just really messy and verbose. In Haskell, this is usually a sign that you want to start applying some ingenious trickery... but I'm having an ingeniety failure here. Suppose, for example, that in one case you want to recursively transform two subexpressions. I end up writing something like transform (...sub1...sub2...) = let sub1s = transform sub1 sub2s = transform sub2 in map (\sub1' -> put sub1' back into main expression) sub1s ++ map (\sub2' -> put sub2' back into main expression) sub2s After you've typed that a few times, it becomes *very* boring! But I can't think of a clean way to abstract it. :-( It's *almost* like you want to use the list monad: transform (...sub1...sub2...) = do sub1' <- transform sub1 sub2' <- transform sub2 return (put sub1' and sub2' back into the main expression) Except that that doesn't quite work properly. As shown above, I actually want to go through all the transformation steps for the first branch, and *then* all the steps for the second branch. Any hints?

8 12

wordsBy in the base libraries?
by Maxime Henrion 23 Oct '07

23 Oct '07

Hello all, What do you think about having a wordsBy function in the standard libraries? It often comes in handy. > wordsBy :: (a -> Bool) -> [a] -> [[a]] > wordsBy p s = case dropWhile p s of > [] -> [] > ':rest -> (s':w) : wordsBy p s'' > where (w, s'') = break p rest This version takes care of avoiding a redundant character check that is present in the standard definition of the words function, originally discovered by Neil Mitchell, see his blog post here: http://neilmitchell.blogspot.com/2007/07/equational-reasoning-in-haskell.ht… Then we can define the words function naturally: > words :: String -> [String] > words = wordsBy isSpace Cheers, Maxime

5 11

Re: [Haskell-cafe] Function Types
by PR Stanley 23 Oct '07

23 Oct '07

f x = x x :: a f x :: b therefore f :: a -> b x = a and x = b therefore a = b therefore f :: a -> a Simple mappings are easy to work out. It's the more detailed stuff I'm not sure about. f g x y = g x (y x) Cheers, Paul At 03:15 23/10/2007, you wrote: >On 10/22/07, PR Stanley ><<mailto:prstanley@ntlworld.com>prstanley(a)ntlworld.com> wrote: >Hi >What are the rules for calculating function types? >Is there a set procedure ? >Thanks, Paul > > >There must be a set procedure, since otherwise the compiler could >not function! =) > >Seriously, though, I'm not exactly sure what you're asking >for. Could you maybe provide a few examples of the kind of thing >you're asking about? > >-Brent

4 3